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Page 1: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

Users may download and print one copy of any publication from the public portal for the purpose of private study or research.

You may not further distribute the material or use it for any profit-making activity or commercial gain

You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Downloaded from orbit.dtu.dk on: Jan 19, 2021

Identifying simple and cost-effective gear solutions for an effective implementation ofthe new EU Common Fisheries Policy (CFP)

Melli, Valentina

Publication date:2019

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Melli, V. (2019). Identifying simple and cost-effective gear solutions for an effective implementation of the newEU Common Fisheries Policy (CFP). Technical University of Denmark.

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Identifying simple and cost-effective gear solutions for an effective implementation of the new EU Common Fisheries Policy (CFP)

PhD Thesis

By Valentina Melli

DTU Aqua National Institute of Aquatic Resources

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Identifying simple and cost-effective gear solutions for

an effective implementation of the new EU Common

Fisheries Policy (CFP)

Ph.D. Thesis, 2018

Valentina Melli

Technical University of Denmark

National Institute of Aquatic Resources

Section for Management Systems - Fisheries Technology, Hirtshals, Denmark

”It is not the strongest of the species that survives, nor the most intelligent, it is

the one that is most adaptable to change” - Charles Darwin

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Preface

The present thesis is submitted in partial fulfilment of the requirements for obtaining a

Doctor of Philosophy (Ph.D.) degree. The thesis consists of a review and four supporting

papers. Three papers are published and one is a manuscript ready for publication.

I wish to express my sincere gratitude to my supervisors: Dr Ludvig A. Krag, Dr Junita D.

Karlsen and Prof Henrik Gislason from the Technical University of Denmark – National

Institute of Aquatic Resources (DTU Aqua). Their support and guidance have been

invaluable for me and this dissertation. Further, I would like to thank Dr Bent Herrmann

from SINTEF for the endless hours spent developing analytical facilities for my analyses.

I also owe a debt of gratitude to:

- all my highly appreciated colleagues in the Fisheries Technology group for many

inspiring talk and discussions, and for being always there whenever I needed;

- the amazingly skilled DTU Aqua Fisheries technicians for measuring thousands

and thousands of fish and Nephrops with me, helping to deal with any technical

challenge during the experimental trials and most of all, for being always

interested and passionate about the work;

- Helle Andersen for helping me extract the data, with precious care and precision;

- my colleague and friend Tiago Malta, for sharing with me every very step of these

three years. I couldn’t have asked for a better person to share the office with;

- my dear partner Marco for following me in this adventure and all over the world.

You made me brave.

Finally, I would like to acknowledge the financial support granted by the European

Maritime and Fisheries Fund and the Ministry of Environment and Food of Denmark that

made the research described in the supporting papers possible. Projects: FlexSelect –

Scaring lines, an innovative and flexible solution for the Nephrops fishery (Grant

Agreement No 33113-I-16-068) and Vision - Development of an optimal and flexible

selective system for trawl by use of new technology and underexploited fish behaviour

(Grant Agreement No 33113-I-16-015).

Hirtshals, December 2018

Valentina Melli

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Table of contents

Preface ................................................................................................................. 3

Dansk resumé (abstract in Danish) ....................................................................... 9

Abstract ............................................................................................................... 11

1. Introduction ..................................................................................................... 13

2. The outcomes of a discard ban: a global overview ......................................... 17

3. Case study fishery: why the Nephrops trawl fishery? ...................................... 19

3.1 Fishing dynamics ....................................................................................... 20

3.1.1 Nephrops catchability .......................................................................... 21

3.1.2 Fish catchability ................................................................................... 22

3.2 Bycatch reduction measures before the landing obligation........................ 24

3.3 Bycatch reduction under the landing obligation ......................................... 24

4. Gear modifications for the Nephrops-directed mixed trawl fishery .................. 27

4.1 Anterior modifications ................................................................................ 28

4.1.1 Doors, sweeps and bridles .................................................................. 29

4.1.2 Counter-herding and anterior fish excluder devices ............................ 29

4.1.3 Headline height ................................................................................... 29

4.1.4 Topless or cutaway trawl ..................................................................... 30

4.1.5 Netting tapering ................................................................................... 31

4.2 Posterior modifications .............................................................................. 33

4.2.1 Trawl body mesh size .......................................................................... 33

4.2.2 Horizontal separator panel .................................................................. 34

4.2.3 Grids .................................................................................................... 35

4.2.4 Square mesh panels (SMP) ................................................................ 35

4.2.5 Sieve panels ........................................................................................ 36

4.2.6 Horizontally divided codends ............................................................... 36

4.2.7 Codend configuration .......................................................................... 37

5. Flexible gear modifications ............................................................................. 39

5.1 Flexible anterior modifications ................................................................... 40

5.2 Flexible posterior modifications ................................................................. 42

6. Towards haul-by-haul control over selectivity ................................................. 45

7. Conclusions and future work ........................................................................... 47

Reference List ..................................................................................................... 49

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List of papers

Paper I: Melli, V., Karlsen, J. D., Feekings, J. P., Herrmann, B., Krag, L.

A., 2018. FLEXSELECT: counter-herding device to reduce

bycatch in crustacean trawl fisheries. Canadian Journal of

Fisheries and Aquatic Sciences, 75: 850–860. https://doi:

10.1139/cjfas-2017-0226

Paper II: Melli, V., Krag, L.A., Herrmann, B., Karlsen, J.D., 2018.

Investigating fish behavioural responses to LED lights in trawls

and potential applications for bycatch reduction in the Nephrops-

directed fishery. ICES Journal of Marine Science, 75: 1682–1692.

https://doi.org/10.1093/icesjms/fsy048

Paper III: Melli, V., Krag L.A., Herrmann, B., Karlsen J.D., 2019. Can active

behaviour stimulators improve fish separation from Nephrops

(Nephrops norvegicus) in a horizontally divided trawl codend?

Fisheries Research. https://doi.org/10.1016/j.fishres.2018.11.027

Paper IV: Melli, V., Herrmann, B., Karlsen J.D., Feeking, J.P., Krag L.A.

Predicting optimal combinations of bycatch reduction devices in

fishing gears: a meta-analytical approach. Manuscript.

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Dansk resumé (abstract in Danish)

Med den nye Fælles fiskeripolitik (FFP), sigtede den Europæiske Union (EU)

mod at fjerne discarden af kommercielle arter, gennem indførelsen af en

forpligtelse til at lande hele fangsten (”landings forpligtelsen”). Denne fangst-

baserede tilgang, hvor alle størrelser af regulerede arter skal landes og

modregnes kvoten, er designet til at tilskynde fiskere til at undgå uønsket fangst.

For at kunne imødekomme dette, er der behov for et større udvalg af redskaber,

der matcher ændringer både i forekomsten af arter og størrelser men også i

skipperens aktuelle ønske om en fangstsammensætning, der kan maksimere

profitten inden for kvoten. Fleksible redskabsløsninger, som det enkelte fartøj kan

ændre fra slæb til slæb for at tilpasse deres størrelses- og arts-selektion, kan føre

til en effektiv implementering af FFP’en samtidig med at fiskeriets økonomiske

bæredygtighed opretholdes. Disse løsninger er specielt vigtige i flerartsfiskerier,

hvor ”choke”-arter kan begrænse udnyttelsen af mere produktive bestande. Dette

studie fokuserer derfor på det danske flerartsfiskeri efter Jomfruhummer

(Nephrops norvegicus), der har en af de højeste rater af uønsket fangst og derfor

ventes at blive stærkt påvirket af den nye FFP. Vi undersøgte fleksible

redskabsmodifikationer som kunne støtte alternative fangststrategier, såsom

reduktionen af bifangst af fisk både over og under det kommercielle mindstemål,

eller en selektiv tilbageholdelse af kun den mest værdifulde bifangst.

Afhandlingen består af en sammenfatning og 4 artikler.

Artikel 1 indeholder udvikling og afprøvning af det første studie af skræmmeliner i

jomfruhummerfiskeriet. Skræmmeliner er en fleksibel anordning der let kan

monteres og afmonteres foran selve trawlen og er designet til at lede fisk ud af

trawlsporet. Effektiviteten af skræmmelinerne varierede med fiskenes art og

størrelse men ikke med tidspunktet på dagen. Resultaterne viste at fangst af

potentielt uønskede fiskearter kunne undgås uden at fangsten af jomfruhummer

blev påvirket.

Artikel 2 fokuserer på et lovende design, den horisontalt delte fangstpose, som

kan føre til en fleksible opdeling af fangsten. Vi prøvede at anvende visuel

stimulering til at forbedre artsopdelingen og sammenlignede dette med et

baseline-redskab. Ved brug af ”Light Emitting Diodes” (LED) undersøgte vi om

enten positiv eller negativ phototaxis, kunne bruges til at øge den vertikale

separering af fisk fra jomfruhummer.

Artikel 3 fortsætter undersøgelsen af den horisontalt delte fangstpose men ved

brug af andre typer af adfærdsstimulering. Vi undersøgte om, og i hvilken

udstrækning, det er muligt til at forbedre den vertikale opdeling af fisk fra

jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks

undvigelsesadfærd. Vi undersøgte to slags adfærdsstimulatorer: et gardin af

kæder ved indgangen til den nedre fangstpose og en serie af flydeliner foran den

opdelte fangstpose.

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Artikel 4 beskriver en meta-analytisk tilgang til at forudsige størrelsesselektionen

af et redskab med flere forskellige bifangst-reducerende anordninger samt til at

sammenligne deres effektivitet under forskellige fangstscenarier. Vi brugte denne

teoretiske tilgang på trawlfiskeriet efter jomfruhummer for at identificere den mest

egnede kombination af bifangstanordninger og den alternative fangststrategi som

denne kombination ville kunne understøtte. Denne meta-analytiske tilgang kan

accelerere processen med at identificere optimal brug af fleksible

redskabsløsninger og dermed udvide fiskerens muligheder i håndteringen af EU

landingsforpligtelsen.

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Abstract

With the new Common Fisheries Policy (CFP) the European Union (EU) aimed at

eliminating the discard of commercial species, introducing the obligation to land

all catches (“landing obligation”). This catch-based approach, where all sizes of

regulated species have to be landed and counted against quota, is designed to

encourage fishermen to minimize unwanted catches. Therefore, fishermen are

now in need of fishing gear options to cope with the variability in unwanted

catches and maximize their profit within the allowed catch limits. Flexible and

specialized gear solutions, which can be used on a haul-by-haul basis to adjust

size and species selectivity, can lead to an effective implementation of the CFP

while maintaining the economic viability of the fishery. These solutions are

particularly urgent in mixed trawl fisheries, where “choke” species can limit the

exploitation of more productive stocks. Therefore, this study focused on the

Danish Nephrops (Nephrops norvegicus) directed mixed trawl fishery, one of the

economically most important fisheries in Europe. This multispecies fishery has

one of the highest rates of unwanted catches, and is expected to be strongly

affected by a fully implemented and controlled landing obligation. We investigated

flexible gear modifications that could support alternative harvest strategies, such

as the reduction of both undersized and commercial sized fish bycatch or the

retention of only the most valuable bycatch species and sizes. The thesis

consists of a review and four papers.

Paper I contains the development and test of the first counter-herding device for

Nephrops-directed trawl fisheries. This flexible anterior modification, easily

mountable and de-mountable on the gear at a haul-by-haul level, was designed

to lead fish out of the trawl path. Its efficiency varied across species and sizes,

but was consistent regardless of diel period. The results showed a major

reduction of catches of potentially unwanted fish species, in particular haddock

(Melanogrammus aeglefinus) and whiting (Merlangius merlangus), with no effect

on Nephrops catches.

Paper II focuses on a horizontally divided trawl codend, which could lead to a

flexible separation of the catch in different compartments of the trawl. We

attempted to use visual stimulation to improve species separation. Using Light

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Emitting Diodes (LED), we investigated if either positive or negative phototaxis

could be used to improve fish vertical separation from Nephrops. The results

showed significant changes in vertical separation but no clear species-specific

phototactic response. Moreover, overall LED lights increased the proportion of

individuals entering the lower compartment together with Nephrops.

Paper III continues the research on the horizontally divided trawl codend, but

applying other types of behavioural stimulators. We investigated if and to which

extent it is possible to improve the vertical separation of fish from Nephrops by

adding active stimulators designed to exploit fish avoidance behaviour. We tested

two types of behaviour stimulators: a chain curtain at the entrance of the lower

compartment and a set of rising float-lines ahead of the point of separation. The

results showed that species separation can be partially improved by the

stimulators, but the effect may not be sufficient to justify the additional complexity

in design with respect to the baseline.

Paper IV describes a meta-analytical approach to predict the size-selectivity of a

gear with a combination of Bycatch Reduction Devices (BRDs) and to compare

their performance under different catch scenarios. We applied this theoretical

approach to the Nephrops-directed trawl fishery, to identify the most pertinent

BRDs combinations and the alternative harvest strategies that they could

support. By including the results obtained in the previous papers, as well as

relevant BRDs available in literature, we predicted the selectivity of up to 100

possible combinations. Their performance was investigated for the target

species, Nephrops, and two bycatch species, cod (Gadus morhua) and haddock.

This meta-analytical approach can accelerate the process of identifying optimal

uses of flexible gear solutions, broadening fishermen’s options when coping with

the EU landing obligation.

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1. Introduction

Mixed trawl fisheries are characterized by complex combinations of harvested

species, varying in productivity (i.e. abundance and state of the stock) and

economic value. Although theoretically all the commercial species caught are

target, in practice, within mixed fishery, individual catch goals vary according to

multiple economic, sociological, technical and legislative factors (Salomon et al.,

2014). In particular, the management framework strongly influence which fraction

of the catch is wanted and which one is unwanted. For example, measures such

as quotas and minimum landing sizes can increase the unwanted fraction

(Graham et al., 2007; Feekings et al., 2012). These unwanted catches, also

known as “bycatch”, include undersized and/or damaged target species, valuable

species whose quota is not available or has been exhausted, and low valued or

non-commercial species (Kelleher, 2005). In most mixed fisheries, the unwanted

fraction can equal or even exceed the wanted one (Hall and Mainprize, 2005;

Kelleher, 2005). Unless otherwise regulated, these unwanted catches are

discarded at sea, often dead or injured (Evans et al., 1994). Because discarding

unwanted catches is generally considered as an ecological and economical

waste of marine resources, most mixed fisheries around the world are managed

through mandatory technical measures (Kennelly, 2007). These technical

measures, termed Bycatch Reduction Devices (BRDs; Kennelly, 2007), are

designed to restrict the amount of unwanted catches, by modifying the gear

selectivity. The term “selectivity” expresses the ability of a gear to retain the

individuals encountered, on the basis of factors such as species, size and

behaviour (MacLennan, 1992). For trawl gears, the selectivity is mostly

determined by the characteristics of the codend, i.e. the final part of the gear

where the catch accumulates (Glass, 2000). In particular, regulation of the

selectivity of trawl gears has historically focused on mesh size and shape (Glass,

2000; Herrmann et al., 2009). Nevertheless, BRDs can be introduced in the

codend or ahead of it to select out undersized individuals and/or unwanted

species. However, depending on the management framework in place, the BRDs

can compromise the economic viability of the fishery and limit its capacity to cope

with spatial and temporal variability in catches.

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In the European Union (EU), the management system consists of total allowable

catches (TACs) for the most valuable marine commercial species, determined

annually on the basis of estimated Maximum Sustainable Yields, i.e. “the

maximum annual catch which on average can be taken from an exploited stock

without deteriorating its productivity” (Salomon and Holm-Müller, 2013). As most

fishing grounds in the EU are shared among different Member States, TACs are

assigned per fishing area and then divided among the countries. Moreover, to

prevent unknown and unrecorded discarding of unwanted catches from

compromising the efficiency of the quota system (Punt et al., 2006; Crowder and

Murawski, 1998), the new EU Common Fishery Policy (CFP) introduced a

landing obligation for important harvested stocks (EU, 2013; 2016). The so-called

“discard ban” compels fishermen to land all catches, both wanted and unwanted.

Consequently, unwanted catches now count against fishermen’s quotas.

Moreover, this creates additional costs for the industry due to the processing of

the unwanted fraction of the catch (Hall et al., 2000; Hall and Mainprize, 2005).

Both sorting time and handling costs will likely increase as a bigger part of the

catch has to be separated and stored; on a limited storage space this could force

fishermen to increase the number of journeys to the harbour. Moreover, in mixed

fisheries, whenever the quota for one species is exhausted, and the catch of that

species cannot be avoided, fishing activities have to stop. These “choke” species

can potentially lead to the under-exploitation of more productive stocks, with

consequences on the economy of the fishery (Ulrich et al., 2011; Baudron and

Fernandes, 2015). Therefore, one of the main expected outcomes of a discard

ban is to strongly incentivize fishermen to couple selectivity with economy (Hall

and Mainprize, 2005; Graham et al., 2007). Indeed, in the frame of a landing

obligation, it is in fishermen’s interest to avoid or reduce the amount of unwanted

catches by improving the selectivity of the fishing gear, for example adopting

BRDs.

To be implemented effectively, with less undesirable economic impacts on the

industry, a landing obligation needs to be combined with flexible technical

regulations to increase fishermen’s ability to adjust the selectivity of their gears

(EU, 2016; Eliasen et al., 2019). The legislation of BRDs is often too rigid and

follows a “one-gear-fits-all” approach, where technical solutions are applied at the

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fishery or regional level. In contrast, since the amount of unwanted catches is

mostly determined by the combination of gear, fishing practice and quota

availability, and since these may differ among vessels, the economic

consequences of the landing obligation can be expected at a vessel level.

Therefore, each vessel should be able to choose from a “toolbox” of BRDs to

better match the gear selectivity with specific catch goals. Moreover, in mixed

fisheries one BRD is rarely enough to cope with the spatial and temporal

variability in catch composition, as well as inter-annual variation in quota limits.

More gear options need to be identified to support alternative harvest strategies

(Eliasen et al., 2019). A toolbox of flexible gear modifications, which can be

temporarily applied to the gear without requiring major structural changes, could

enable a more dynamic adjustment of the gear selectivity at the haul-by-haul

level.

The present PhD thesis aimed at: i) developing a gear solution to prevent

potentially unwanted catches from entering the trawl, ii) determining to which

extent target and bycatch species can be separated inside the trawl, and iii)

investigating the potential of combinations of such gear solutions to achieve

optimal selectivity profiles. We used as a case study the Danish Nephrops

(Nephrops norvegicus) directed mixed trawl fishery, one of the most economically

important fisheries in Europe and the Northeast Atlantic, and the most

challenging in terms of bycatch reduction (Kelleher, 2005). In the present review,

an overview of BRDs available for the case-study fishery is presented and used

to discuss the definition of a “flexible gear modification”. According to this

definition, Paper I presents the development and testing of a flexible anterior

modification. Paper II and III advance the knowledge on a flexible posterior

modification. Finally, Paper IV addressed the question of predicting optimal

BRDs combinations, using the results obtained in Paper I-III, as well as previous

gear modifications described for the case-study fishery.

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2. The outcomes of a discard ban: a global overview

Landing obligations, better known as discard bans, have been used as a

management tool all over the world in order to achieve full accountability of all

catches. Positive long-term outcomes have been described for countries outside

the EU that have been managing discard bans for decades (Diamond and

Beukers-Stewart, 2011; Condie at al., 2014; Karp et al., 2019). For example, a

discard ban for Pacific cod (Gadus macrocephalus) and pollock (Theragra

chalcogramma) has been enforced in the US Alaskan groundfish fishery since

1998 (Graham et al., 2007). In response to the ban, the fishery adopted gear

modifications to improve the selectivity of the fishing gears used and cooperation

among the vessels was observed in signalling bycatch hotspots to be avoided.

This led to a reduction of the discard rates of both pacific cod and pollock to just

0.4% and less than 1%, respectively (Graham et al., 2007). Another example is

represented by the British Columbia groundfish trawl fishery, in which the

discarding of Sebastes spp. is prohibited. Here, the management system has

introduced both incentives for the fishermen to match catches to quotas, in the

form of individual transferable quotas, and deterrents for illegal discarding, such

as a 100% coverage monitoring programme (Branch and Hilborn, 2008).

Although in these examples the discard ban is limited to few commercial species,

there are successful cases of discard bans where all commercial species are

included. This is the case of the Norwegian discard ban, which started for cod

and haddock in 1987 and was then extended gradually to include all living marine

resources in the following 30 years (Gullestad et al., 2015). Besides enforcing the

ban, the core of the Norwegian discard ban involves a set of “pragmatic

exemptions” (Gullestad et al., 2015) to increase flexibility and sustain the viability

of those fishermen that can demonstrate responsible behaviour and conduct (e.g.

allowed discard of live individuals and damaged catch in “small quantities”;

Gullestad et al., 2015).

Successful examples of discard ban have in common specific features, such as a

high level of surveillance, with serious consequences for those that violate the

rules (Hall et al., 2000; Branch and Hilborn, 2008), but also incentives for the

industry to comply to the ban (Hall and Mainprize, 2005; Stockhausen et al.,

2012) and an overall set of measures aimed at reducing the amount of unwanted

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catches in the first place, thus simplifying their management on land (Condie et

al., 2014; Salomon et al., 2014). In particular, flexibility in gear-based technical

measures and legislation has been identified as key to obtain a reduction of

bycatch rates without causing the fishery to collapse (Condie et al., 2014; EU,

2016; Eliasen et al., 2019, Karp et al., 2019).

Consequently, for an effective implementation of the landing obligation, together

with its enforcement, it is necessary to address the question of to what extent the

selectivity of a specific fishery can be adjusted. As described in Section 1, this

question is of particular importance in mixed species fisheries, where choke

species can strongly affect the capitalization of quotas.

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3. Case study fishery: why the Nephrops trawl fishery?

The Nephrops-directed mixed trawl fishery is one of the most profitable fisheries

in Denmark, with approximately 184 vessels targeting Nephrops for at least part

of the year of (2017 data; Danish Fisheries Agency). The main fishing areas are

in the North Sea (ICES Division IVa and IVb), Skagerrak and Kattegat (ICES

Division IIIa; Fig. 1). Total landings of Nephrops in 2017 were above 4,000

tonnes, for a value of approximately 250 million DKK (33 million Euro;

http://www.statistikbanken.dk). In addition, these vessels landed approximately

2,000 tonnes of fish, including cod (Gadus morhua), saithe (Pollachius virens),

hake (Merluccius merluccius), haddock (Melanogrammus aeglefinus), plaice

(Pleuronectes platessa), witch flounder (Glyptocephalus cynoglossus) and

monkfish (Lophius piscatorius), among others. Therefore, because of its highly

morphologically diverse catch and the recovering status of some gadoid stocks in

the area, this fishery has been classified as “very high risk” in terms of non-

compliance to the landing obligation (Anon, 2015).

Figure 1. Geographic position and ICES classification of the main fishing areas for the case-study fishery. Illustration by Dr Thomas Noack.

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3.1 Fishing dynamics

Most vessels operating in the Danish Nephrops-directed mixed trawl fishery have

quota to land fish species which can contribute up to 2/3 of the profit of the

fishery (Danish Fisheries Agency). Consequently, the Danish fishery adopts the

so-called Combi trawls, which were designed to maximize the retention of both

Nephrops and fish species. Contrary to strictly Nephrops-directed trawls, Combi

trawls include longer sweeps, a higher headline height and an extension between

the trawl body (i.e. tapered section) and codend (Fig. 2a). All these elements

contribute to increasing fish catchability (Winger et al., 2010).

(a)

(b)

Figure 2. Schematic illustration of the trawl design and twin-rig configuration used by the Danish Nephrops-directed mixed trawl fishery. (a) Nephrops trawl scheme, modified with permission by SEAFISH Industry Authority. (b) Twin-rig scheme; artwork by Marco Nalon.

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The trawlers, varying in length mostly between 15 and 30 m

(http://www.statistikbanken.dk), tow in general two identical trawls in a twin-rig

configuration (Fig. 2b). This system has been proven to increase catches of

benthic and demersal species, in particular Nephrops (Sangster and Breen,

1998).

3.1.1 Nephrops catchability

Nephrops norvegicus, also known as Norway Lobster, Langoustine or Scampi, is

a widely distributed arthropod crustacean of the order Decapoda. It lives on the

continental shelf of the North East Atlantic and in the Mediterranean Sea, on

muddy substrates, at depths ranging from 15 to 800 m (Chapman, 1980; Ungfors

et al., 2013). A specific type of seabed, i.e. fine cohesive mud, is essential for this

species, as Nephrops construct deep and complex burrows which are used as

refuges and for reproduction (Chapman and Rice, 1971; Rice and Chapman,

1971). When inside the burrows, Nephrops are unlikely to be caught by trawl

nets. Therefore, emergence due to foraging and mating activities, determines the

catchability of the species (Bell et al., 2006). This varies depending on biological

and environmental factors, such as molting cycle, female reproductive stage,

ambient light level, season, area, and tides (Chapman, 1980).

When outside the burrows, Nephrops react to disturbance, in particular physical

contact, by rapidly flipping the tail which propels them backwards (Newland and

Chapman, 1989). Reaction distance has been observed to vary between 0 and

55 cm, depending on the orientation of the individual with respect to the towing

direction of the gear (Newland and Chapman, 1985; Newland and Chapman,

1989). Average swimming speed (tail-flips) after tactile stimulation is 1−1.5 knots,

and the distance covered is limited to approximately 1-2 m (Newland and

Chapman, 1989). Thus, with fishing gears towed generally at 2–3 knots,

Nephrops in the path of the trawl are quickly overtaken by the footrope (Main and

Sangster, 1985; Newland and Chapman, 1989). After being stimulated, Nephrops

can, on average, rise vertically about 20–50 cm above the seabed (Newland and

Chapman, 1985). Therefore, considering standard headline heights of

approximately 1–2 m, and even though bigger individuals have been observed to

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rise to up to 85 cm from the seabed, very few Nephrops escape above the

headline of the Combi trawl (Main and Sangster, 1985).

3.1.2 Fish catchability

Demersal fish species, sympatric in Nephrops fishing grounds, include both

roundfish and flatfish species. To be available to capture by the trawl, these

species have to be in the trawl path (i.e. area swept by the footrope). Therefore,

to effectively target fish, a trawl has to first concentrate them into the trawl path, a

result that can be achieved by exploiting fish behavioural responses to the

forward, spreading components of the trawl (Winger et al., 2010). This

mechanism is generally referred to as “herding”. Fish in the herding area (i.e.

between the doors) are stimulated by the doors and sweeps, which interact with

the seafloor producing vibrations sediment resuspensions (Glass and Wardle,

1989; Engås and Ona, 1990; Winger et al., 2010). Most species react to these

stimuli as they would in case of an approaching predator (Fernӧ and Huse,

2003). Roundfish species, in general, tend to swim away from the approaching

threat (i.e. doors and sweeps) while keeping it at the edge of their visual field.

This produce a movement described as “fountain manoeuvre” (Fig. 3a; Winger et

al., 2010). This results in individuals swimming directly into the trawl path,

exposing them to capture. Among the factors influencing the efficiency of

roundfish herding, two are known to play a fundamental role: the length of the

sweeps, with herding efficiency increasing at increasing lengths, and the angle of

the sweeps with respect to the towing direction (Winger et al., 2010). For cod and

haddock, sweeps lengths between 20 and 120 m (Engås and Godø, 1989) and

sweeps angles between 10 and 20 degrees (Strange, 1984) were found to

significantly increase catches.

In contrast, flatfish and benthic species, specialized in camouflage, are reticent to

flee and start swimming away only after direct or near contact with the doors or

sweeps (Fig. 3b; Main and Sangster, 1981). Once they flee, they move away

perpendicularly from the stimulus and either attempt to keep a constant distance

from the pursuing threat or burst to gain distance and then resettle on the

seafloor (Ryer, 2008). As a result of this slow herding process, longer sweeps (up

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to 400 m) and small sweeps angles are necessary to leave enough time for the

individuals to reach the trawl path (Ryer et al., 2010).

Finally, the herding process was found to be size-dependent for many species,

and its efficiency to vary according to the towing speed, as a result of the species

and size-dependent differences in swimming capacity (Winger et al., 2010; He,

2011). In particular, those individuals that do not possess the energy or have the

capacity to maintain swimming speeds at least as fast as the towing speed would

be overtaken before reaching the trawl path and be exposed to capture (Winger

et al., 2010).

Figure 3. Schematic illustration of the herding process for (a) roundfish species (Winger et al., 2010) and (b) flatfish species (Main and Sangster, 1981).

Once in proximity of the trawl mouth, fish have been observed to turn around and

attempt to keep position ahead of the footrope (Main and Sangster, 1981; Glass

and Wardle, 1989; Wardle, 1993; Arimoto et al., 2010; Winger et al., 2010).

Depending on the species, size and the level of exhaustion, each individual can

either rise vertically and escape over the headline, be overtaken by the footrope,

or turn again and swim directly into the trawl (Kim and Wardle, 2003; Winger et

al., 2010).

Finally, for the most relevant bycatch species of the Nephrops-directed fishery,

fish catchability is known to vary at different light levels, as the behavioural

reactions described are mainly vision-dependent (Walsh and Hickey, 1993; Ryer

et al., 2010). Indeed, herding has been observed to cease at light levels below

fish visual thresholds (Wardle, 1993; Kim and Wardle, 1998a).

(a) (b)

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3.2 Bycatch reduction measures before the landing obligation

Nephrops-directed trawl fisheries adopt a minimum mesh size of 70 or 90 mm

(depending on region). However, the poor selective properties of these mesh

sizes in relation to the Minimum Conservation Reference Size (MCRS) of the fish

species caught by the Danish fleet lead to high catches of undersized individuals

(Kelleher, 2005; Krag et al., 2008). To appropriately select out these undersized

individuals, a minimum mesh size of 120 mm is necessary (Graham and Ferro,

2004). Following concerns about the state of gadoids stocks, and in particular

cod (EC, 2008), since 2013 Danish trawlers targeting Nephrops are required to

use either of the following options:

a species-selective grid with 35 mm-spaced vertical bars inserted in a 70

mm square mesh codend, at least 8 m from the codline, to exclude the fish

bycatch, both undersized and commercial sized;

a size selective trawl (termed SELTRA trawl) consisting of a 90 mm

diamond mesh codend with a 3 m long escape panel inserted in the upper

netting of codend, starting at least 7 m before the codline. Depending on

the fishing area, the panel can be of either square meshes (140 mm, 3

opening angle ratio in Skagerrak; 180 mm, 4 opening angle ratio in

Kattegat) or diamond meshes (270 mm, both areas; Madsen et al., 2012;

ICES, 2014).

The escape panel is effective in reducing the catch of undersized individuals

while retaining commercial sized individuals (Frandsen et al., 2009; Briggs et al.,

2010). Therefore, it is the BRD adopted by most of the Danish Nephrops-directed

mixed trawl fishery. However, the release efficiency of an escape panel can be

more variable than that of a grid, because the escape panel relies on the

individuals actively contacting it. This varies according to size and species-

specific behaviour as well as position of the SMP in the gear (Krag et al., 2014;

Herrmann et al., 2015a; Nikolic et al., 2015).

3.3 Bycatch reduction under the landing obligation

The introduction of the landing obligation implies, in theory, that fishing activities

have to stop when the first quota is exhausted. Among the mix of species that

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compose the bycatch in the Danish Nephrops-directed mixed trawl fishery, there

are two that represent a high risk of choking the fishery: cod and plaice (North

Sea Advisory Council, 2018). Cod capture below the MCRS (30 and 35 cm in

ICES Division IIIa and IVa-b, respectively) has been significantly reduced with the

adoption of the SMP (North Sea Advisory Council, 2018). However, unwanted

catch rates remain variable and have been observed to increase at higher cod

abundances (North Sea Advisory Council, 2018). Moreover, cod above the

MCRS used to be frequently discarded when of low-value (Category 4 and 5;

http://www.hanstholmfiskeauktion.dk/prices?lang=en). Therefore, with the full

implementation of the landing obligation in 2019, a quota deficit for cod for all the

State Members fishing in these areas is predicted to impact most mixed demersal

fisheries, including the Nephrops-directed one (North Sea Advisory Council,

2018). In contrast, there is currently a surplus in plaice quota but this species has

been classified as a potential economic choke species, i.e. a species where the

high abundance of undersized catches that has now to be sorted and stored on-

board can make the trip uneconomic (North Sea Advisory Council, 2018). Finally,

additional species can be troublesome depending on the specific fishing area and

gear design; Denmark has currently a quota deficit for hake, saithe and whiting,

all species whose catches can be abundant in ICES Division IVa and IVb (North

Sea Advisory Council, 2018).

Therefore, the current BRDs are not sufficient to prevent the impact of choke

species on the Danish Nephrops-directed mixed trawl fishery. In particular, the

SELTRA codends were not designed to prevent the catch of commercial-sized

cod and fishermen would have to adopt a grid to avoid their capture once out of

quota. This would obviously eliminate any other valuable fish bycatch as well as

potentially cause a loss of target Nephrops (Frandsen et al., 2009).

Consequently, fishermen could now be willing to voluntarily adopt BRDs which

provide alternative harvest patterns. In particular, simple BRDs, that do not

require major changes in fishing dynamics, are more likely to be adopted by

fishermen than more complex ones (Broadhurst, 2000). Furthermore, flexible

solutions that can be used to change the selectivity at the haul-by-haul level,

when required by the catch composition, would represent a valuable tool to cope

with the EU landing obligation.

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4. Gear modifications for the Nephrops-directed mixed trawl

fishery

Prior to discussing which gear modifications can lead to a more flexible

selectivity, it is beneficial to explore the different types of modifications that can

be introduced in a Combi trawl. In particular, this section will focus on BRDs

developed for the Nephrops-directed fisheries, or applicable to them, with the aim

of reducing fish bycatch while maintaining Nephrops catches. For decades gear

technologists have developed BRDs which exploit species differences in terms of

morphology, size and behaviour to improve gears selectivity (Catchpole and

Gray, 2010; Graham, 2010). Dozens of BRDs and gear modifications are

documented in literature (Broadhurst, 2000; Catchpole and Revill, 2008; Graham,

2010) and private and public institutions are now collecting and organizing in

databases the sea trial results to aid the industry in identifying gear designs and

BRDs options (e.g. http://www.discardless.eu/selectivity_manual;

http://www.seafish.org/geardb/; https://tool.gearingup.eu/). The BRDs involve

modifications to different components of the trawl and are hereafter organized in

two main groups depending on their aim: preventing the catch of unwanted

individuals (anterior modifications) or select these out after they entered the trawl

(posterior modifications; Fig. 4).

Figure 4. Schematic illustration of the area of interest of each group of gear modifications. Artwork by Marco Nalon.

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4.1 Anterior modifications

The capture process begins well ahead of the trawl, when individuals initially

detect the noise produced by the vessel, warps, doors and gear components

(Winger et al., 2010). According to Fernö and Huse (2003), the timing, type and

intensity of the response is then governed by the same behavioural trade-offs

that determine prey fleeing from predators (i.e. optimal escape theory; Ydenberg

and Dill, 1986). In general, each individual that detects a threat must sequentially

decide (1) whether and when to flee, (2) in which direction to flee, (3) how fast to

flee, and (4) how far to flee. If we consider as benefit the continuation of the

previously ongoing activity and as drawback the risk of being predated, the

behavioural response can be expressed as the result of the individual attempt to

minimize costs and maximize benefits (Fernö, 1993; Godin, 1997). This decision-

making process continues while the distance between the individual and the

predator shrinks. Once the perceived risk of being predated exceeds the benefits

of keeping position, the individual starts to flee. In general, the escape begins

with a slow adjustment in swimming direction away from the approaching

stimulus (Olsen et al. 1983; Winger et al., 2010). Therefore, the direction of the

stimulus perceived as a threat is what determines the direction of the escape. In

a standard trawl, the stimuli created by the early spreading components (i.e.

doors and sweeps) are those identified as the approaching threat (Kim and

Wardle, 1998a). Thus, fish swim away from these components and are eventually

herded towards the trawl mouth. Once they reach the trawl mouth, new stimuli

are perceived, e.g. the footrope and netting, which stimulate the individuals to

turn around. Here fish have been observed to either maintain distance ahead of

the pursuing trawl or to escape in different directions, e.g. above the headline or

below the footrope (Kim and Wardle, 1998a; Winger et al., 2010).

Anterior gear modifications are those that either mitigate these initial stimuli, thus

reducing the efficiency of the herding process, add stimuli to re-direct fish escape

out of the trawl path, or facilitate escaping opportunities once in the trawl mouth.

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4.1.1 Doors, sweeps and bridles

As described above, doors, sweeps and bridles are the elements of the trawl that

first trigger fish escape response and direct it towards the trawl path. In particular,

their interaction with the sediment, in terms of sand cloud and vibrations

produced, create multisensorial stimuli that trigger fish response (Winger et al.,

2010). Therefore, modifications that raise either of these components off the

seafloor (Fig. 5, a and b) have successfully reduced the herding of some fish

species (Rose et al., 2010; Ryer et al., 2010; He et al., 2014; Sistiaga et al.,

2015; 2016; BIM, 2018). Moreover, species-specific herding efficiency can be

modified by shortening the length of the sweeps and bridles, thus increasing their

angle with respect to the towing direction (Mathai et al., 1984). Previous studies

have demonstrated that at wider angle of attachment of the sweeps fish have

less time to reach the trawl path and, thus, herding is less efficient (Strange,

1984). For example, at angles greater than 20 degrees, catches of cod and

haddock were significantly reduced (Strange, 1984).

4.1.2 Counter-herding and anterior fish excluder devices

Although the visual and tactile stimuli produced by doors and sweeps increase

fish catchability, the same type of stimuli, simply orientated in a different direction,

can cause an early escape response and increase fish chances of avoiding

capture. For example, higher-order multi-net configurations such as quad-rig

systems (i.e. four gears towed in parallel) catch less fish due to the additional

presence of wires to connect the different gears, which lead the fish to the outer

extremities of the catching zone (Broadhurst et al., 2013a; b). Similarly, additional

elements, such as diagonal wires, ropes and plastic banners (Fig. 5, c and d),

can be added in the herding area to re-direct fish escape away from the trawl

path (Ryer, 2008; McHugh et al., 2014, 2015; Paper I; BIM, 2018). These

devices, termed counter-herding devices, will be discussed in details in Section 5.

4.1.3 Headline height

Once in the trawl mouth, fish are stimulated by the footrope to swim in the towing

direction until fatigued or until the costs of maintaining position exceed the

benefits (Kim and Wardle, 2003; Breen et al., 2004; Winger et al., 2010).

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Depending on the species, size and individual fitness, some fish will attempt to

swim upwards and escape over the trawl headline. Intuitively, higher headline

heights increase fish catchability (Wardle, 1986; Johnson et al., 2008; Broadhurst

et al., 2016). Therefore, to intentionally reduce the trawl efficiency in catching

species that attempt to escape upwards in the trawl mouth (i.e. haddock; Wardle,

1986), one can simply lower the headline height. Previous studies demonstrated

that the optimal headline height derives from a trade-off between having sufficient

height to maximise the capture of the target crustacean, which can swim upwards

when stimulated by the footrope, and lowering it sufficiently to minimise catches

of unwanted fish species (Eayrs, 2002; Madhu et al., 2015). However, length-

dependent swimming capacities and behaviours may reduce the efficacy of this

gear modification for smaller individuals.

4.1.4 Topless or cutaway trawl

Once in the trawl mouth, the netting of the trawl wings and mouth has been

observed to stimulate avoidance behaviour in fish (Kim and Wardle, 2003).

Depending on species and light level, the colour contrast of the twine with respect

to the background can stimulate fish to keep away from the netting even if the

mesh size would allow them to swim through (Kim and Wardle, 1998b, Winger et

al., 2010). Therefore, by perceiving the upper netting panel of the trawl mouth,

fish are stimulated to stay in proximity of the seafloor until they turn into or are

overtaken by the trawl. Consequently, trawls designed with the footrope located

ahead of the headline, remove this stimulus and can enhance fish upward

escape (Fig. 5 e). These designs, termed cutaway trawls or topless trawls, have

significantly reduced catches of roundfish and rockfish species (Thomsen, 1993;

Hannah et al., 2005; He et al., 2007; Chosid et al., 2008; Campbell et al., 2010;

Krag et al., 2015; Eayrs et al., 2017). However, inconclusive results were

obtained regarding the efficacy of the cutaway trawl on cod, with some studies

effectively reducing cod catches (Thomsen, 1993; Pol et al., 2003; Chosind et al.,

2008), and others finding no significant difference (Revill et al., 2006; Krag et al.,

2015). The efficiency of the cutaway trawl was found to depend on the headline

height, with no effect on cod detected with a headline height of 4.5 m (Krag et al.,

2015).

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4.1.5 Netting tapering

The entry of fish into the trawl net is highly variable within and between species

and depends on multiple factors. One of these is how the trawl funnel appears to

the fish. Large trawls with a steeper-angle tapering (i.e. inclination of the netting)

that slowly reduces the funnel section and leads into an extension instead of

directly into the codend can create the illusion of an open path and incentivize

fish to turn around and swim into the trawl (Winger et al., 2010). In contrast,

smaller trawls with a wider-angle tapering leading directly into the codend, could

result in the netting being more evident to the fish. Moreover, altering the tapering

in the netting can have two subtle consequences on the trawl selectivity: i) a

wider-angle tapering may increase the probability for an individual to contact the

netting and, thus, to be selected out; and ii) the angle at which individuals contact

the netting with a wider-angle tapering may favour escapement (Broadhurst et al.,

2012). Although relatively few studies were conducted to determine the effect of

trawl length and netting tapering angle on fish catchability (Broadhurst et al.,

2012; 2015), the different features of commercial gears targeting fish and those

not targeting them are, intuitively, the result of fishermen’s experience of these

effects.

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Figure 5. Examples of anterior gear modifications. (a) Semi-pelagic doors from Sistiaga et al. (2015); (b) Floating sweeps from He at al. (2014); (c) Anterior fish excluder from McHugh et al. (2017); (d) Hypothetical floating counter-herding device from Ryer (2008); and (e) Picture of a model topless trawl tested in a flume tank from He et al. (2007).

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4.2 Posterior modifications

Once fish have entered the trawl, their vertical position in the trawl section is

species-dependent and can range from a few centimetres to several meters from

the lower netting panel (Winger et al., 2010). Benthic species such as monkfish

and flatfish typically stay in close proximity to the lower netting (e.g. Rose, 1995;

Bublitz, 1996), whereas demersal species, e.g. gadoids, tend to distribute

vertically across the trawl section (e.g. Main and Sangster, 1981; Thomsen,

1993). Moreover, the vertical distribution of fish inside the trawl has been proved

to vary, for some species, throughout the journey towards the codend (Holst et

al., 2009; Fryer et al., 2017). Upon reaching the narrow section immediately

ahead of the codend (i.e. trawl extension), some species, including haddock and

cod, can begin to swim erratically, in random directions (Grimaldo et al., 2008; He

et al., 2008). Indeed, as exhaustion sets in and crowding increases orderly

behaviours may be disrupted and substituted by randomly orientated burst-

swimming (Winger et al., 2010). This behaviour is likely to cause collision with

netting or other individuals. Finally, once in the codend, most fish are considered

to be exhausted and highly stressed, having endured prolonged continuous

swimming and attempted to avoid contact with the netting and other individuals.

Here, depending on their residual energy, individuals may attempt to keep

position ahead of the accumulated catch, try to escape through the meshes or

become part of the accumulated catch (Watson, 1989; Wardle, 1992; O’Neill et

al., 2003).

Posterior gear modifications, which include some of the most studied and

implemented BRDs, exploit behavioural differences in species vertical distribution

and swimming capacity, as well as morphological differences between and within

species, to separate and/or select out unwanted catches (see reviews by Glass,

2000; Graham, 2006).

4.2.1 Trawl body mesh size

To reduce the catch of undersized roundfish, while retaining the most valuable

larger sizes, an effective strategy can be to substitute the standard upper netting

mesh size with a larger one. This provides an escape opportunity for those fish

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that rise while falling back into the trawl, but still within a size-selection process

that would retain larger individuals. Several studies tested this modification in

different fisheries (Thomsen, 1993; Briggs, 2010; Campbell et al., 2010; Kynoch

et al., 2011; Krag et al., 2014; Bayse et al., 2016). In particular, considering the

species of interest of this study, the following examples obtained significant

reductions of bycatch species: Thomsen (1993) reduced cod catches by 38% in a

whitefish fishery by replacing the 135 mm mesh size at the rear end of the

tapered section with 540 mm mesh size netting; Campbell et al. (2010)

significantly reduced catches of cod below 78 cm by replacing the 160 mm mesh

size netting with a 300 mm one, and Krag et al. (2014) obtained a significant

reduction on several important bycatch species of the Nephrops-directed fishery

by using a 800 mm mesh size section in the tapered area before the trawl

extension.

4.2.2 Horizontal separator panel

Differences in vertical escaping patterns at the mouth of the trawl can be

exploited to segregate species into two or more different compartments, by

inserting horizontal netting panels (Fig. 6 a). This design, termed separator trawl,

was originally tested by Strzysewski (1972) with a separator starting 1.5 m above

the footrope in a demersal herring trawl. Subsequently, separator trawls were

developed and tested for a variety of fisheries including the Nephrops-directed

mixed trawl fishery. Fryer et al. (2017) reviewed such studies, and analysed the

main factors affecting species-specific separation efficiency. Depending on the

species, the position of the separator panel, both vertically and horizontally in the

trawl, were found to affect the proportion of species entering each compartment

(Fryer et al., 2017). Among the species analysed, all the fish, with the exception

of monkfish, were found to be affected by the height of the separator, whereas

only cod was additionally affected by the horizontal distance from the footrope. A

higher proportion of cod entered the upper compartment when the separator

started at the extension level. Moreover, plaice was found to be affected by the

time of the day, with a higher proportion of individuals entering the upper

compartment during the night (Fryer et al., 2017).

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4.2.3 Grids

In crustacean fisheries, grids are often applied to mechanically sort the small-

sized target crustacean, which passes through the grid into the codend, from

larger animals, which are diverted out of the trawl through an escape window.

Both rigid (e.g. Isaksen et al., 1992; Polet, 2002; Graham, 2003; Fonseca et al.,

2005) and flexible grids (e.g. Loaec et al., 2006) have been developed for

crustacean fisheries. A Nephrops trawl with a Swedish grid (35 mm bars space)

catches 80–100% less weight of commercial sized fish (Catchpole et al., 2006;

Valentinsson and Ulmestrand, 2008; Frandsen et al., 2009; Drewery et al., 2010).

Intuitively, this use of grids is not applicable if the marketable fish represent a

desired catch. Nonetheless, grids can be applied to the Nephrops-directed mixed

trawl fishery to separate fish into a different codend instead of releasing them

(Fig. 6 b; Anon, 2001; Graham and Fryer, 2006; Grimaldo et al., 2008). Moreover,

simple frames with only few bars can be used to stimulate fish, both behaviourally

and mechanically, to enter the upper codend (Krag et al., 2009a).

4.2.4 Square mesh panels (SMP)

The idea of SMPs came from maintaining mesh opening to assist fish escape

(Fig. 6 c). Indeed, due to their structure, square meshes stay open irrespective of

the longitudinal tension, unlike diamond mesh which tends to close as

longitudinal strain is applied (Robertson, 1986). Due to their relative simplicity,

they have been applied in a multitude of configurations and mesh sizes. From the

trawl body (e.g. Briggs, 2010), to the trawl extension (e.g. Krag et al., 2008), to

the codend (e.g. Herrmann et al., 2015a), SMPs have been inserted in various

positions in the trawl. However, SMPs have proved more effective when placed in

a position where the likelihood of fish contacting the meshes is higher, e.g. before

the catch accumulation zone in the codend or at the passage between tapered

section and extension/codend (Fig. 6 d; Graham and Kynoch, 2001; Graham et

al., 2003). Additional stimulation can also be added to accentuate fish contact

with the SMP or square mesh section. For example, Grimaldo et al. (2018) tested

floats and Light Emitting Diodes (LED) lights on free moving rope and increased

the escape rate of haddock; Kim and Wang (2010) tested a fluttering net panel

and a set of free ropes, successfully stimulating the escapement of juvenile red

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sea bream (Pagrus major) in laboratory conditions; and Krag et al., 2016 used a

system of floats and ropes to discourage fish from moving through and prolong

their stay in the SMP section, thus increasing their probability of contacting the

meshes and escaping.

4.2.5 Sieve panels

A sieve panel consists of a netting panel attached inside the trawl body and/or

extension with a continuous inclination, generally an upward inclination in

Nephrops-directed fishery. Assuming that Nephrops will be concentrated in

proximity of the lower netting (Fryer et al., 2017; Karlsen et al., 2019) and that a

large enough mesh size is used in the sieve panel, most of the target species will

pass through the sieve and be retained in the codend. In contrast, bycatch

species will attempt to avoid contact with the meshes and follow the rising netting

towards an escape widow or a separate compartment. Therefore, similarly to

grids, sieve panels with the fore edge of the panel attached to the trawl lower

netting (Fig. 6 e) can be used in the Nephrops-directed fishery to select out

unwanted species too big to pass through the panel (Santos et al., 2018a;

Cosgrove et al., 2019). In contrast, sieve panels with the fore edge partly raised

would allow fish species generally found in close proximity of the lower netting,

such as monkfish and flatfish, to be retained together with Nephrops

(Valentinsson and Ulmestrand, 2008).

4.2.6 Horizontally divided codends

Among the horizontally divided trawls, the ones with the separator starting in the

trawl extension effectively separate most important bycatch species for

Nephrops-directed mixed trawl fisheries (Fryer et al., 2017). The relative height of

the compartments can be adjusted to maximize the probability of fish entering the

upper compartment while minimizing that of Nephrops. Multiple studies have

quantified the vertical separation efficiency of horizontally divided trawl codends

for both target and bycatch species, and how the addition of behavioural

stimulators can alter it (Holst et al., 2009; Krag et al., 2009b; Karlsen et al., 2019;

Paper II; Paper III). From simple frames (Krag et al., 2009b; Karlsen et al., 2019),

to LED lights (Paper II), floats and chains (Paper III), these studies modified the

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species-specific separation into the compartments. In particular, these stimulators

were significantly effective on undersized individuals, despite them being often

considered too exhausted to respond to stimulation (Paper II; Paper III).

4.2.7 Codend configuration

Among all the components of the trawl that can be modified to reduce bycatch,

none is as studied and broadly implemented as the codend. It is well-known that

the mesh size, shape, twine material and twine thickness influence, both

mechanically and behaviourally, species possibility to escape through them

(O’Neill, 2003; Herrmann et al., 2015b). Moreover, the circumference (i.e. No. of

meshes) in the codend can affect the intensity of the water flow inside the

codend, and provide fish additional opportunities to escape through the codend

meshes by reducing the speed required to maintain position within the codend

(Rose, 1995; Broadhurst et al., 1999; O’Neill et al., 2003; Jones et al., 2008).

Therefore, codends can be modified in multiple ways to better select out

unwanted catches. In particular, the mesh size can be increased (e.g. Beutel et

al., 2008; Frandsen et al., 2011); the codend can be entirely or partly constructed

from square meshes (e.g. Frandsen et al., 2011; Wienbeck et al., 2014); the

number of diamond meshes in the codend circumference can be reduced to

enhance their opening (e.g. Broadhurst and Kennelly, 1996); and hanging ropes

(i.e. ropes shortened with respect to the stretched length of the codend) can be

inserted to prevent the codend from stretching while the catch accumulates and

thus maintaining mesh opening (e.g. Robertson and Shanks, 1989).

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Figure 6. Examples of posterior gear modifications. (a) Separator trawl from Ferro et al. (2007); (b) Grid in combination with a horizontally divided trawl codend from Graham and Fryer (2006); (c) Square mesh codends model from SEAFISH Industry Authority; (d) From left to right: SMP before catch accumulation point, SMP at the end of the tapered section, and grid with escape window, from Drewery et al. (2010); and (e) Sieve panel in combination with a horizontally divided trawl codend from Santos et al. (2018a).

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5. Flexible gear modifications

The previous section highlighted the wide array of options to modify the

selectivity of a Nephrops-directed mixed demersal trawl and reduce the bycatch

of fish. However, the few that have been introduced into legislation are applied to

entire fleets, without considering vessel-specific catch goals. Some gear

modifications (e.g. topless trawls, reduced headline height, and sieve panels

combined with escape windows) will always be more effective at sorting out

larger individuals due to within-species physical and behavioural limits in

swimming ability and escape responses (Revill et al., 2006; Drewery et al., 2010;

Krag et al., 2015). However, this loss of valuable catch would not be desirable

when fish quota is available. Therefore, these modifications are unlikely to be a

popular tool for all fishermen and at all times, as the cost–benefit balance of

preserving quota by eliminating the bigger individuals is likely to vary according to

quota levels and species market value fluctuations. In contrast, BRDs that

minimize the catch of undersized individuals, such as SMPs, can be perceived as

more “environmentally friendly options” but are unable to prevent the catch of

commercial sized bycatch species from choking the fishery.

Moreover, some BRDs have not been implemented or adopted voluntarily by the

fishermen due to unacceptable losses of target species (Frandsen et al., 2009;

Ingólfsson, 2011; Santos et al., 2018a). However, with the landing obligation, a

temporary loss of target species can become acceptable to the fishermen, as the

alternative solution (i.e. buying extra quota or stopping fishing) would ultimately

be more costly.

Fishermen face a much more complex reality than the one represented and

addressed by the legislation or by individual gear modifications. Even within

fishery, the fleet consists of different sized vessels using different sized gears of

varying designs. The specific location, time of day or season, weather conditions,

vessel configuration, skipper ability, quota combination, etc., all affect the catch

composition and amount of unwanted catches (Feekings et al., 2012).

Consequently, fishermen need to be able to address selectivity issues on a day-

to-day or even haul-to-haul basis. This could be achieved via gear modifications

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designed to be simple in their application to the trawl and cost-effective in terms

of time required for their implementation.

According to the above, this thesis focused on “flexible” gear modifications, which

could provide the means to alter the selectivity of the trawl at the haul level:

5.1 Flexible anterior modifications

Among the anterior modifications previously described, counter-herding and

anterior fish excluder devices are some of the most flexible and simple options.

Because they consist of additional components added to the trawl, they do not

normally require any change to either the geometry or fishing dynamics of the

gear (McHugh et al., 2014, 2015; Paper I). Anterior fish excluder devices were

developed and tested by McHugh et al. (2014; 2015) in the Australian school

prawn (Metapenaeus macleayi) fishery. Here, otter trawls have only short bridles

and no sweeps, thus the additional components placed between the doors have

the function of making the approaching trawl more evident (McHugh et al., 2015).

In contrast, the use of counter-herding stimuli to lead fish out of the trawl path has

firstly been investigated in this study (Paper I). The potential of this type of

devices had been previously discussed but not tested due to concerns about

constraining the door spread (Ryer, 2008). In particular, engineering challenges

were expected in handling the variable tensions on the components of the

counter-herding device, for example at variations in spread due to bottom

topography and sediment characteristics. However, a careful consideration of the

materials and geometry of the counter-herding designs has proven sufficient in

preventing such problems (Paper I). Some adjustments of the spreading

mechanism (e.g. weight of the doors) can be required to prevent a reduction in

door-spread (Paper I). Nonetheless, the results imply only a minor reduction in

spread, which does not compromise the trawl geometry and can result in

improved bottom contact of the footrope (Broadhurst et al., 2014) thus increasing

Nephrops catches.

The potential of these devices has just started to be explored. Although positive

results were achieved in terms of bycatch reduction with both anterior fish

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excluder (McHugh et al., 2014, 2015) and counter-herding devices (Paper I), only

few geometries and materials have been investigated. The mechanism that

determines the efficacy of these devices has still to be clearly understood. Due to

the forward position in the trawl and to the sediment resuspension, typical of

crustacean-directed fisheries, video observations of fish responses to counter-

herding devices are difficult to obtain. Therefore, it is unsure if fish are re-directed

out of the trawl path, as hypothesized (Fig. 7a), or if they rise vertically and

escape above the headline (Fig. 7b; Paper I). Most likely, the response varies

across species and sizes, as well as according to environmental parameters

affecting fish perception of the counter-herding device.

Figure 7. Schematic illustration of the possible swimming direction of fish in response to the counter-herding device. (a) horizontal response, and (b) vertical response. Artwork by Marco Nalon.

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The results available show species-specific (McHugh et al., 2014, 2015; Paper I)

and size-dependent (Paper I) responses to counter-herding and anterior fish

excluder devices. Therefore, these BRDs could be used at the haul-by-haul level

to adjust both species and size composition of the catch. Different materials

should be tested to optimize the performance of the device (e.g. bouncing or

sweeping on the seafloor) and/or to make it more visible. For example, Ryer

(2008) postulated that a floating counter-herding device could be used to

selectively reduce roundfish catches and retain flatfish. Moreover, different

geometries of the counter-herding device (i.e. distance from the trawl mouth

and/or angle of attachment of the lines) should be investigated to determine if its

species and size-dependent effectiveness can be adjusted. Indeed, since

modifications to sweeps lengths and angles can significantly affect the herding

efficiency, similar effects can be expected for the counter-herding process (Ryer

et al., 2010; Winger et al., 2010).

5.2 Flexible posterior modifications

Among the array of posterior modifications, those that lead to an effective

separation of the main bycatch species from Nephrops are particularly suitable

for the necessities and goals of the Nephrops-directed mixed trawl fishery.

Indeed, the separation would not only benefit the quality of the catch (Karlsen et

al., 2015) but also allow fishermen to control their consumption of fish quota by

modifying the configuration of the upper codend. For example, large diamond or

square meshes could select out the undersized individuals of most commercial

northeast Atlantic fish species and retain the most valuable sizes (Graham and

Ferro, 2004; Frandsen et al., 2011; Wienbeck et al., 2014). After quota

exhaustion or in case of low-valued bycatch, the upper codend could even be left

open.

The main limit of these designs is the species and size-dependent separation

efficiency. Sieve panels and grids can effectively separate the catch on the basis

of size but their efficiency is limited for undersized individuals, which can pass

through together with Nephrops (Santos et al., 2018a; Cosgrove et al., 2019).

Moreover, because of the complex morphological components that determine

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Nephrops selectivity (Frandsen et al., 2010) a rather large mesh size is required

for the sieve panel to maintain Nephrops catches in the lower codend (Santos et

al., 2018a). In contrast, horizontally divided trawl codends are very efficient in

separating Nephrops into the lower compartment (Holst et al., 2009; Karlsen et

al., 2019; Paper II; Paper III). Nonetheless, because they depend strongly on

species behaviour to achieve the separation, they can be enhanced by behaviour

stimulators. Limited research has been conducted on the application of behaviour

stimulators in combination with a horizontal separator panel (Paper II; Paper III),

while most studies have focused on increasing contact probability with escape

panels or windows. Fluttering lines, net panels, and floats have been tested as a

mean to trigger erratic swimming or avoidance reactions (Kim and Wang, 2010;

Herrmann et al., 2015a; Krag et al., 2016; Grimaldo et al., 2018; Paper III). Visual

stimuli such as black canvas or dark twine have been used to slow down fish

passage through the trawl and increase their probability of encountering a BRD

(Glass and Wardle 1995; Glass et al. 1995; He et al. 2008). LED lights have also

been used to highlight escape routes. For example, Lomeli and Wakefield (2014)

demonstrated that Chinook salmon (Oncorhynchus tshawytscha) escape rate

through escape windows increased when these were illuminated by blue LED

lights. However, although owning a great potential for species-specific

behavioural responses, the application of LED lights in trawls require further

studies. Most often the results of their application was negative (increased or

similar bycatch rate), for example when LED lights were attached in

correspondence of a grid (Hannah et al., 2015; Larsen et al., 2017), a SMP

(Grimaldo et al., 2018) or a horizontally divided trawl codend (Paper II).

Future research should develop the knowledge of species behavioural reactions

and of the main drivers of such responses. A more systematic understanding

would aid the identification of the appropriate type of stimulator, depending on the

relative position in the trawl, the average environmental conditions and the main

species of interest. Nonetheless, the results available are promising and the

continuous collection of video observations during the capture process will

improve the understanding and aid the exploitation of fish behaviour inside the

trawl.

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6. Towards haul-by-haul control over selectivity

Once flexible gear modifications have been identified for a specific fishery, they

can be combined with the existing mandatory BRDs or with some of the more

complex and structural modifications described in Section 4.

Because each BRD is bound to have some limitations in efficiency, more and

more research has been conducted on the advantage of combining sequential

BRDs (Brinkhof et al., 2018; Hermann et al., 2018; Larsen et al., 2018; Paper IV).

Especially in mixed trawl fisheries, where the bycatch composition is often too

diverse to be sorted or selected out through one single selective process,

sequential processes can achieve multiple objectives, such as the reduction of

both undersized and commercial sized bycatch (Larsen et al., 2018; Paper IV).

Moreover, recent studies have highlighted that the performance of gear designs

under different, realistic catch scenarios can affect the viability of that design in a

specific fishery (Sala et al., 2015; Santos et al., 2018b; Paper IV). Therefore, by

combining sequential selective processes, the vulnerability of one BRD to specific

catch scenarios (e.g. high density of individuals around MCRS) could be

compensated by the previous or following processes (Paper IV).

In principle, as long as the BRDs do not compromise the structure of the trawl,

multiple BRDs can be combined. However, because each BRD is likely to cause

a small loss of target species, the sum of these losses can lead to unacceptable

impacts on the viability of the fishery (Paper IV). Moreover, to maximize the

advantage of adopting multiple sequential BRDs and offset the additional

complexity in gear design, the choice of BRDs should be limited to highly efficient

designs, targeting different species and size-groups.

To identify the most promising BRD combinations in well-studied fisheries such

as the Nephrops-directed mixed trawl fishery, where high numbers of BRDs have

been developed, a theoretical approach is recommended (Paper IV).

Nonetheless, future research should verify experimentally the predicted

combined selectivity of these promising combinations and determine if and to

what extent they could represent viable options for the fishery.

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7. Conclusions and future work

The thesis was able to define, develop and test simple and flexible gear

modifications to reduce unwanted fish catches in the Danish Nephrops-directed

mixed trawl fishery. The study contributed with additional knowledge to the

development of both anterior (Paper I) and posterior BRDs (Paper II; III),

delivering new effective devices for the Nephrops-directed mixed trawl fishery

and advancing the understanding of species behaviour outside and inside the

trawl.

In particular, a new efficient, unique and flexible device (FLEXSELECT) was

developed and is been currently tested and adapted to other fisheries. This

counter-herding device has not only a broad applicability to several mixed trawl

fisheries around the world, but is also an ideal example of how trawl selectivity

can be substantially modified using simple and cost-effective solutions. The

results obtained in this study answered the question of feasibility and value of this

type of devices for mixed trawl fisheries. However, it raised even more questions

regarding their functioning and possible further developments. Therefore,

experimental research on counter-herding devices has just begun and it will

require both technological developments (i.e. test of different materials and

geometries) and behavioural studies to understand species-specific and length-

dependent response mechanisms.

In contrast, species separation into horizontally-divided trawl codends has

perhaps reached its maximum efficiency for the Nephrops-directed mixed trawl

fishery; at least until the understanding of species-specific behavioural responses

to stimulation is further developed. In this study, I used both tactile and visual

stimulation, as well as investigating one of the most expected responses to

artificial illumination: phototaxis. Overall, the results indicate that in the narrow

section of a Nephrops-directed trawl, behavioural responses to stimulation are

limited; when they occur, they end up often increasing the proportion of

individuals entering the lower compartment. Therefore, although the results are of

interests for future applications of both LED lights and active stimulators in other

fisheries and positions in the trawl, their development would benefit from a more

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systematic understanding of species behaviour. This is the case, in particular, for

LED lights applications, because multiple technical parameters of the lights (e.g.

intensity, colour, pattern, and orientation) influence species responses (Nguyen

and Winger, 2019). The effect of each of these parameters needs to be

investigated individually before further interpretations of the behavioural

responses can be attempted.

Finally, the combined performance of the gear modifications tested in this study

and of other pertinent BRDs for the Nephrops-directed fishery was predicted to

support alternative harvest strategies. In particular the inclusion of flexible gear

modifications, such as the counter-herding device and the horizontally divided

trawl, would create a multi-purpose trawl, where selectivity could be adjusted to

match catch goals. This multi-purpose trawl could function as a strictly Nephrops

trawl or a mixed demersal trawl, with different species- and size-selectivity,

depending on the combination of BRDs introduced. Even though, before the

landing obligation, the cost of multiple BRDs in terms of target loss was

unacceptable, it is now the best option to maximize quota capitalization and

reduce the risk of being impacted by choke species.

With the new EU landing obligation, the role of selectivity as a management tool

and the potential uptake of BRDs by the industry are strengthening, and the

concept of one-gear-fits-all used in the legislation of fishing gears may soon be

retired. A more flexible legislation will lead to a new concept of gear design,

where the trawl can integrate multiple BRDs and fishermen can choose from a

toolbox of gear modifications to achieve the desired catch profile.

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64

Papers

Due to copyright reasons, papers I to III are not included in this version of the

thesis and can be found at the following links:

Paper I: Melli, V., Karlsen, J. D., Feekings, J. P., Herrmann, B., Krag, L.

A., 2018. FLEXSELECT: counter-herding device to reduce

bycatch in crustacean trawl fisheries. Canadian Journal of

Fisheries and Aquatic Sciences, 75: 850–860. https://doi:

10.1139/cjfas-2017-0226

Paper II: Melli, V., Krag, L.A., Herrmann, B., Karlsen, J.D., 2018.

Investigating fish behavioural responses to LED lights in trawls

and potential applications for bycatch reduction in the Nephrops-

directed fishery. ICES Journal of Marine Science, 75: 1682–1692.

https://doi.org/10.1093/icesjms/fsy048

Paper III: Melli, V., Krag L.A., Herrmann, B., Karlsen J.D., 2019. Can active

behaviour stimulators improve fish separation from Nephrops

(Nephrops norvegicus) in a horizontally divided trawl codend?

Fisheries Research. https://doi.org/10.1016/j.fishres.2018.11.027

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PAPER IV

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Predicting optimal combinations of bycatch reduction devices in

fishing gears: a meta-analytical approach

Melli1* V., Herrmann2,3 B., Karlsen1 J.D., Feekings1 J.P. and Krag1 L.A.

1DTU Aqua, National Institute of Aquatic Resources, North Sea Science Park, DK-9850,

Hirtshals, Denmark

2SINTEF Ocean, Willemoesvej 2, DK-9850 Hirtshals, Denmark

3University of Tromsø, Breivika, N-9037 Tromsø, Norway

*[email protected]

Abstract

Global efforts to reduce the capture of non-target species and/or undersized individuals have led to the development of a vast array of bycatch reduction devices (BRDs), in particular for mixed trawl fisheries due to their high bycatch rates. Some of these BRDs could likely benefit from being combined. However, the number of BRDs available would generate a prohibitive number of combinations to be tested in scientific trials. Therefore, in this study we proposed a meta-analytical approach to predict the species-specific, size-selectivity of a trawl with combinations of relevant BRDs, originally tested independently. We applied the method to the well-studied Nephrops (Nephrops norvegicus) directed mixed trawl fishery in the Skagerrak and Kattegat seas and included eight different BRDs: a counter-herding device, a modification of the netting in the trawl body, a horizontal separation and multiple codend configurations. This generated a total of 100 possible combinations. We predicted the size-selectivity of each combination for the target species, Nephrops, and two bycatch species of different economic value, cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). Furthermore, we illustrated how to compare and investigate the performance of the combinations obtained, from both single- and multi-species perspectives, under different catch scenarios. As a result, we identified the most pertinent BRD combinations for the case-study fishery and the alternative harvest strategies that they could support. From the original set of combinations, the meta-analytical approach facilitated the identification of the 15 most pertinent options, of which one would minimize the catch of the two bycatch species considered and another maintain commercial catches of cod. Finally, we identified which interchangeable combinations would lead to a more flexible and dynamic trawl selectivity.

Keywords Bycatch reduction devices, sequential selectivity, optimal gear design, mixed demersal fishery, cod (Gadus morhua), Nephrops norvegicus

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1. Introduction

Addressing the issue of unwanted catches in mixed species trawl fisheries is one

of the major challenges of fisheries science and management (Kelleher, 2005;

Graham, 2010; Pérez Roda et al., 2019). For decades, efforts to reduce the

capture of non-target species and/or undersized individuals have involved the

development of technical modifications of the fishing gear; herein termed Bycatch

Reduction Devices (BRDs; Kennelly and Broadhurst, 2002). Some BRDs consist

of modifications to the physical components of the trawl, e.g. mesh size and

characteristics, overall gear geometry, footrope and headline configuration

(Fujimori et al., 2005; Krag et al., 2010; Broadhurst et al., 2012; Herrmann et al.,

2015; Brinkhof et al., 2017). Others use the available knowledge of species-

specific behavioural responses during the catching process (Winger et al., 2010)

to modify species catchability or provide specific escape routes (Krag et al., 2008;

2015; Sistiaga et al., 2015; Lomeli et al., 2018; Melli et al., 2018a). Some BRDs

combine both aspects to exploit differences in size, shape and behaviour among

species to select out the unwanted catches (e.g. Graham and Fryer, 2006; Kim

and Wang, 2010; Karlsen et al., 2018). Together with severe management

measures (e.g discard bans, quotas and reduced fishing effort), BRDs have

contributed to reduce global discard (Zeller et al., 2017). However, specific

fisheries are still bounded to high discard rates (Pérez Roda et al., 2019).

Among the mixed trawl fisheries, those that target crustaceans are the most

challenging in terms of bycatch reduction because they catch a mix of species

with substantial morphological differences and adopt a small mesh size to retain

the target crustacean (Kelleher, 2005; Pérez Roda et al., 2019). Thus, they have

been widely studied and many BRDs have been developed for these fisheries.

This is the case, for example, for the Nephrops (Nephrops norvegicus) directed

mixed trawl fisheries in the northeast Atlantic and penaeid fisheries around the

world (see for review Broadhurst, 2000; ICES, 2004; Catchpole and Revill, 2008).

Dozens of BRDs are documented in literature for these fisheries, with various

degrees of effectiveness in reducing the bycatch and impacts on the target

species. However, because of the morphological and behavioural differences

among the species caught, achieving the desired reduction of bycatch in these

fisheries is rarely (if ever) obtained via one single modification. Among the vast

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array of BRDs available, some could benefit from being combined with others.

However, given the number of modifications available, testing all the possible

combinations at sea would be extremely expensive and time consuming.

Therefore, a cost-efficient approach is to first identify the most promising

combinations that are worth further and detailed investigation.

The aim of this study is to use a meta-analytical approach to (i) utilize all the

information available from previously described BRDs (ii) demonstrate how to

compare and identify the most promising BRD combinations by modelling the

combined selectivity of a trawl with multiple pertinent BRDs; and (iii) predict the

performance of each BRD combination under different real catch scenarios, both

single- and multi-species. We used as a case study BRDs developed for the

Nephrops-directed mixed trawl fishery in the Skagerrak and Kattegat seas. This

meta-analytical approach would not only help to re-evaluate interesting designs

that were never implemented into legislation, but would also provide the tools

necessary to determine if and when their combination would represent an optimal

choice for the case-study fishery. The optimal combination will depend on the

specific catch composition, in terms of species and sizes, and on fishermen’s

individual catch goals (Engås and Soldal, 1992; Maynou and Sardà, 2001;

Feekings et al., 2012). For example, when quotas are available, fishermen may

aim at reducing undersized individuals, while when quotas become restrictive, an

additional temporary reduction in marketable sizes might be necessary.

Therefore, we identified the most promising combinations that could match the

gear selectivity with either of these harvest strategies.

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2. Case-study fishery

Nephrops, also known as Norway lobster, langoustine or scampi, is one of the

economically most important fishery resources in Europe. Due to its wide

geographical range, it is fished using a wide variety of gear types, under different

technical legislations, and in different environmental conditions (ICES, 2004).

Therefore, to maintain a certain level of similarity in fishing dynamics in our case

study, we focused on the ICES sub-division IIIa. Nephrops catches in this area

are regulated by quota, where Denmark is responsible for taking the majority of

the quota (ICES, 2014). A Minimum Conservation Reference Sizes (MCRS) of 32

mm carapace length is set for EU countries fishing in this area (ICES, 2016). The

main fishing technique used to target Nephrops is demersal otter trawls, often

towed in twin- or multi-rig configurations. It is typically conducted at depths

between 30 and 200 m, on muddy grounds. Besides Nephrops, the fishery

catches several economically important fish species, including cod (Gadus

morhua) and haddock (Melanogrammus aeglefinus). These bycatch species can

be a desirable catch, as they contribute to the economic value of the fishery.

However, they are also subjected to quota management and to the EU landing

obligation (EU, 2013). In addition, cod is regulated by the EU long-term

management plan which aims at restoring depleted cod stocks (EC, 2008). To

achieve this objective, trawlers targeting Nephrops are required since February

2013 to use either a species-selective grid in combination with a 70 mm square

mesh codend (termed Swedish grid due to its main adoption by the Swedish

fleet) or a size selective codend (termed SELTRA) which consists of a 90 mm

diamond mesh codend with either a square mesh panel (SMP; 140 mm in

Skagerrak, 180 mm in Kattegat) or a diamond mesh panel (270 mm; ICES,

2014).

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3. Meta-analytical approach

The selectivity of trawl gears is often described by the species- and size-selective

properties of the codend (e.g. mesh size and shape, twine thickness, and codend

circumference). However, multiple selective processes occur during the catching

process. For an individual encountering a trawl to end up being retained in the

codend, it has to be retained during each step of the capture process. Therefore,

we considered the overall selection process in the trawl as sequential. By

assuming each step to be independent from the others, we could combine

population-independent size selectivity estimates for each individual step of the

process. This enabled us to predict the size selectivity of a trawl with a

combination of BRDs tested separately in previous independent experiments. We

selected BRDs pertinent for the case-study fishery for which the data-collection

method allowed estimating absolute species-specific size selectivity, i.e. covered-

codend experiments (Wileman et al., 1996) and paired gears experiments

including a non-selective mesh size (Krag et al., 2014). Furthermore, we selected

BRDs that could be applied independently in different sections of the trawl,

without interfering with each other.

3.1 Predicting the overall trawl selectivity

For a Nephrops, cod or haddock of length l, the likelihood of entering a specific

section of the trawl requires that it is retained by the previous sections. We

divided the trawl in four sections i (Fig. 1), each with an individual retention

probability r(l)i, and modelled the overall retention probability 𝑟𝐶𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) for an

individual of length l, assuming that it was available for the gear, as the product of

the size selection processes in each section of the trawl:

𝑟𝐶𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) = ∏ 𝑟(𝑙)𝑖4𝑖=1 = 𝑟𝐻𝑒𝑟𝑑𝑖𝑛𝑔(𝑙) × 𝑟𝐵𝑜𝑑𝑦(𝑙) × 𝑟𝐸𝑥𝑡𝑒𝑛𝑠𝑖𝑜𝑛(𝑙) × 𝑟𝐶𝑜𝑑𝑒𝑛𝑑(𝑙) (1)

where rHerding(l), rBody(l), rExtension(l) and rCodend(l) were the size selectivity in the

respective sections of the trawl.

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Figure 1 Schematic drawing of the four independent trawl sections considered in this

study.

3.2 BRDs included in the study

Three species were considered in this study: the target species, Nephrops, and

two bycatch species, cod and haddock. For each of these species, we included in

the meta-analysis seven datasets involving a total of six independent BRDs and

the design used as baseline (i.e. C0, 90 mm diamond mesh codend). The dataset

used as baseline selectivity for haddock differed respect to Nephrops and cod

due to lack of data for this species in Krag et al., (2013; Table 1). Two of the

datasets were from studies using paired gears, while the rest were analysed

according to a covered-codend design.

The three first sections of the trawl (i.e. Herding, Body and Extension) involved

one modification each. When predicting the selectivity of a trawl with combined

BRDs, they were either present (termed H1, B1, E1) or absent (H0, B0, E0). The

last section of the trawl (i.e. Codend) involved four options, numbered from C0 to

C3, Moreover, we included the option of leaving the codend open (C4) by

considering zero retention for those individuals entering that codend.

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Table 1 Summary of the datasets included in the meta-analysis.

3.2.1 Herding section

The counter-herding device developed by Melli et al. (2018a) reduced the

retention of fish bycatch by leading fish outside the trawl path (Fig. 2 a). No

significant effect was detected on the catches of Nephrops, whereas the design

was very effective on haddock and less on cod. Length-dependent effects were

detected for both roundfish species.

3.2.2 Body section

The large mesh diamond panel (800 mm) tested by Krag et al. (2014) in the

upper netting of the trawl body exploited fish behaviour inside the trawl. Some

species are known to rise towards the upper netting (Winger et al., 2010) and,

thus, have a higher probability to escape through the large meshes (Fig. 2 b).

The results showed a strong effect on some roundfish, such as haddock, but a

lower effect on cod and flatfish.

3.2.3 Extension section

The data used for this section were those collected for the horizontally divided

trawl codend used as baseline in Melli et al. (2018b) and Melli et al. (2019). The

separator panel and frame influence the vertical distribution of species in the

trawl extension to segregate them in different compartments (Fig. 2c). The results

Reference Trawl section ID Type of data Description

Melli et al., 2018a HERDING H0/H1 Paired gears Counter-herding device

Krag et al., 2014 BODY B0/B1 Paired gearsTrawl with 800 mm diamond meshes

in the upper netting of trawl body

Melli et al., 2018b

and Melli et al.,

2019

EXTENSION E0/E1 Covered-Codend Horizontally divided trawl codend

Krag et al., 2013 CODEND C0 Covered-Codend90 mm diamond mesh codend;

cod and Nephrops

Krag et al., 2016 CODEND C0 Covered-Codend90 mm diamond mesh codend;

haddock

Krag et al., 2015 CODEND C1 Covered-Codend 120 mm diamond mesh codend

Krag et al., 2013 CODEND C2 Covered-Codend90 mm diamond mesh codend with

120 mm square mesh panel

Krag et al., 2015 CODEND C3 Covered-Codend120 mm diamond mesh codend with

180 mm square mesh panel

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showed that Nephrops enter the lower compartment, with the exception of a few

bigger individuals. In contrast, most fish entered the upper compartment.

However, while haddock showed a strong preference for the upper compartment,

the vertical distribution of cod was found to be length-dependent, where larger

individuals entered the upper compartment in higher proportions than smaller

ones (Melli et al., 2019).

3.2.4 Codend section

A total of four codend designs were included in the meta-analysis: 90 mm

diamond (C0) from Krag et al. (2013) for Nephrops and cod and Krag et al.

(2016) for haddock; 120 mm diamond (C1) from Krag et al. (2015); 90 mm

diamond with a 120 mm SMP (C2) from Krag et al. (2013); and 120 mm diamond

with 180 mm SMP (C3) from Krag et al. (2015; Table 1; Fig. 2d). The most

important characteristics that determine their selectivity are summarized in Table

2. Using the horizontal separation in the Extension section, we could apply each

codend in either the lower or upper position and in combination with each other.

Table 2 Summary of codend specifications. Circum. = circumference in the codend; Twine thickness = twine thickness of the netting; SMP = square mesh panel; m= metre, mm = millimetre.

CodendLength

(m)

Circum.

(No.

meshes)

Codend

mesh size

(mm)

Twine

thickness

SMP

mesh

size (mm)

SMP

Length

(m)

Cover

mesh size

(mm)

C0 7 100 95.1 4 mm, Double - - 40

C1 6 92 127.4 5 mm, Double - - 40

C2 7 100 94.8 4 mm, Double 126.1 3 40

C3 6 92 126.9 5 mm, Double 180 3 40

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Figure 2 Schematic drawings of the BRDs included in the study. a) Counter-herding device from Melli et al., 2018a; b) Large meshes in the upper netting of the trawl body from Krag et al., 2014; c) Horizontally divided trawl codend from Melli et al., 2018b; d) C0: 90 mm diamond codend from Krag et al., 2013; C1: 120 mm diamond codend from Krag et al., 2014; C2: 90 mm diamond codend with 120 mm SMP from Krag et al., 2013; C3:120 mm diamond codend with 180 mm SMP from Krag et al., 2014.

3.3 Estimation of size-selectivity from original datasets

For each of the datasets included in the meta-analysis, and for each species

separately, we estimated the size-dependent retention probability r(l) with the

size represented by the length l of the species (Wileman et al. 1996). Two

different approaches were followed, depending on the type of experimental data

originally collected (i.e. covered-codend or paired gears).

3.3.1 Covered-codend

Several parametric models were tested to describe the size selection, r(l, v)

where v is a vector consisting of the parameters of the model. The values of the

parameters v were then estimated so that the experimental data (averaged over

hauls) would be most likely observed, assuming that the model was able to

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describe the data sufficiently well. We considered a total of nine models. The four

models Logit, Probit, Gompertz and Richard described in Wileman et al. (1996);

the dual sequential selection models applied by Zuur et al. (2001), Sistiaga et al.

(2010) and Krag et al. (2016); and the triple logistic model described by Noack et

al. (2017). The dual sequential and triple logistic models imply that two and three

selective processes, respectively, are expected to contribute to the size selection

process. These processes are assumed sequential, meaning that the proportion

of individuals exposed to the second process is assumed to consist of those that

were not exposed to the first process and additionally those that were, but were

retained. For the dual sequential model, we assumed the first process to be

modelled by a logistic curve, while considering all the four classical models (Logit,

Probit, Gompertz and Richard) for the second process. To identify the best model

for each species and dataset, we followed the procedure of inspecting goodness

of fit as described by Wileman et al. (1996), selecting the model with the lowest

Akaike information criterion (AIC) value (Akaike, 1974).

3.3.2 Paired gears

For the BRDs in the Herding and Body sections, where the experimental data

were not expected to follow an s-shaped selectivity model, we used the flexible

polynomial model of order four often applied to catch comparison of paired gears

data (Krag et al., 2014; Melli et al., 2018a). This provided 31 additional models

that were considered as candidates for describing the experimental data. The

model with the lowest AIC was selected to either describe the size-dependent

retention rate, r(l), according to Krag et al. (2014), or the catch comparison rates,

cc(l), according to Melli et al. (2018a). In the latter, cc(l) was used to estimate the

catch ratio, cr(l), using the relationship between cr and cc (Herrmann et al. 2017).

The catch ratio expresses the relative selectivity of the design when compared to

the control trawl. This step was required because the device tested by Melli et al.

(2018a) could not only reduce the retention of individuals but also increase it.

Thus, a retention rate limited to 1.0 would not fully represent the absolute effect

of the device. Using the catch ratio, a value of 1.0 implies that there is no

difference in catch respect to the control, whereas values above and below 1.0

imply increased or reduced catches, respectively.

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3.3.3 Uncertainty estimation

The model chosen for each dataset, its parameters and fit statistics are

summarised in Appendix 1. Regardless of the model selected, once the model

was chosen for each dataset and each species, a double-bootstrap method with

1000 repetitions was applied to consider both within and between hauls variation

in size selectivity (Millar, 1993). For each design, the result of the bootstrap

method was a new set of data that was analysed using the identified selection

model. This bootstrap set, besides being used to estimate Efron 95% confidence

intervals (CIs; Efron, 1982) for the specific design, was an essential step to

estimate of uncertainties for the combined selectivity (see section 3.4).

3.4 Estimation of combined selectivity

Considering the BRDs included in this study, and because the modification

introduced in the Extension section is a separation into compartments, rExtension(l)

which expresses the probability of an individual of length l to enter the lower

compartment, Eq. (1) becomes:

𝑟𝐶𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) = 𝑟𝐻𝑒𝑟𝑑𝑖𝑛𝑔(𝑙) × 𝑟𝐵𝑜𝑑𝑦(𝑙) × [𝑟𝐸𝑥𝑡𝑒𝑛𝑠𝑖𝑜𝑛(𝑙) × 𝑟𝐶𝑜𝑑𝑒𝑛𝑑𝐿(𝑙) + (1.0 − 𝑟𝐸𝑥𝑡𝑒𝑛𝑠𝑖𝑜𝑛(𝑙)) ×

𝑟𝐶𝑜𝑑𝑒𝑛𝑑𝑈(𝑙)] (2)

where rCodendL(l) is the size selectivity of the lower codend and rCodendU(l) of the

upper one. When no separation is included in the trawl rExtension(l) equals one,

meaning that all individuals enter the only codend available. When no BRD is

inserted in the Herding and Body sections, rHerding(l) and rBody(l) are assumed to

equal one, meaning that individuals that enter that section are retained as they

would in a standard trawl.

According to Eq. (2), we defined as baseline of this study a trawl with no BRD in

the Herding area (rHerding(l)=1.0), no BRD in the Body section (rBody(l)=1.0), no

separation in the Extension (rExtension(l)=1) and a 90 mm diamond codend (C0). All

the possible combinations of BRDs were obtained by substituting the size

selectivity in the pertinent sections in Eq. (2).

To estimate 95% Efron CIs for each rCombined(l), we used the bootstrap sets

obtained in section 3.3.3 for each original design. Because these bootstrap sets

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were obtained independently, a new bootstrap set of results for rCombined(l) was

created using:

𝑟𝐶𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙)𝑖 = 𝑟𝐻𝑒𝑟𝑑𝑖𝑛𝑔(𝑙)𝑖 × 𝑟𝐵𝑜𝑑𝑦(𝑙)𝑖 ×

[𝑟𝐸𝑥𝑡𝑒𝑛𝑠𝑖𝑜𝑛(𝑙)𝑖 × 𝑟𝐶𝑜𝑑𝑒𝑛𝑑𝐿(𝑙)𝑖 + (1.0 − 𝑟𝐸𝑥𝑡𝑒𝑛𝑠𝑖𝑜𝑛(𝑙)𝑖) × 𝑟𝐶𝑜𝑑𝑒𝑛𝑑𝑈(𝑙)𝑖] 𝑖 ∈ [1 … 1000] (3)

where i denotes the bootstrap repetition index (Herrmann et al. 2018). In Eq. (3)

the 1000 bootstrap sets generated from the original datasets were either

multiplied or summed to obtain the new bootstrap set for the combined

configuration. Based on this final bootstrap set, 95% Efron Percentile CIs for

rCombined(l) were estimated.

All the analyses were conducted with the software SELNET (Herrmann et al.,

2012).

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4. Identification of the most promising combinations

Once the selectivity of the BRD combinations was modelled, its applicability and

relevance for the case-study fishery was investigated through different tools.

These tools allowed us to select out all the combinations that did not produce the

desired effects, in terms of target and bycatch catches, and ultimately determine

which BRD combinations have the highest potential for optimizing the selectivity

of the Nephrops-directed fishery in Skagerrak and Kattegat.

In particular, we used three tools to visualize and summarize the performance of

the combinations: delta selectivity, cumulative population caught, and

performance indicators.

4.1 Delta selectivity

The first tool to investigate the species-specific, population-independent

performance of a BRD combination entailed comparing it to the size-selectivity of

the baseline design. If rB(l) is the size selectivity of the baseline trawl, and rC(l)

the size selectivity of the combination of interest, then the difference in selectivity,

Δr(l) is:

Δ𝑟(𝑙) = 𝑟C(𝑙) − 𝑟B(𝑙) (4)

Uncertainties for Δr(l) were estimated using the approach described in (section

3.1), but by subtracting the two independently generated bootstrap sets. In

general, Δr(l) spans between -1 and 1, where values above 0.0 imply that the

combination has higher retention probability for individuals of length l than the

baseline design, while values below 0.0 imply a lower retention probability.

Δr(l) was estimated separately for each species of interest and used to identify

the species-specific length-range significantly affected by the combination of

BRDs. Ideally, for a good combination, Δr(l) would be close to or above 0.0 for

target species and below 0.0 for unwanted species.

4.2 Cumulative population caught

For the second tool, we investigated the performance of each combination under

three realistic population scenarios for each species (P1-P3). The three

population structures nPop(l) for each species were estimated using the original

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datasets included in this study, by pooling data over hauls for hauls with more

than 20 individuals. The data included for each population are summarized in

Appendix 2. For covered-codend datasets, data from codend and cover were

summed to reflect the population entering the trawl. In contrast, for paired gears

datasets, only data from the control trawl were used to obtain the population

entering the trawl. Within species, the populations differed in length-range

represented, density of each length-class and mode/s (i.e. most frequent length

class represented). For each population, uncertainties (95% Efron CIs) were

obtained based on a double bootstrap method. This considered both the

between-hauls variability in the structure of the population entering the codend

and within-haul variability deriving from limited numbers of fish or Nephrops

entering the codend in that specific haul, as well as the potential effect of

subsampling. Specifically, the double bootstrap procedure accounted for

between-hauls variability by selecting hauls h with replacement from the h

number of hauls selected from the dataset. Within-haul uncertainty was

accounted for by resampling with replacement the fish or Nephrops length-

measured, followed by raising the numbers according to the subsampling ratios

within each compartment. The number resampled for each compartment in this

inner bootstrap loop equalled the total number of individuals length-measured in

the respective compartment in the selected haul. 1000 bootstrap repetitions were

conducted and used to calculate the 95% Efron CIs for the population nPop(l).

Using the size-selection curves predicted in section 3.4 for each BRD

combination, and applying them to nPop(l), we obtained simulated catches,

nCatch(l). We visualized the population caught as the cumulative distribution

function for the catch:

𝐶𝐷𝐹_𝑛𝐶𝑎𝑡𝑐ℎ(𝐿) = ∑ {𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) × 𝑛𝑃𝑜𝑝(𝑙)}𝐿𝑙=0 (5)

For each 𝐶𝐷𝐹_𝑛𝐶𝑎𝑡𝑐ℎ(𝐿) we calculated 95% CIs based on the bootstrap sets for

𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) and 𝑛𝑃𝑜𝑝(𝑙) using the approach previously described for 𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙).

Ideally, a good BRD combination would lead to catching more commercial sized

than undersized individuals regardless of the population structure. Because

𝐶𝐷𝐹_𝑛𝐶𝑎𝑡𝑐ℎ(𝐿) expresses the retention rate up to a certain length, the rate at the

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species-specific MCRS denotes the proportion of catch that is undersized for a

given population scenario. Moreover, BRD combinations whose efficiency was

significantly affected by the population structure had 𝐶𝐷𝐹_𝑛𝐶𝑎𝑡𝑐ℎ(𝐿) with non-

overlapping CIs for the different scenarios.

4.3 Performance indicators

For the third tool, we converted the number of individuals per length class into

weights and used them to calculate summary indicators (Sala et al., 2015). This

is particularly useful to evaluate the usability of a BRD combination in a fishery

because quotas are typically expressed in weight, not in number of individuals.

For cod and haddock, we used the length-weight relationship available on

fishbase.org for ICES Division IIIa. For Nephrops we used the data from the Data

Collection Framework (DCF) and International Bottom Trawl Survey (IBTS)

programs in Skagerrak and Kattegat. The specific values of the factors a and b

used for the length-weight conversion are provided in the Supplementary

Material.

Using the size-selection curves predicted in section 3.1 for each BRD

combination, and applying them to the population expressed in weight, 𝑤𝑙 ×

𝑛𝑃𝑜𝑝(𝑙), we obtained simulated catches in weight, 𝑤𝑙 × 𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) × 𝑛𝑃𝑜𝑝(𝑙).

𝑤𝑙 is the weight for length class l obtained by the species specific relationship

𝑤𝑙 = 𝑎 × 𝑙𝑏. These were then summarized by calculating three different indicators

(wP−, wP+, and WDiscardRatio), for each of the species-specific nPop(l)

separately. wP− and wP+ were used to express the percentage of weight retained

for individuals below and above the species-specific MRCS, respectively, for a

specific combination of BRDs. Ideally, a selective gear would have a low wP− for

both the target and bycatch species. In contrast, wP+, should be high for the

target species and either high or low for the bycatch species, depending on the

catch goals of the individual fisherman. The wDiscardRatio was calculated to

express the percentage of weight of undersized individuals respect to the total

weight of the catch. Normally, a low wDiscardRatio would imply that the gear is

well suited to the catch scenario. However, BRDs that strongly reduce the weight

of the commercial sized bycatch, thus enhancing high quota savings, will have a

relatively high value of wDiscardRatio. The indicators were calculated by:

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𝑤𝑃− = 100 ∑ {𝑎×𝑙𝑏×𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙)×𝑛𝑃𝑜𝑝(𝑙)}𝑙<𝑀𝐶𝑅𝑆

∑ {𝑎×𝑙𝑏×𝑛𝑃𝑜𝑝(𝑙)}𝑙<𝑀𝐶𝑅𝑆

𝑤𝑃+ = 100 ∑ {𝑎×𝑙𝑏×𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙)×𝑛𝑃𝑜𝑝(𝑙)}𝑙>MCRS

∑ {𝑎×𝑙𝑏×𝑛𝑃𝑜𝑝(𝑙)}𝑙>𝑀𝐶𝑅𝑆 (6)

𝑤𝐷𝑖𝑠𝑐𝑎𝑟𝑑𝑅𝑎𝑡𝑖𝑜 = 100 ∑ {𝑎 × 𝑙𝑏 × 𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) × 𝑛𝑃𝑜𝑝(𝑙)}𝑙<𝑀𝐶𝑅𝑆

∑ {𝑎 × 𝑙𝑏 × 𝑟𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑(𝑙) × 𝑛𝑃𝑜𝑝(𝑙)}𝑙

We used the MRCS for the ICES division IIIa: 32 mm carapace length for

Nephrops, 30 cm and 27 cm total length for cod and haddock, respectively. All

indicators (wP−, wP+ and wDiscardRatio) were estimated with uncertainties for

each species and population scenario, using the bootstrap set for rCombined(l) and

nPop(l). Specifically, by first calculating the values for the indicators based on

each bootstrap repetition result for rCombined(l) and nPop(l) synchronous in (5) to

obtain a bootstrap set for the indicator values. Finally, based on the resulting

bootstrap set, 95% Efron CIs were obtained for each of the indicators.

Because uncertainties are typically wider at the tails of the length range

represented in the data, and since the conversion into weights accentuate the

influence of the larger and less represented length classes when estimating the

indicators, we restricted the length range for each of the species analysed

according to the strength of the data in the original datasets. In particular, we set

the minimum length class as the smallest length class including at least five

individuals in all the datasets. Similarly, we determine the maximum length of the

range as the largest length class with at least five individuals in all the datasets.

The length range was, therefore, restricted to 20.5–76.5 cm and 18.5–43.5 cm for

cod and haddock, respectively, and to 20.5–59.5 mm for Nephrops. This

approach prevented the less represented length classes from compromising the

information contained in the main bulk of data.

4.4 Multispecies comparison of the best combinations

Once a subset of the overall combination was identified, we could simulate and

compare their performance under a multispecies catch scenario. Following the

process described in section 4.2, but restricting the number of hauls selected to

those including all the three species of interest, we estimated a multispecies set

of populations, nPop(l). We then used these populations to calculate the

indicators described above. Each indicator was used to grade the overall

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performance of the combinations, and identify the best options, depending on the

hypothetical catch goals (e.g. maximum quota saving or maximum economic

output).

5. Results and discussion

By using this meta-analytical approach we could identify hidden potential for

improving selectivity in the case-study fishery, i.e. Nephrops-directed trawl

fishery, deriving from the combination of previously described, pertinent BRDs.

Nonetheless, the predicted selectivity curves and performances of the BRD

combinations are theoretical, assuming that when combined the BRDs would

perform as they do when applied individually. However, for this assumption to be

true, the combined application of the BRDs in the trawl would need to be carefully

planned. For example, when inserting a codend with a SMP as the lower codend

of a horizontally divided trawl, the escape probability through the meshes of the

SMP would likely be affected by the obstruction represented by the upper

codend. Therefore, to perform experimentally as predicted, the design would

need to prevent proximity between the two codends. This additional complexity

may be justified if the improvement in selectivity of the BRD combination is

substantial. The major outcome of this meta-analysis is indeed the identification

the most promising combinations that could be worthwhile the time and cost

outlay associated with experimental investigation.

5.1 Prediction of combined BRDs selectivity

From the datasets included in this study, we obtained a total of 100 possible

combined designs for Nephrops and cod. Since data for haddock were

unavailable for C2 (i.e. 90 mm diamond mesh size codend with a 120 mm SMP),

the number of possible combinations for haddock was 64. For all the species,

four combinations had rCombined(l) equal to 0.0, relative to the theoretical option of

fishing with an open codend (C4) when no separation in the extension was

included (E0). Thus, the number of combined selectivity curves was 96 for

Nephrops and cod (Fig. 3 and 4) and 60 for haddock (Fig. 5). In each figure, the

combinations that were identified as the most promising for the case-study fishery

at the end of the elimination process described in section 5.2 are highlighted.

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By combining multiple BRDs we were able to produce alternative selectivity

patterns (Fig. 3, 4 and 5), respect to the traditional S-shaped selection curve of

trawl gears (Dickson et al., 1995; Wileman et al., 1996). In the simple codends

C0 and C1 (i.e. with no SMP) the retention probability increased with the size of

the individuals until reaching the 100% retention level. The inclusion of a SMP did

already alter the shape of the selectivity curve, to the point that the curve

appeared to be split into two sections with different steepness (e.g. Fig. 3,

H0B0E0C3). This is caused by the sequential selection processes of the SMP

and codend, respectively. Consequently, the addition of selection processes (e.g.

BRDs) increased the level of complexity of the selectivity curves (e.g. Fig. 4,

H1B1E0C0), sometimes increasing the retention of the smaller length classes

and/or decreasing the retention of the larger ones (e.g. Fig. 4, H1B1E0C0).

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Fig

ure

3. P

red

icte

d s

ele

ctiv

ity c

urv

es (s

olid

lines) a

nd

95

% E

fron

CIs

(da

sh

ed

lines) o

f the

96

co

mb

ina

tion

s fo

r Ne

ph

rop

s. G

rey rib

bo

ns h

igh

ligh

t the

most p

ertin

ent

co

mb

inatio

ns s

ele

cte

d a

fter th

e e

va

lua

tion

pro

cess. T

he

figure

is re

ad

as a

ma

trix w

ith e

ach

plo

t inclu

din

g c

urv

es w

ith a

co

mbin

atio

n o

f the

BR

Ds in

dic

ate

d a

s c

olu

mn

an

d ro

w title

s. C

olu

mn

s a

dd

the

ho

rizo

nta

l sep

ara

tion

(E1

) with

the

spe

cifie

d c

ode

nd

as lo

we

r co

den

d. R

ow

s a

dd

BR

Ds in

the

traw

l bo

dy (B

1), h

erd

ing

are

a (H

1), o

r bo

th

(H1B

1). B

RD

s in

unspe

cifie

d s

ectio

ns a

re a

bse

nt (i.e

. H0

; B0; E

0). T

he

first p

lot (u

ppe

r left c

orn

er) s

ho

ws th

e s

ele

ctiv

ity o

f a tra

wl w

ith th

e fo

ur s

ing

le c

od

en

ds: C

0=

Red;

C1=

Gre

en

; C2=

Blu

e; C

3=

Pu

rple

. In c

olu

mn

2 to

6, th

e c

olo

ur o

f each

cu

rve

co

rresp

onds to

the

rela

tive

co

de

nd

in th

e u

pp

er p

ositio

n. T

he

colo

ur P

ink is

use

d fo

r the

op

en

co

de

nd

(C4

).

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Fig

ure

4

. P

red

icte

d se

lectivity cu

rve

s (s

olid

lin

es)

an

d 9

5%

E

fron

C

Is (d

ashe

d lin

es)

of

the

96

com

bin

atio

ns fo

r co

d.

Gre

y rib

bon

s h

igh

ligh

t th

e m

ost

pe

rtin

ent

co

mb

ination

s s

ele

cte

d a

fte

r th

e e

va

lua

tio

n p

rocess.

Th

e f

igure

is r

ead

as a

ma

trix

with

ea

ch

plo

t in

clu

din

g c

urv

es w

ith

a c

om

bin

atio

n o

f th

e B

RD

s in

dic

ate

d a

s c

olu

mn

an

d r

ow

title

s.

Colu

mn

s a

dd

the

ho

rizo

nta

l sep

ara

tio

n (

E1

) w

ith

th

e s

pe

cifie

d c

ode

nd

as lo

we

r co

den

d.

Ro

ws a

dd

BR

Ds in t

he

tra

wl b

od

y (

B1

), h

erd

ing

are

a (

H1

), o

r b

oth

(H1B

1).

BR

Ds in

unspe

cifie

d s

ectio

ns a

re a

bse

nt

(i.e

. H

0;

B0;

E0

). T

he

first

plo

t (u

ppe

r le

ft c

orn

er)

sho

ws t

he

se

lectivity o

f a

tra

wl w

ith

th

e f

ou

r sin

gle

cod

en

ds:

C0

=R

ed;

C1=

Gre

en

; C

2=

Blu

e;

C3=

Pu

rple

. In

co

lum

n 2

to

6,

the

colo

ur

of

each

cu

rve

co

rre

sp

onds t

o t

he

re

lative

co

de

nd

in

th

e u

pp

er

positio

n.

Th

e c

olo

ur

Pin

k i

s u

se

d f

or

the

op

en

co

de

nd

(C

4).

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Fig

ure

5. P

redic

ted

se

lectiv

ity c

urv

es (s

olid

lines) a

nd

95%

Efro

n C

Is (d

ash

ed lin

es) o

f the

60

com

bin

atio

ns fo

r ha

dd

ock. G

rey rib

bo

ns h

ighlig

ht th

e m

ost p

ertin

en

t

co

mb

inatio

ns s

ele

cte

d a

fter th

e e

va

lua

tion

pro

cess. T

he

figure

is re

ad

as a

ma

trix w

ith e

ach

plo

t inclu

din

g c

urv

es w

ith a

co

mbin

atio

n o

f the

BR

Ds in

dic

ate

d a

s c

olu

mn

an

d ro

w title

s. C

olu

mn

s a

dd

the

ho

rizo

nta

l sep

ara

tion

(E1

) with

the

spe

cifie

d c

ode

nd

as lo

we

r co

den

d. R

ow

s a

dd

BR

Ds in

the

traw

l bo

dy (B

1), h

erd

ing

are

a (H

1), o

r bo

th

(H1B

1). B

RD

s in

unspe

cifie

d s

ectio

ns a

re a

bse

nt (i.e

. H0

; B0; E

0). T

he

first p

lot (u

ppe

r left c

orn

er) s

ho

ws th

e s

ele

ctiv

ity o

f a tra

wl w

ith th

e fo

ur s

ing

le c

od

en

ds: C

0=

Red;

C1=

Gre

en

; C3=

Pu

rple

. In c

olu

mn

2 to

6, th

e c

olo

ur o

f each

cu

rve

co

rrespo

nds to

the

rela

tive

co

de

nd

in th

e u

pp

er p

ositio

n. T

he

co

lou

r Pin

k is

use

d fo

r the

op

en

co

den

d(C

4).

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Before interpreting the combined selectivity patterns, in particular of the most

complex designs, it is important to observe their uncertainties. For each predicted

selectivity curve, the 95% Efron CIs reflected the strength of the data and the

consistency (between-hauls variation) of the effect in the original datasets. Thus,

combinations of BRDs with high binomial noise in one or more of the original

datasets resulted in wide CIs. In particular, this is the case for the tails of the

length-range of each species, where the dataset with the most restricted length

range limited the inferential power for that combination. This result prevented

predictions which were not supported by the original experimental data.

Examples can be observed in Fig. 3, where the combined selectivity curves of H1

and H1B1 for Nephrops resembled a bell-shaped curve (Dickson et al., 1995;

Lövgren et al., 2016), with a high retention of the central length classes and a low

retention of the smaller and larger classes. However, as expressed by the wide

CIs, the effect on the larger classes is inconclusive and should not be interpreted.

Moreover, many of the combined selectivity curves involving the counter-herding

device (H1) exceeded retention rates of 1.0 (e.g. Fig. 3 columns 1–4 of rows 3–4,

and Fig. 4 all curves of rows 3–4). This was caused by the use of the catch ratio

to describe the effect of the counter-herding device, which in some cases

increased the number of individuals entering the trawl.

Of the variety of selectivity patterns that could be achieved by combining the

BRDs included in this study, only few combinations would be viable for the case-

study fishery. In particular, high retention rates of commercial sized Nephrops

were an essential requirement for a Nephrops-directed fishery. Indeed, the BRD

combinations highlighted tended to all have a similar selective pattern in the main

length-range of Nephrops (Fig. 3). In contrast, the desirable effects on the two

bycatch species, cod and haddock, were more complicated to evaluate, since

they can vary depending on the specific catch goals. Some of the highlighted

BRD combinations strongly reduced the retention of haddock but not that of cod;

others reduced the undersized bycatch but retained the commercial sized cod;

others reduced both the undersized and commercial sized fractions of both

species. In the following section we have illustrated the elimination process

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undertaken to identify the most pertinent (highlighted) combinations for the case-

study fishery that could be worth further experimental investigations.

5.2 Identification of the most promising combinations

To determine the most relevant combinations, we inspected both the absolute

selectivity and the performance under different catch scenarios of each

combination, relative to the three species of interest. This elimination process

was iterative, as each of the tools used (section 4) provided different information

about the efficiency and applicability of the BRD combinations. However, the

indicators (third tool) proved to be the most efficient measure to determine if the

BRD combination could represent a viable option for the case-study fishery. In

particular, all the BRD combinations predicted to cause a major loss of

commercial sized Nephrops, under any of the considered population scenarios,

were excluded. In contrast, the other two tools provided a more detailed and

length-based information, and were used in few cases where the absolute

selectivity curve (section 5.1) and the indicators were not sufficient to clearly

select or eliminate the BRD combination from the pool.

Hereafter we illustrated the information provided by each tool through examples,

which are meant to clarify the desired properties of a pertinent BRD combination

rather than describing the selectivity pattern of all the combinations obtained.

5.2.1 Delta selectivity

The species-specific absolute selectivity of BRD combinations was compared to

that of the baseline gear to identify significant changes in selectivity (Fig. 6). In

the example, where the combination H0B0E1C1C0 is compared to the baseline,

the Delta selectivity highlighted significant major losses of commercial sized

Nephrops (red curve). Moreover, the combination resulted in a moderate but

significant reduction of cod between 20 and 40 cm (green curve), but no

significant change in haddock catches (blue curve).

Indeed, the combination H0B0E1C1C0 included a horizontally divided codend

with a lower codend of 120 mm diamond meshes and an upper codend of 90 mm

diamond meshes. Intuitively, this combination would not be applicable to a

Nephrops-directed fishery, as most Nephrops enter the lower compartment (Melli

et al., 2018b; Melli et al., 2019; Karlsen et al., 2018) and would not be retained

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with a 120 mm diamond mesh size (Krag et al., 2008). Similarly, most haddock

enter the upper compartment and would not be appropriately selected out by the

90 mm diamond mesh size (Graham and Ferro, 2004; Melli et al., 2018b). In

contrast, the reduction in cod between 20 and 40 cm highlighted how this length-

range enters in greater proportion the lower compartment and would, therefore,

benefit from encountering a 120 mm mesh size.

Figure 6 Predicted selectivity with 95% Efron CIs (solid lines with ribbons) of the combination H0B0E1C1C0 for Nephrops (red), cod (green) and haddock (blue). Delta selectivity with 95% Efron CIs (solid line with dashed lines) of the H0B0E1C1C0 selectivity respect to the baseline (H0B0E0C0).

5.2.2 Cumulative population caught

In terms of bycatch reduction, the cumulative population caught was used to

inspect the variability in discard ratios of BRD combinations, under different

population scenarios (Fig. 7). Indeed, when implementing a BRD to reduce the

bycatch of commercial species, a first objective is generally to reduce the catch of

undersized individuals, more than that of valuable sizes. However, the efficiency

of some of the BRD combinations in selecting out undersized individuals was

found to be strongly affected by the structure of the population encountered (Fig.

7). For example, in the third population scenario (P3), where the mode in the

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population is close to the MCRS for cod (30 cm), approximately 75% of the

population caught with the combination H1B0E1C2C1 consisted of undersized

individuals. This was caused by the BRDs included in this combination

(H1B0E1C2C1), which were a counter-herding device, a horizontally divided

codend, a lower codend of 90 mm diamond mesh size with a 120 mm SMP and

an upper codend with 120 mm diamond meshes. Although this combination could

have been expected to be a viable option for the case-study fishery, the results

showed that it is less performing than other combinations with a lower amount of

BRDs involved. This is because none of the BRDs included in H1B0E1C2C1

were very effective in improving the selectivity for cod around 30 cm, which enter

more frequently the lower compartment and have a lower escape rate through

SMPs (Krag et al., 2015; Melli et al., 2018a). Accordingly, combinations with

population-dependent efficiencies in reducing undersized catches were

considered less desirable than those with lower but more stable efficiency.

Figure 7 On the left, cumulative population caught with 95% Efron CIs (solid lines with ribbons) with the combination H1B0E1C2C1 under three population scenarios of cod. The vertical dashed line indicates the MCRS (30 cm). On the right, structure of the three populations used with 95% Efron CIs (solid lines with ribbons).

5.2.3 Performance indicators

Finally, the performance of the BRD combinations was investigated from a

management-fishermen perspective by calculating the weight indicators to

determine how fishermen’s quotas and incomes may be affected. This tool is

particularly useful to overview the performance of the combinations under

realistic catch scenarios and to summarize a large quantity of information,

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including some of those conveyed by the other tools (e.g. proportion of

undersized caught). All the approximately 1000 indicators for all the BRD

combinations and for each of the population scenarios (P1-P3 per species and a

multispecies scenario) are presented as Supplementary Material. Here, to

illustrate the interpretation of the indicators, we present a subset of BRD

combinations (Table 3). The subset includes the indicators for cod population

scenarios for: the baseline design, two combinations that minimally affected the

retention of cod (red), two combinations with a moderate effect (yellow) and two

combinations that minimized cod catches (green). The baseline design retained

on average between 66.6 and 78.2% of the weight of undersized cod (wP-). The

addition of a horizontally divided codend with a SMP in the lower codend

(H0B0E1C2C0) or of the counter-herding device (H1B0E0C0) did not significantly

reduce the weight of cod below the MCRS (Table 3). In contrast, pairing the

counter-herding device with a large mesh panel in the trawl body (H1B1E0C0)

reduced significantly both undersized (wP- between 33.0 and 34.2%) and

commercial sized catches (wP+ between 33.3 and 38.8%) because of their

complementary efficacy (Krag et al., 2014; Melli et al., 2018a). An even lower

retention of undersized cod could be achieved by combining the counter-herding

device with a horizontally divided trawl codend having a 90 mm mesh size and

120 mm SMP in the lower compartment and 120 mm mesh size in the upper

compartment (H1B0E1C2C1). Moreover, this combination retained on average a

higher percentage of commercial cod (in weight), a result that could be desirable

within the case-study fishery (Table 3). However, in all these examples, the

wDiscardRatio (i.e. percentage of weight discarded respect to the total caught)

was significantly and substantially higher in the scenario with high density of

individuals around the MCRS (P3; Table 3).

BRD combinations highly effective on cod (e.g. H0B1E0C2 and H1B1E1C2C4),

not only reduced the percentage of weight retained below and above the MCRS,

but also had lower wDiscardRatio for the most critical scenario (P3).

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Table 3 Indicators for the baseline design (H0B0E0C0) and six examples of combinations for cod, under three population scenarios (P1, P2 and P3). 95 % Efron CIs are shown within parenthesis. The examples are ordered according to their mean wP-, colours are used to highlight the efficiency of the combination in reducing catches of cod: red = low effect; yellow = medium effect; green = high effect.

5.3 Most promising BRD combinations for the case-study fishery

After inspecting the performance of the BRD combinations, we identified 15

combinations for Nephrops and cod, out of the original set of 96 combinations,

which could be applicable to the case-study fishery (Table 4). Of these 15

combinations, only 10 included predictions for haddock, due to the lack of data

for the 90 mm diamond mesh size codend with a 120 mm SMP (C2). All the

selected combinations retained on average a weight of commercial sized

Nephrops within 15% from the baseline design. This derived mainly from having

a lower codend of 90 mm diamond mesh size, whenever the horizontal

separation was introduced. Only one of the selected BRD combinations had a

wP− (%) wP+ (%) wDiscardRatio (%)

P1 66.6 (53.9 – 77.6) 98.7 (98.0 – 99.3) 2.8 (1.6 – 4.9)

P2 78.2 (70.5 – 86.1) 96.0 (94.3 – 97.2) 8.1 (5.4 – 11.1)

P3 69.0 (57.0 – 78.5) 94.0 (91.7 – 96.4) 59.9 (48.3 – 66.8)

P1 52.7 (41.4 – 63.0) 95.5 (93.7 – 97.1) 2.3 (1.3 – 4.0)

P2 63.6 (56.2 – 71.2) 90.7 (88.0 – 93.2) 7.0 (4.7 – 9.8)

P3 54.7 (44.4 – 63.2) 86.7 (83.0 – 91.7) 56.2 (43.2 – 63.7)

P1 55.8 (41.7 – 67.5) 63.4 (50.0 – 81.4) 3.6 (1.8 – 6.3)

P2 61.7 (50.1 – 73.8) 63.2 (51.3 – 79.0) 9.5 (6.0 – 13.3)

P3 57.6 (43.4 – 68.6) 62.9 (52.2 – 77.8) 65.0 (52.4 – 71.7)

P1 33.6 (23.4 – 42.7) 38.8 (28.3 – 51.9) 3.6 (1.7 – 6.8)

P2 33.0 (24.7 – 42.4) 35.9 (27.8 – 48.1) 9.0 (5.4 – 13.6)

P3 34.2 (24.0 – 43.6) 33.3 (26.4 – 44.9) 67.6 (51.8 – 76.0)

P1 12.4 (7.6 – 16.6) 55.0 (41.9 – 71.3) 1.0 (0.5 – 1.8)

P2 15.8 (10.9 – 20.5) 47.4 (36.3 – 61.3) 3.5 (2.0 – 5.4)

P3 12.6 (7.8 – 16.5) 40.9 (32.2 – 55.5) 38.6 (22.9 – 49.7)

P1 6.1 (3.2 – 10.9) 52.9 (43.4 – 64.6) 0.5 (0.2 – 1.0)

P2 9.4 (5.8 – 15.6) 43.1 (35.9 – 55.1) 2.3 (1.2 – 3.9)

P3 6.2 (3.0 – 10.6) 34.6 (25.3 – 49.6) 26.9 (11.5 – 40.9)

P1 1.2 (0.6 – 2.4) 8.3 (5.1 – 11.5) 0.6 (0.3 – 1.5)

P2 1.8 (1.0 – 3.1) 6.8 (4.4 – 9.7) 2.8 (1.3 – 5.2)

P3 1.3 (0.6 – 2.3) 5.4 (3.3 – 8.6) 32.1 (14.9 – 49.3)

H0B1E0C2

H1B1E1C2C4

H0B0E1C2C0

H1B0E0C0

H0B0E0C0

H1B1E0C0

H1B0E1C2C1

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different lower codend, C2, in combination with a 90 mm diamond codend as

upper codend (Table 4). Furthermore, out of the 15 BRD combinations selected,

10 included the counter-herding device (Melli et al., 2018a) and six the large

mesh size in the upper netting of the trawl body (Krag et al., 2014). Only three of

the selected combinations included the maximum level of complexity (i.e. No. of

BRDs) possible in this study. This was mainly caused by the potential loss of

commercial sized Nephrops associated with each additional BRD introduced in

the trawl, which could eventually add up to an unacceptable level. However,

these were also the BRD combinations predicted to be most effective in reducing

the overall bycatch (Fig. 8).

Table 4 Description of the BRDs included in the 15 most pertinent combinations. H=Herding area; B=Trawl body; E=Trawl extension.

When comparing the performance of the selected BRD combinations under a

multispecies catch scenario (see Appendix 2 for the structures of the populations

considered and the Supplementary Material for the indicators values), we

identified some clear potential harvest strategy for the fishing vessels operating in

the Skagerrak-Kattegat seas (Fig. 8). In Figure 8, the #0 indicates the baseline

design; under the catch scenario considered, the undersized bycatch retained by

the baseline design consisted of 75.3% (66.2–84.0) for cod and a highly variable

H B E Lower codend Upper codend

H0B0E1C0C1 1 - - YES 90 mm diamond 120 mm diamond

H0B0E1C0C2 2 - - YES 90 mm diamond 90 mm + 120 mm SMP

H0B0E1C0C3 3 - - YES 90 mm diamond 120 mm + 180 mm SMP

H0B1E0C0 4 - YES - 90 mm diamond -

H0B1E1C0C2 5 - YES - 90 mm diamond 90 mm + 120 mm SMP

H1B0E0C0 6 YES - - 90 mm diamond -

H1B0E1C0C1 7 YES - YES 90 mm diamond 120 mm diamond

H1B0E1C0C2 8 YES - YES 90 mm diamond 90 mm + 120 mm SMP

H1B0E1C0C3 9 YES - YES 90 mm diamond 120 mm + 180 mm SMP

H1B0E1C0C4 10 YES - YES 90 mm diamond open

H1B0E1C2C0 11 YES - YES 90 mm + 120 mm SMP 90 mm diamond

H1B1E0C0 12 YES YES - 90 mm diamond -

H1B1E1C0C1 13 YES YES YES 90 mm diamond 120 mm diamond

H1B1E1C0C2 14 YES YES YES 90 mm diamond 90 mm + 120 mm SMP

H1B1E1C0C3 15 YES YES YES 90 mm diamond 120 mm + 180 mm SMP

BRDs includedCombination ID

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percentage of haddock (10.7–67.7%). Moreover, catches of commercial sized

bycatch were 97.4% (96.4–98.2) and 62.0% (26.0–92.0) for cod and haddock,

respectively. Respect to the baseline design, most of the identified BRD

combinations had desirable catch profiles: they did not affect significantly the

weight of commercial sized Nephrops retained; and they caught less than 50% of

the weight of undersized bycatch, both cod and haddock (highlighted sections in

Fig. 8). One exception, the combination #6 (H1B0E0C0), was predicted to retain

on average 60.6% (48.3–73.0) of the weight of undersized cod. In terms of

commercial sized, and thus valuable, bycatch a desirable catch profile could be

to either strongly reduce catches or to maintain them as high as possible,

depending on quota availability and market values. However, all the BRD

combinations identified as most promising similarly minimized the percentage of

commercial sized haddock retained, with the exception of combination #1

(H0B0E1C0C1). Nonetheless, this effect may be desirable in a Nephrops-

directed fishery when considering handling and storage costs, as well as the

relatively low market price for haddock

(http://www.hanstholmfiskeauktion.dk/prices?lang=en).

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Figure 8 Two species comparisons of the performance of the most promising BRD combinations (15 for Nephrops and cod, and 10 for haddock) under the multispecies catch scenario. On the left column, percentage (in weight) of undersized bycatch retained (wP-). On the right column, percentage (in weight) of commercial sized bycatch retained (wP+). The first two rows show the percentage (in weight) of bycatch with respect to the percentage (in weight) of target catches (i.e. commercial sized Nephrops). Highlighted sections indicate desirable performances. MCRS = Minimum Conservation Reference Size.

If fishermen were to preserve their fish quotas by reducing both undersized and

commercial sized roundfish catches and obtain relatively clean Nephrops

catches, the optimal choice of BRD combinations would be #15. Indeed, by

including a BRD in each of the four sections of the trawl considered in this study,

this combination achieved overall retention below 25% and 1% of the weight of

cod and haddock, respectively (Fig. 8). This would allow fishermen to continue

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fishing for Nephrops even when approaching exhaustion of roundfish quotas. In

contrast, if fishermen were to minimize the bycatch of undersized roundfish, while

maintaining the majority of the income deriving from commercial sized cod, for

example when cod quota is available, the BRD combinations #2 (H0B0E1C0C2)

and #7 (H1B0E1C0C1) could represent the best options (Fig. 8). Although many

other BRD combinations achieved similar results, these two had the advantage of

maintaining on average the same percentage retained of undersized Nephrops

as the baseline design (see Supplementary Materials for the Indicators values).

In particular, #2 retained 83.0 % (78.3–87.6) of commercial cod catches and

although data for haddock were not available for this BRD combination, haddock

catches can be expected to be low due to its high escape rate through 120 mm

SMPs (Krag et al., 2008; Fryer et al., 2014). Moreover, other BRD combinations

could be preferred if fishermen were to shift from one harvest strategy (e.g.

maximum income) to the other (e.g. quota saving) without having to return to the

harbour. For example, the BRD combination #2 can be converted into

combination #8 by simply adding the counter-herding device and to #10 by

leaving the upper codend open. Therefore, when encountering a bycatch hotspot,

fishermen could drastically reduce roundfish catches, without having to change

fishing area or interrupt fishing activities. The combination of BRDs can allow

fishermen to address selectivity issues on a day-to-day or even haul-to-haul

basis, increasing their ability to adapt to changes in species availability and

annual quotas. Furthermore, because trawl catches are highly variable and the

proportion and composition of the unwanted fraction varies according to multiple

factors (Engås and Soldal, 1992; Feekings et al., 2012), this approach would

make trawl selectivity more flexible and dynamic.

6. Conclusions

The meta-analytical approach described in this study offers the opportunity to fully

exploit the knowledge available concerning bycatch reduction measures for well-

studied fisheries and fishing areas. It creates the means to predict and compare

the performance of combinations of multiple BRDs, from both single- and

multispecies perspectives, and to identify solutions that could support flexible

harvest strategies. We hope that this approach will initiate further discussion

about new multi-purpose trawl designs, where a flexible selectivity can be

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achieved by inserting or removing BRDs, depending on the populations

encountered and the individual catch goals.

The meta-analysis allowed the identification of interesting BRD combinations for

the Nephrops-directed fishery which would be worth experimental validation. For

future references, to optimize the predictive power of the analysis, some caution

should be used when choosing which BRD types and experimental datasets to

include: 1) the choice should be limited to BRDs with substantial effects; limited

effects would result in inconclusive predictions; 2) within species, homogeneity of

length-range among the studies included is essential, as the dataset with the

most restrictive range will affect the overall uncertainties; 3) a multi-species

approach including target and bycatch species is always recommended,

especially when considering mixed fisheries.

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Krag, L.A., Herrmann, B., Karlsen, J.D., Mieske, B., 2015. Species selectivity in

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172: 243–249.

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Krag, L.A., Herrmann, B., Feekings, J., Karlsen, J.D., 2016. Escape panels in

trawls – a consistent management tool? Aquatic Living Resources, 29: 306.

Lomeli, M.J., Waldo Wakefield, W., Herrmann, B., 2018. Illuminating the

Headrope of a Selective Flatfish Trawl: Effect on Catches of Groundfishes,

Including Pacific Halibut. Marine and Coastal Fisheries, 10: 118–131.

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FLEXSELECT: counter-herding device to reduce bycatch in crustacean

trawl fisheries. Canadian Journal of Fisheries and Aquatic Sciences, 75:

850–860.

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behavioural responses to LED lights in trawls and potential applications for

bycatch reduction in the Nephrops-directed fishery. ICES Journal of Marine

Science, 75: 1682–1692.

Melli, V., Krag L.A., Herrmann, B., Karlsen J.D., 2019. Can active behaviour

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in a horizontally divided trawl codend? Fisheries Research.

https://doi.org/10.1016/j.fishres.2018.11.027

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mesh better selective than larger mesh? A perspective on the management

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199.

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APPENDIX 1

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In this appendix we describe the models used for the size-selectivity in each of

the original datasets and species included in the meta-analyses. Moreover, we

report the fit statistics for each model fit.

In case of poor fit statistics (p-value <0.05; deviance >>DoF), the model curve

plots and the residuals were examined to determine whether there were

structural problems in describing the experimental data with the model or if it

could be a case of data overdispersion (Wileman et al., 1996). When no

systematic structure was detected, we considered the low p-values to be a

consequence of overdispersion in the data. Such cases are frequent, especially

when subsampling occured, and have been reported before in all the original

studies included in this meta-analysis (Krag et al., 2013; Krag et al., 2014; Krag

et al., 2015; Krag et al., 2016; Melli et al., 2018a; Melli et al., 2018b; Melli et al.,

2019).

1. Paired gears datasets

1.1 Herding area and trawl body

Data for these two Bycatch Reduction Devices (BRDs) were collected using

paired gears, i.e. a modified test trawl towed in parallel with a control trawl. For

each species, length-dependent count data for each gear were used to estimate

the size-dependent catch comparison rate cc(l) with 95% Efron confidence

intervals (Efron, 1982). The catch comparison rate cc(l) expresses the probability

of a catching an individual of length l with the test trawl given that it was available

to either trawl.

To model cc(l) we used a highly flexible model, often applied to this type of

experiments (Krag et al., 2014; Melli et al., 2018a):

𝑐𝑐(𝑙, 𝒗) = exp (𝑓(𝑙,𝒗))

1.0+exp (𝑓(𝑙,𝒗)) (1)

where f is a polynomial of the fourth order with coefficients v0,…,v4 so v =

(v0,…,v4). We used f (l,v) in the following form:

𝑓(𝑙, 𝒗) = ∑ 𝑣𝑖 × (𝑙

100)

𝑖4𝑖=0 = 𝑣0 + 𝑣1 ×

𝑙

100+ 𝑣2 ×

𝑙2

1002 + ⋯ + 𝑣4 × 𝑙4

1004 (2)

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where the length l is divided by 100 to improve the numerical stability of the

model fitting by preventing numerical overflow due to lengths being raised to

powers in the polynomials. Leaving out one or more of the parameters v0…v4 in

equation (4) provided 31 additional models that were considered as potential

models to describe cc(l,v). We then applied model averaging to describe cc(l,v),

ranking the models according to how likely they were compared to each other

(Burnham and Anderson, 2002). The individual models were ranked and

weighted according to their Akaike's Information Criterion (AIC) values (Akaike,

1974; Burnham and Anderson, 2002; Herrmann et al., 2017) and models with

AIC values within +10 the value of the model with the lowest AIC, were

considered to contribute to cc(l,v) (Katsanevakis, 2006; Herrmann et al., 2017).

Fit statistics highlighted overdispertion in the data for both cod and haddock in

the dataset used for the Herding area (Melli et al., 2018a) and for cod in the trawl

body dataset (Table 1).

Tabel 1. Fit statistics for the modelled catch comparisons.

Cod Nephrops Haddock

p-value Deviance DoF

p-value Deviance DoF

p-value Deviance DoF

Herding area

0.03* 100.75 76 0.06 53.49 39 0.01* 61.50 39

Trawl Body

0.01* 109.94 76 0.55 45.03 47 0.62 35.75 39

2. Covered-codend datasets

2.1 Trawl Extension

The BRD introduced in the trawl extension was a horizontal separation into two

compartments; all individuals that entered the trawl were assumed to be caught

in either the upper or lower compartment because of the mesh size used (40 mm

T90) that is non-selective for the species considered. We were interested in

estimating the length-dependent probability for an individual to enter the upper

compartment, cUPPER(l). According to Krag et al. (2014), we used a length-

dependent model containing four parameters (c1, c2, L50C, and SRC):

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cUPPER(𝑙) = 𝑐1 + (c2 ― 𝑐1) ×exp [(

ln(9)

𝑆𝑅𝐶×(𝑙−𝐿50𝐶)]

1.0+ exp [(ln(9)

𝑆𝑅𝐶×(𝑙−𝐿50𝐶)]

(3)

In a Eq. (3) the probability for an individual to enter the upper compartment,

cUPPER(l), follows a logistic curve within two asymptoms, c1 and c2. The constants

c1 and c2 are constrained to the interval [0.0; 1.0] and represent the asymptotic

probabilty of entering the upper compartment for the largest and smallest

individuals, respectively. L50C is the length at which cUPPER(l) is the mean of c1

and c2. SRC defines how quickly cUPPER(l) shifts from a value close to c1 to a value

close to c2 with increasing length in the vicinity of L50C. Thus, if SRC is close to

0.0, the change in cUPPER(l) will appear over a small length range, whereas if SRC

has a value far from 0.0 the change in cUPPER(l) will cover a wider length span.

Model fits statistics (p-value, deviance, DoF, R2) and parameters for cUPPER(l) of

each species are summarized in Table 2.

Tabel 2. Fit statistic for the modelled cUPPER(l)

Parameters Cod Nephrops Haddock

L50C 16.29 35.97 12.92

SRC 4.91 5.11 2.38

c1 0.76 0.22 0.77

c2 0.30 0.11 0.00

p-value 0.31 0.34 0.03*

Deviance 79.59 47.21 57.96

DoF 74 44 39

2.2 Codends

For each species and each codend separately, we tested different parametric

models to estimate the retention rate at length, r(l, v), where v is a vector

consisting of the parameters of the model. We chose the model with the lowest

individual Akaike information criterion (AIC) value (Akaike, 1974).

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2.2.2 Nephrops

The triple logistic model (Eq. 4) was found to describe best the size selectivity of

Nephrops in the codends C0, C1 and C3 with the retention probability described

by:

𝑟(𝑙, 𝑐1, 𝐿501, 𝑆𝑅1, 𝑐2, 𝐿502, 𝑆𝑅2, 𝐿503, 𝑆𝑅3) = 𝑐1 × 𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿501, 𝑆𝑅1) + 𝑐2 ×

𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿502, 𝑆𝑅2) + (1.0 − 𝑐1 − 𝑐2) × 𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿503, 𝑆𝑅3) (4)

The triple logistic model is constructed by assuming that there are three different

selective processes which contribute to the overall selectivity, i.e. it is the sum of

three logit models in which the weights of the contributions add up to 1.0 (Noack

et al., 2017). These processes are determined by the multiple possible contacts

modes of Nephrops with the codend meshes (Frandsen et al., 2010). In the triple

logistic model, a fraction of individuals, c1, will be subjected to one logistic size

selection process with parameters L501 and SR1; another fraction c2 will be

subjected to a second logistic size selection process with parameters L502 and

SR2; the remaining fraction (1.0–c1–c2) will be subjected to a third logistic curve

with parameters L503 and SR3. The contact ratio parameters c1 and c2 indicate

the probability for an individual to have its selectivity determined by the first and

second process, respectively (Herrmann et al., 2013). Thus, they are numbers

between 0.0 and 1.0.

In contrast, the selectivity of Nephrops in the codend C2 was found to be

described best by a Dual sequential selection curve (Eq. 5) with the first process

modelled by a logistic curve and the second by the size selection model

“Gompertz” (Wileman, 1996). This model implies that the selectivity of the codend

is the result of two sequential selective processes. The first process is described

by a logistic selection curve with parameters L501 (i.e. length of fish with a 50%

retention probability) and SR1 (i.e. difference in length between fish with 75% and

25% retention probabilities) while the second process is described by a

“Gompertz” selection curve, with parameters L502 and SR2. Because the two

processes are sequential, the proportion of individuals that are exposed to the

second process is assumed to consist of those that did not attempt to escape in

the first process and additionally those that attempted to, but were retained.

Therefore, c1 represents the assumed length-independent probability that the

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size selection of the individual will be defined by both selection processes (double

escape attempt), while 1.0 – c1 represents the probability of the individual

encountering only the second process. Thus, c1 is a number between 0.0 and

1.0.

𝑟(𝑙, 𝑐1, 𝐿501, 𝑆𝑅1, 𝐿502, 𝑆𝑅2) = (1.0 − 𝑐1) × 𝐺𝑜𝑚𝑝𝑒𝑟𝑡𝑧(𝑙, 𝐿502, 𝑆𝑅2) + 𝑐1 ×

𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿501, 𝑆𝑅1) × 𝐺𝑜𝑚𝑝𝑒𝑟𝑡𝑧(𝑙, 𝐿502, 𝑆𝑅2) (5)

Model fits statistics (p-value, deviance, DoF, R2) and parameters for the size

selectivity of Nephrops are summarized in Table 3.

Tabel 3. Fit statistics of the modelled size-selectivity for Nephrops in the four codends C0, C1, C2 and C3.

Parameters C0 C1 C2 C3

L50 31.14 47.91 34.98 54.70

SR 10.20 17.13 19.21 65.97

1/δ - - - -

L501 48.72 52.37 28.80 66.10

SR1 4.03 22.53 5.11 13.60

L502 30.97 47.33 33.79 44.63

SR2 7.10 5.00 25.54 0.10

L503 0.10 33.75 - 0.58

SR3 76.31 1.12 - 0.09

c1 0.09 0.63 0.73 1.78

c2 0.75 0.27 - 0.10

Model 4 4 5 4

p-value 0.96 0.42 0.06 0.93

Deviance 30.44 43.10 64.83 29.39

DoF 46 42 49 42

2.2.1 Cod

A Dual sequential size selection curve was found to describe best the selectivity

of cod in the 90 mm diamond mesh size codend (C0) and in the 120 mm

diamond codend with a 180 mm Square Mesh Panel (SMP; C3). For both

codends the two selective processes were modelled using a logistic curve and a

“Probit” curve, respectively (Eq. 6).

𝑟(𝑙, 𝑐1, 𝐿501, 𝑆𝑅1, 𝐿502, 𝑆𝑅2) =

(1.0 − 𝑐1) × 𝑃𝑟𝑜𝑏𝑖𝑡(𝑙, 𝐿502, 𝑆𝑅2) + 𝑐1 × 𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿501, 𝑆𝑅1) × 𝑃𝑟𝑜𝑏𝑖𝑡(𝑙, 𝐿502, 𝑆𝑅2) (6)

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Although a dual sequential size selection model is often expected when the

codend include a SMP (e.g. C3), a second selective process can occur also in

simple codends (e.g. C0) for example during haul-back of the gear (Madsen et

al., 2012).

Similarly, the selectivity of cod in a 90 mm diamond mesh size codend with a 120

mm SMP (C2) was found to be described best by a Dual sequential size selection

curve, but with both processes modelled by a logistic curve (Eq. 7).

𝑟(𝑙, 𝑐1, 𝐿501, 𝑆𝑅1, 𝐿502, 𝑆𝑅2) =

(1.0 − 𝑐1) × 𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿502, 𝑆𝑅2) + 𝑐1 × 𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿501, 𝑆𝑅1) × 𝐿𝑜𝑔𝑖𝑡(𝑙, 𝐿502, 𝑆𝑅2) (7)

Finally, the selectivity of cod in a 120 mm diamond mesh size codend (C1) was

described best by the classical size selection model “Richard” (Wileman, 1996).

This is described not only by the parameters L50 and SR, but also by an

additional parameter (1/δ) that describes the asymmetry of the curve.

Model fits statistics (p-value, deviance, DoF, R2) and parameters for the size

selectivity of cod are summarized in Table 3.

Tabel 3. Fit statistics of the modelled size-selectivity for cod in the four codends C0, C1, C2 and C3.

Parameters C0 C1 C2 C3

L50 22.21 37.67 39.27 66.27

SR 7.57 13.35 14.14 13.84

1/δ - 0.39 - -

L501 19.63 - 44.01 68.08

SR1 3.68 - 4.86 7.31

L502 19.29 - 29.83 36.63

SR2 14.96 - 6.67 27.01

L503 - - - -

SR3 - - - -

c1 0.98 - 0.53 0.73

c2 - - - -

Model 6 Richard 7 6

p-value 0.90 1.00 0.98 0.79

Deviance 56.90 49.59 43.04 73.40

DoF 72 88 64 84

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2.2.3 Haddock

The dataset used to estimate haddock size selectivity in a 90 mm diamond mesh

size codend included a 270 mm SMP (Krag et al., 2016). Therefore, data for

haddock in this codend were considered to have a binomial distribution, because

individuals escaping from both the SMP and the codend were collected in the

same cover. Following Krag et al. (2016), we estimated the selectivity of the

codend indirectly based on the length-dependent retention data for the combined

selection of SMP and codend. Indeed, the overall selectivity of a codend with a

SMP is generally modelled as Dual selection model with two logistic curves (Eq.

7), where the first selection process is described by L50SMP and SRSMP and the

second by L50codend and SRcodend. Therefore, in the meta-analysis we considered

only the parameters estimated for the logistic curve describing the selectivity of

the 90 mm diamond mesh size codend.

Finally, the selectivity of haddock in the codend C1 and C3 was found to be

described best by the size selection model “Richard” (l, L50, SR, 1/δ) and

“Gompertz” (l, L50, SR), respectively (Wileman, 1996).

Model fits statistics (p-value, deviance, DoF, R2) and parameters for the size

selectivity of haddock are summarized in Table 5.

Tabel 4. Fit statistics of the modelled size-selectivity for haddock in codends C0, C1 and C3.

Parameters C0 C1 C3

L50 52.96 29.61 111.39

SR 21.10 8.69 78.50

1/δ - 2.94 -

L501 53.01 - -

SR1 0.10 - -

L502 28.01 - -

SR2 7.98 - -

L503 - - -

SR3 - - -

c1 0.67 - -

c2 - - -

Model 7 Richard Gompertz

p-value <0.01* 0.98 0.89

Deviance 71.87 24.49 23.41

DoF 38 41 33

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References

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Krag, L.A., Herrmann, B., Karlsen, J.D., 2014. Inferring fish escape behaviour in

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trawls – a consistent management tool? Aquatic Living Resources, 29: 306.

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Madsen, N., Herrmann, B., Frandsen, R.P., Krag, L. A., 2012. Comparing

selectivity of a standard and turned mesh T90 codend during towing and

haul-back. Aquatic Living Resources, 25: 231–240.

Melli, V., Karlsen, J.D., Feekings, J.P., Herrmann, B., Krag, L.A., 2018a.

FLEXSELECT: counter-herding device to reduce bycatch in crustacean

trawl fisheries. Canadian Journal of Fisheries and Aquatic Sciences, 75:

850–860.

Melli, V., Krag, L.A., Herrmann, B., Karlsen, J.D. 2018b. Investigating fish

behavioural responses to LED lights in trawls and potential applications for

bycatch reduction in the Nephrops-directed fishery. ICES Journal of Marine

Science, 75: 1682–1692.

Melli, V., Krag L.A., Herrmann, B., Karlsen J.D., 2019. Can active behaviour

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283–291.

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APPENDIX 2

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In this appendix we describe the populations used when investigating the

performance of each combination under realistic catch scenarios for each of the

species considered. The populations were generated using the original datasets

included in this study, by pooling data over hauls for hauls with more than 20

individuals (Table 1).

Table 1. Summary of the data used to generate each population the three population

scenarios for each of the species analysed.

For the multispecies scenario, hauls from the dataset by Krag et al. (2014)

containing more than 20 individuals for all the species considered were included

(Table 2).

Table 2. Summary of the data used to generate the multispecies scenario.

Species Population Original dataset No. of hauls No. of individuals

P1 Krag et al., 2016 8 6438

P2 Krag et al., 2014 22 12172

P3 Melli et al., 2019 4 7014

P1 Krag et al., 2015 25 3018

P2 Melli et al., 2018a 12 2333

P3 Melli et al., 2019 6 3835

P1 Melli et al., 2018b; 2019 14 5753

P2 Krag et al., 2014 22 4793

P3 Krag et al., 2015 15 4550

Nephrops

Cod

Haddock

Species Original dataset No. of hauls No. of individuals

Nephrops Krag et al., 2014 22 12172

Cod Krag et al., 2014 22 4803

Haddock Krag et al., 2014 22 4793

Page 121: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

Fig. 1 illustrates the structure of the resulting populations (P1-P3 for each species

and the Multispecies scenario), as well as the 95% Efron (Efron, 1972)

Confidence Intervals obtained by the bootstrapping procedure.

Figure 1. Frequencies of the length classes represented in each single-species and

multispecies population scenario. Lengths are Carapace Length (mm) for Nephrops and

Total Length (cm) for cod and haddock.

References

Efron, B., 1982. The jackknife, the bootstrap and other resampling plans. SIAM

Monograph No. 38, CBSM-NSF.

Krag, L.A., Herrmann, B., Karlsen, J.D., 2014. Inferring fish escape behaviour in

trawls based on catch comparison data: model development and evaluation

based on data from Skagerrak, Denmark. PloS one, 9: e88819.

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Krag, L.A., Herrmann, B., Karlsen, J.D., Mieske, B., 2015. Species selectivity in

different sized topless trawl designs: Does size matter? Fisheries Research,

172: 243–249.

Krag, L.A., Herrmann, B., Feekings, J., Karlsen, J.D., 2016. Escape panels in

trawls – a consistent management tool? Aquatic Living Resources, 29: 306.

Melli, V., Karlsen, J.D., Feekings, J.P., Herrmann, B., Krag, L.A., 2018a.

FLEXSELECT: counter-herding device to reduce bycatch in crustacean

trawl fisheries. Canadian Journal of Fisheries and Aquatic Sciences, 75:

850–860.

Melli, V., Krag, L.A., Herrmann, B., Karlsen, J.D. 2018b. Investigating fish

behavioural responses to LED lights in trawls and potential applications for

bycatch reduction in the Nephrops-directed fishery. ICES Journal of Marine

Science, 75: 1682–1692.

Melli, V., Krag L.A., Herrmann, B., Karlsen J.D., 2019. Can active behaviour

stimulators improve fish separation from Nephrops (Nephrops norvegicus)

in a horizontally divided trawl codend? Fisheries Research.

https://doi.org/10.1016/j.fishres.2018.11.027

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SUPPLEMENTARY MATERIAL

Page 124: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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Page 125: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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39

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9.1

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32

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H1B

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8.2

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38

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54

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8.5

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8.8

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5.7

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0.5

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H1B

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3C

43

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3.3

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3.8

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35

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0.6

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37

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5.4

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4.0

(1.4

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5.4

(7.5

- 51

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32

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2.7

- 44

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35

.9 (8

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4.3

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H1B

0E

1C

4C

06

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1.0

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8.9

(12

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4.6

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.8)

5.4

(2.4

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20

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2.4

- 30

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1.2

(0.5

- 2.7

)5

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0.0

)1

6.8

(10

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3.0

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3.6

(4.5

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H1B

0E

1C

4C

10

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.0)

8.5

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0.1

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.8)

11

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7.9

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0.8

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2.0

)5

.5 (0

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6.9

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H1B

0E

1C

4C

24

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15

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9.6

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3.7

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6.7

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3.4

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11

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H1B

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34

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8.6

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0.6

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9.9

(5.7

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2.0

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7.8

(4.7

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23

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6.0

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H1B

1E

0C

03

9.5

(23

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9.9

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7.2

(67

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06

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0.5

(0.2

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3.2

(18

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3.8

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5.0

(67

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23

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1.5

(0.8

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1.3

(15

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2.2

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9.8

(62

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01

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16

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5.9

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H1B

1E

0C

15

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6.4

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8.8

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4.2

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5.1

(1.0

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51

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0.0

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0.4

(0.1

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4.9

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2.7

(22

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5.0

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8.5

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H1B

1E

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22

7.8

(17

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3.5

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8.8

(52

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3)

0.4

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2.5

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8.4

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7.0

(54

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1.3

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0.9

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6.2

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2.0

(48

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9.6

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4.8

(5.9

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H1B

1E

0C

32

7.8

(13

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1.1

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9.5

(27

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1.6

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.4)

28

.1 (8

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5.5

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5.7

(30

.0 - 6

3.7

)2

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.9 - 4

.9)

28

.1 (5

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5.6

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7.0

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Page 126: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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28

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70

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55

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23

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3.5

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0.5

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1.3

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5.2

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17

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1.5

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0.1

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4.8

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0.3

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6.5

(6

0.7

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6.6

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6.9

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25

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H1B

1E

1C

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33

8.1

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2.7

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6.3

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7.4

(5

9.8

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4.8

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0.3

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32

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17

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52

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84

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60

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11

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0.9

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30

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14

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50

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71

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56

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90

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18

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7.3

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7.0

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H1B

1E

1C

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43

4.8

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0.1

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2.3

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9.3

(5

3.1

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5.3

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0.3

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.0)

29

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16

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48

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75

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52

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96

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1.7

(0

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3.2

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7.6

(1

3.2

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4.2

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0.7

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8.2

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27

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H1B

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1C

1C

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5.2

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0.7

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8.6

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5.1

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0.1

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8.4

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19

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60

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37

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84

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0.6

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1.6

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3.1

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9.2

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1.6

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0.6

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2.5

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8.5

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H1B

1E

1C

1C

28

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4.1

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8.7

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4.9

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2.1

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8.6

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0.1

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7.1

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16

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56

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35

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79

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0.6

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1.6

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2.4

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6.6

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8.2

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8.0

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8.4

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2.2

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8.2

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H1B

1E

1C

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38

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3.8

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8.4

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8.8

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7.2

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1.7

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0.1

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7.7

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17

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50

.0 (

30

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71

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0.7

(0

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1.8

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2.4

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7.1

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3.2

(2

3.7

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3.9

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0.5

(2

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20

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H1B

1E

1C

1C

45

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1.2

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4.1

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0.7

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0.5

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3.6

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0.0

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4.5

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13

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40

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23

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57

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0.5

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1.7

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0.6

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3.0

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6.0

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7.8

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0.9

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0.3

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H1B

1E

1C

2C

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8.6

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4.6

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2.6

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5.7

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7.3

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0.2

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23

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13

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39

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80

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57

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10

5.5

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0.7

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22

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11

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37

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65

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51

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82

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14

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6.2

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H1B

1E

1C

2C

12

5.1

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5.5

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9.0

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2.8

(4

8.0

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6.2

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0.2

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20

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11

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34

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71

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50

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95

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1.3

(0

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2.5

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9.0

(9

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32

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56

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44

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73

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14

.8 (

5.8

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3.3

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H1B

1E

1C

2C

32

7.8

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7.3

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2.9

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2.8

(4

8.4

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5.9

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0.2

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23

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12

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37

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70

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49

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93

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1.4

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2.8

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1.7

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7.0

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6.4

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24

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4.6

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19

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10

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33

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60

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42

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79

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1.4

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8.5

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31

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49

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39

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64

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16

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6.4

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4.9

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H1B

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1C

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5.0

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5.9

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10

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46

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55

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76

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8.5

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45

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45

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33

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60

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24

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4.0

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H1B

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1C

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2.3

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7.8

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9.5

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8.3

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0.3

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25

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8.1

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7.0

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0.7

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5.8

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0.8

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25

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6.0

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1.8

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6.5

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6.4

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1.6

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6.5

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36

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H1B

1E

1C

3C

22

7.8

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4.3

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0.4

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5.6

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3.1

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7.8

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0.3

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27

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2.3

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5.5

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27

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3.8

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1.0

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7.1

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5.1

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34

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H1B

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1C

3C

42

4.4

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1.5

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6.9

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1.4

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1.5

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1.5

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0.4

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24

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24

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0.1

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40

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H1B

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1C

4C

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17

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11

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23

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19

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11

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28

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15

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H1B

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11

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12

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11

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Page 127: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

WP

-W

P+

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78

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96

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73

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8.1

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3.0

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6.7

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1.4

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2.9

(53

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3.8

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5.0

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H0B

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4.7

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2.3

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Page 128: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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53

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61

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43

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70

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18

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5.2

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6.7

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8.0

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0.4

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9.6

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12

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37

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30

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48

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2.7

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5.3

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H0B

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46

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38

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56

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0.5

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7.5

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1.2

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1.2

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5.9

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H0B

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41

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6.1

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8.2

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0.4

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1.2

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2.6

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3.3

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1.9

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9.0

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H0B

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1.8

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31

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25

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40

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35

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7.7

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30

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22

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37

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31

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48

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62

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45

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0.6

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3.0

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0.3

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8.4

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11

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32

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27

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42

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26

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19

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36

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17

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H0B

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40

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32

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0.5

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7.3

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4.7

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33

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19

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14

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24

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15

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10

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20

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2.6

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10

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H1B

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3.4

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0.0

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1.8

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61

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50

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73

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63

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51

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79

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9.5

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57

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43

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68

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62

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52

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77

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65

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52

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71

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H1B

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3.7

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55

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42

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71

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1.1

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0.7

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7.3

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6.4

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13

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7.8

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3.7

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13

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15

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41

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31

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55

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30

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15

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19

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13

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27

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47

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24

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3.9

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27

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20

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34

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51

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39

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65

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6.3

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H1B

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17

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35

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25

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48

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1.5

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21

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14

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28

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46

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37

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60

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48

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32

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58

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H1B

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1C

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31

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2.7

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3.8

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20

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14

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26

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29

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22

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38

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10

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19

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13

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25

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26

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19

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36

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59

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42

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70

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H1B

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1C

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41

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19

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15

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10

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20

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3.8

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7.5

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15

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10

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20

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65

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53

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74

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H1B

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1C

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3.6

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1.4

(4

8.1

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9.1

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50

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40

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61

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59

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47

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74

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8.4

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46

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34

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56

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57

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47

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72

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62

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48

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69

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H1B

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1C

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15

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54

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41

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70

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0.8

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1.5

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4.5

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21

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47

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36

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61

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41

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32

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55

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33

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19

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H1B

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2.1

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0.5

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9.4

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13

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25

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18

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34

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3.8

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- 1

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4.6

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5.8

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H1B

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13

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15

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41

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24

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H1B

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1C

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2.4

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8.1

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50

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40

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59

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48

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8.3

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11

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45

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33

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55

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57

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48

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61

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47

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H1B

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11

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55

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41

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71

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12

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2.9

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H1B

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25

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18

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8.4

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31

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48

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38

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52

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42

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45

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33

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51

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42

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64

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50

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36

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40

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31

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12

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7.8

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48

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64

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Page 129: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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Page 130: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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Page 131: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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Page 132: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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Page 133: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

WP

-W

P+

Dis

ca

rd R

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WP

-W

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ca

rd R

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WP

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Page 134: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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Page 135: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

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Page 136: Identifying simple and cost-effective gear solutions for an …€¦ · jomfruhummer, ved at tilføje stimulatorer designet til at aktivere fisks undvigelsesadfærd. Vi undersøgte

Technical University of Denmark

DTU AquaKemitorvetDK-2800 Kgs. Lyngby

www.aqua.dtu.dk