anastasija martjanova bachelor project report 2015

2015

Bachelor thesis

presented to the

Roskilde university

partial fulfilment of

the requirements for

the degree of Bachelor

of Science

Head of Department : Garry Banta

Department of Environmental, Social and Spatial Change

Roskilde University

Internal supervisor : Benny Hansen

Department of Environmental, Social and Spatial Change

Roskilde University

External supervisor : Mads Christoffersen

DTU Aqua National Institute of Aquatic Resources

Source : “Glass Eel” Chris Bowser NYSDEC

Habitat behaviour and substratum

selection by elver (Anguilla Anguilla) By Anastasija Martjanova

Student Number: 50713

Anastasija Martjanova

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ABSTRACT

For more than a century the life history of the European eel (Anguilla Anguilla) has been

studied. European eel was earlier thought as a catadromous species; however studies

conducted during the last couple of years proved that some part of the population never

reaches the freshwater, and stays in the coastal brackish water during all their growth phase.

This leads to a conclusion that eels are not truly catadromous species. The purpose for this

study is to investigate species sediment preference, burial performance and general behaviour

characteristics.

The present experiments are set up to investigate one aspect of this bury activity: elver

preference for specific coarseness of sediment. Laboratory experiments with 127 juvenile

European eels (70-130mm) were conducted at Den Blå Planet, National Aquarium of Denmark,

within 10 working days. The largest amount of eels was about 7 cm long and weighted 1.5-2.0

g. Highest rate of eels that were presented in the water column was observed after one hour of

the experiment start, whereas burial activity were observed continuously during all experimental

time. Conclusively, just 15% of juvenile eel preferred sand with vegetation and 12% preferred

small gravel. Whereas the major part of the eels (72%) preferred larger subtract such as

medium gravel and large gravel.

Such information will be of importance for field estimation of elver abundances in the

coastal areas, and a general evaluation of habitat applicability for juvenile eel. Sediment for

juvenile eels is not only a first line of defence and protection from currents, but also a place

where they spend much of their time. Decrease and damage of natural habitat of juvenile eels

in brackish water by anthropogenic factors likely is one of the main reasons of eels’ population

decline. The results of this study have implication for coastal zone management, demonstrating

the importance of habitat and subtract in the early stages of eel life ecology.


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PREFACE

This paper is a Bachelor project report written by Anastasija Martjanova from Roskilde

University (RUC).The project was conducted in cooperation with DTU Aqua National Institute of

Aquatic Resources, particularly with Peter Munk and Mads Christoffersen. It has been set up to

acquire more information on habitat preference and use by the elvers of Anguilla anguilla. This

project addresses the readers such as students and persons with a background in fish biology

and environment biology at least at Bachelor level. The paper concerns a research project on

the information on habitat preference and use by the elvers. All laboratory work has been

conducted at the Den Blå Planet, the National Aquarium of Denmark, which has been recently

opened in Kastrup. Laboratory work was conducted under the supervision of Mads

Christoffersen. The project is a result of ten weeks of experimental work, which includes

preparation for laboratory experiment, designing and establishment of the project materials and

equipment.

It includes analysis of suggested literature and books, meetings, ten days of laboratory

experiment, processing and handling the data analysis. The project is mainly focused on

answering on the question what type of sediment preference of Anguilla anguilla juveniles, and

followed by descriptive analyze of observation results on eel general behavior during the

experiment, such as: burial activity and presence in a water column. Whole project includes an

abstract of our work, introduction about eel ecology and life cycle, methods and material part

which describe the laboratory work. It is followed by results of our experiments which include

observation and notes about eel behavior and subtract preference; and our descriptive

discussion and analyze of our results and about overall about our project. The project ends with

conclusion and perspectives. Final part includes references and an appendix, where all raw data

and sheets are illustrated for better understanding.


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ACKNOWLEDGMENT

I would like to thank the Head of Studies of the Department of Natural Science of

Roskilde University, Morten Blomhøj for permission to write my Bachelor project alone and

outside of my home institution. It gave me opportunity to study habitat behaviour and subtract

preference of one of most mysterious and interesting species, the European eel. The laboratory

experiment was conducted at Den Blå Planet, National Aquarium of Denmark, which has been

recently opened in Kastrup. It was a great help from employees for taking care of the eels in

the aquarium, helping me feed the eels, and supplying me with all required equipment, as well

as designing the boxes and holding tanks, and taking care of regular technical maintaining staff.

A special thanks to DTU Aqua, National Institute of Aquatic Resources, and Peter Munk

in particular who made me wonder about eel life style and behaviour. Then, fortunately for me

I got introduced to my future supervisor - Mads Christoffersen. Through all the work, starting

with writing the plan for the laboratory work and building the boxes, till the last correction of

my reference list, Mads was much more than just a supervisor: mentor, tutor and most

important, person on who I could to rely as on myself. He supported me in most stressed

moments, always having a sensible advice or right explanation.

Also, I would like to mentioned participation of Peter Munk, who has joined us on our

finish line before submit this paper. Thanks to Peter’s review, helpful discussions and

comments, project found it logical structure, and will have a positive contribution to the

understanding of elver subtract preference.


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Table of Contents

Summary ............................................................................................................................... 5

1. Introduction ............................................................................................................... 10

1.1. Migration and life cycle ............................................................................................ 10

1.2. Conservation status ................................................................................................. 12

1.3. Habitat and behaviour ............................................................................................. 13

1.4. Problem description ................................................................................................ 14

2. Method ...................................................................................................................... 15

2.1. Specimen collection and husbandry .......................................................................... 15

2.2. Sediment preference experiments ............................................................................ 16

2.3. Experiment design .................................................................................................. 18

2.4. Observation strategy ............................................................................................... 19

2.5. Technique of summarizing sediment preference result .............................................. 20

3. Results ....................................................................................................................... 21

3.1. Morphological measurements................................................................................... 21

3.2. Behaviour observation of presence in water column and bury activity ........................ 22

3.3. Subtract preference of juvenile eels ......................................................................... 23

4. Discussion .................................................................................................................. 25

4.1. Behaviour observation of presence in water column and bury activity ........................ 25

4.2. Subtract preference of eel juveniles ......................................................................... 26

4.3. Sampling techniques ............................................................................................... 27

4.4. Implication on management .................................................................................... 28

4.5. Habitat destruction.................................................................................................. 28

4.6. Further studies ....................................................................................................... 29

5. Evaluation of the Project ............................................................................................. 30

6. Conclusion ................................................................................................................. 32

7. Glossary ..................................................................................................................... 33

8. Reference list ............................................................................................................. 34

9. Appendices ................................................................................................................ 39


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Summary

Even at our highly progressive times, and even on our so thoroughly studied Planet

there still is quite big part of living species which lifestyle and biological cycle remain

undiscovered. Due to that fact, as well as due to the rapid population decline, Anguila anguila

species received very high priority in the list of main conservation objectives. Eels as known for

their fascinating life cycle and value: economic and ecological prospective, is a good choice of

conservation study. The studies have a long research history on their distribution and general

ecology. However, even now, this species bring a lot of controversial questions, which bring us

to presented research of this particular species.

Introduction

Eels undergo impressive migrations, and larvae of European eel (Anguilla anguilla) drift

thousands of kilometers from the oceanic spawning area in the Sargasso Sea back to the

coastal areas. At their arrival from the sea, A. Anguillia glass eels (unpigmented juveniles)

typically remain temporally in estuarine areas while they undergo some physical and behavioral

transitions (8) (6). The common belief is that all juveniles subsequently migrate to freshwater

habitats; however recent studies using otoliths microchemistry have provided evidence that the

eel catadromy is facultative, hence a large proportion of eels complete their growth without

ever reaching fresh water (10) (13).

The estuarine habitats might be of much larger importance to eel populations than it is

assumed, and there is a need for improved understanding of the life conditions and habitat use

of the juvenile eels growing in the coastal, estuarine areas.

Little is however known about the life of elvers (the pigmented juvenile eel, following

stage of the glass eel growth) in the coastal areas, thus at DTU Aqua a project has been set up

to avail more information on habitat preference and use of it by the elvers. There are some

questions that would be of interest.

First of all, our main focus were on eel sediment preference and by this project we

would like to answer what type of subtract juvenile eels prefer the most. Next, during

experiment we would observe general eel behavior like swimming or staying still in a water

column without any bury activity. Moreover, bury activity will be noted with specific time frame

during all experiment length.


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Method

During the experiment period all eels were kept in a flow-through holding tank with

water temperature at 17°C and salinity level 14 PSU. All experiments were conducted indoors

with a constant light and air temperature in The Blue Planet. Whole experiment was conducted

in three stages. First, two weeks were used for designing the experiment and finding the

required equipment. For handling substrate and for taking it out of the experimental tank for

eel counting were made eight handmade boxes. This stage was followed by 10 working days of

the actual experiments with six trials when 20 randomly chosen elvers were released into the

two aquariums (10 in each). These experiments took part in duration of two hours; during each

trial of eel behaviour (such as burial behaviour, presence /swimming in water column) in

substrate was observed and noted. Final weeks were used for calculation of the results,

planning and discussion of experiment method, cleaning and maintaining. Each of the six trials

was done by similar method: group of 20 elvers was collected randomly from the large holding

tank, and then the first 10 individuals were released into the Tank 1. Then, after 30 minutes of

acclimatization, first observation was made, as well as the counting of elvers that present in

water column or bury into the sediment. The next 10 elvers were again randomly released, but

now into the Tank 2.

At the end of the experiment each box with juveniles and sediment was taken outside

the experiment tank and placed into a plastic bucket in order to count eels which preferred this

habitat. At the end, after all calculations and observations, all eels were fed. To avoid any

behaviour acquired eels were used only once in the experiment.

Results

In total, 127 eels were measured to estimate their weight and length. Eels’ sizes were

between 7 cm to 14 cm with an average of 10.9 cm. Most abundant group of individuals - 69

eels – was eels from 11cm to 13cm. Weight of observed individuals varied from 0.3 g to 3.13 g,

with an average of 1.6 g. Most populated group - with 55 individuals – was between 1.5 and 2

g.

Results at the end of all experiments showed that the majority of the juvenile eels

preferred to stay totally or almost covered in the habitat; only 4 juvenile eels remained in the

water column. More than 10%, which is 14 individuals, performed bury activity. We noted that

majority of those individuals, at the end of experiment days preferred to burying in medium

gravel (GM) and large gravel (GL), with 5 eels in each, respectively. We observed that there are

two groups (gravel large + gravel medium) and (gravel small + sand and vegetation) which


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were significantly different from each other with 72% supremacy of the first one. However,

future studies needed to include higher amount of eel juveniles in order to prove and show

higher significance of substrate preference.

Discussion

A range of experiments has been carried out on behavioral aspects of the glass eels. For

example, Anguilla japonica clearly showed a nocturnal activity under laboratory conditions.

Light–dark cycle was a determinant factor affecting their photonegative behavior; they show

nocturnal locomotive activity, and feeding behavior (19). Elvers are known to hide under stones

or bury in sediment, but little precise information is available on this important aspects of the

juvenile ecology.

The present experiments are set up to investigate one aspect of this burying activity:

elver preference for specific coarseness of sediment. Our expectations were based on different

research papers and projects which noticed that eels preferred habitat were they could hide

from predators and bury while they rest. Our results prove our expectation what eels would

preferred medium and large coarse subtract due to fact that this type of substrate ideally suited

for elvers living behavior and life condition.

It is known that eel fishing is a big part of fishing industry. Unfortunately, ecologists and

fishermen's have to face the fact that overall number of eel is continuing to decline all over the

world. It is caused by various human harsh influences, from overfishing to habitat loss. Our

project implications are concentrated on the latest factor such as habitat loss.

Habitat destruction as a factor has various causes. Most common in the Baltic Sea are

marine aggregate extractions with negative influence on sand eel, when another report proved

that suctions hopper dredging and trailer dredging have also unfavourable conditions for flora

and fauna in the areas.

There is information which concludes that seemingly habitat loss and destruction is the

fundamental reason of eel population decline. Mortality level of eels caused by habitat loss

needs to be taken into account and compared to a number of damaged eels due to fishing and

hydropower turbines.

Report from FISRWG also showed that eels prefer pools with deep waters and large

substrate, making them highly vulnerable to human activities which lead to habitat degradation.

Human activities such as logging and land development make eel juveniles highly vulnerable to

land degradation, which lead to continuous habitat loss and eel disappearance. It can be


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assumed that excavation and habitat destruction observed in recent years has negative effect

on ecology of eel juvenile and further economic profit from fishing on silver eel.

Perspective discussion

For a better understanding of the life cycle and general eel ecology would be interesting

to investigate the eel sediment preference not only in laboratory but also conduct a field

experiment. From the literature, it seems very likely that the fine coarse substrate is a perfect

habitat which helps young eels be less vulnerable and increase their chances for surviving. At

the moment we could state that, as far as we know, it is only experimental study that

contribute to the understanding the habitat choice of eel juveniles in brackish salinity water.

However, there is different type of disturbance and environment effects on eel juvenile subtract

choice and overall eel behaviour , which has not all been included and checked yet.

Conclusion

Our report shows the laboratory experiment on 127 eel juveniles which were used in our

practical analysis. Eels showed strong preference of medium and large gravel subtract, whereas

small gravel and vegetation were ignored by most of the individuals. However studies cannot

determine whether it is caused by effect of disturbance during laboratory experiment or

because of short experiment length. It is clear that further research on the field is needed to

elucidate the mechanisms of how the juvenile eels make their subtract preference, but also

investigate what factors and how effect their general behaviour in order to improve their life

conditions and habitat use of the juvenile eels growing in the coastal, estuarine areas.

Outline on the Bachelor Project Structure and Study

This project topic and problem give excellent opportunity to study both natural science

disciplines: environment ecology and fish biology. In addition it provides indispensable practical

experience based on learnt theory material. Practice and handling of individuals during

laboratory experiments, showed how substantial the knowledge of basics of species biology is,

and how strongly it affects not only on their behaviour, but also distribution and habitat

preference.

It was very advantageous to make a project with people who have been investigated

biology and population dynamics of this particular species for a long time that allows learning

how to understand general eel biology. It also is able to help uncovering the mystery of eel life

style, which occurred due to the lack of the knowledge about the species life cycle. Comments


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and review the project work and laboratory experiment also give opportunity to understand

how method of handling trials and experiment procedure could be improved in future in order

to get more statistically significant numbers.

There was no conflict of interests in this paper, however overall assigned project provide

many difficulties, starting with design of the equipment, high number of samples (129) to the

limited time frame and contradictive theory background. It was troublesome to contract an

appropriate tools and equipment which convince us that there are appropriate to carry grave

substrate with different coarseness, and also allowed water to pass through. Additionally, it is

very important to carry experimental design which will be appropriate for such tiny and fragile

species, who are vulnerable to many different environmental aspects such as salinity, water

quality, food availability and other. During the experiment, significant part of handling and

releasing species were concentrated on accurateness and carefulness.

After finishing our experiment trials we started our measurements on species weight

and length. However, we were surprised to find only 127 individuals; it means that we were

short by two individuals. However in the discussion part we provide a reasonable explanation

for such an unpleasant situation, which might reveal the cause that lead to that. Another

important note is that due to restricted time frame we received assistance with conducting the

statistical analysis the aid was provided by one of the DTU Aqua Research Centre member.

Statistical analysis supports our main hypothesis of the project, however it also showed that

further studies with higher number of samples are required.


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1. Introduction

European eel life history is complex and atypical among aquatic species.

However in the early 20th century, due to early marine expeditions of the Danish biologist

Johannes Schmidt was discovered major breeding location of the European eel (Anguilla

Anguilla). (1) This means that the eel has to make a spawning migrating close to 5000 km, in

the Sargasso Sea. (2)

1.1. Migration and life cycle

Despite that the life cycle of the eek is still highly unknown, there is no doubt that the

reproduction is taking place somewhere in the Atlantic Ocean, where the smallest larvae have

been found. Figure 1 shows the end product of long term research conducted by Johannes

Schmidt, which proved that eels spawned in the sea and were originally marine fishes. Schmidt

found that both the European and the American eel (Anguilla rostrata) spawned in the Sargasso

Sea. On the map the length in millimetres at each stage of their journey is shown. The edge of

the dotted zone marks the locations, at which leptocephalae metamorphose into elvers. (2)

Munk et al 2010 found that European eels follow eastward route back to Europe, helped by the

Subtropical Counter Current, and that this is important in the life-cycle completion of European

eels. (3)

Despite continuous research of the eels’ distribution, there have been

misunderstandings and lack of knowledge about different parts of the European eel life cycle.

Harden Jones (1968) has been expressing his outline about this issue in his book: ‘‘No adult

eels have ever been caught in the open Atlantic nor eggs definitely identified in the wild.

Figure 1 - Distribution patterns of eel larvae with the size of the larvae in mm

(source: Schmidt 1923)


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Migration routes and spawning conditions for adults are unknown or conjectural, as are many

details of the development, feeding and growth, migration or drift are uncertain’’. (5)

European Eels are famous for their impressive migration during their life cycle, which is

the longest and most oceanographically complex among of all Anguilla species. (5) Figure 2

illustrates all life cycles stages with specific regional biological terminology. There are a number

of phases in an eel’s life that have terminology with specific morphology (Figure 2).

As amphihaline fish species, leaf like leptocephalus (marine larvae) start their migration

to the continental shelf with drifting thousands of kilometers from the oceanic spawning area by

ocean currents. (6)

During migration they believed to feed on ‘marine snow’ – organic matter. (7) By

arriving in coastal waters, the leaf-like larvae undergo metamorphosis, becoming glass eels. By

using passive tidal transport and active migration, they enter estuarine (8). The most significant

factors which influence and defines the transition area is the combination of salinity and water

temperature. (8) Eel-shaped, transparent, colourless juveniles in freshwater or saline waters

growth and become pigmented, reaching the elver stage and then yellow eels (9). Increased

size, yellowish bicolour dorsum, and lighter ventral are main morphological difference between

yellow eel and elver. Yellow eel stage lasts for 5-20 years, when they grow into silver eels

Figure 2 - Life Cycle of European Eel (source Rob Slaupkaukas proposal FOR THE INCLUSION OF THE European EEL (Anguilla anguilla) On CMS Appendix II)


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before embarking on their spawning migration. During this period they undergo significant

enlarging of their eyes, eels lose their ability to feed, the dorsal side turns black, as do the

pectoral fins; the ventral side, in contrast, turns white with silvery reflections. At full sexual

maturation, their snake-like elongated body would have dorsal, pectoral, anal and caudal with

absent ventral fin. (4) At the end they migrate, through the ocean to the Sargasso Sea, where

they spawn and die. (10)

1.2. Conservation status

The European eel is an important resource in conservation, ecological and socio-

economic terms. There is a strong positive affect between habitat health and fisheries because

most of eel catching areas are based at the vegetated coastal habitats. According to “Facts

about Swedish fisheries", catchment of eel approximately 50% of the fishermen profit in area of

Skagerrak and Kattegat, making this habitat and species highly significant not only as an

important biological species, but also economically, providing places for employment and source

of income. (11) (12)

As mentioned in ICES (16) report, there are various factors which have influence on the

eel life cycle ecology and distribution of the eel. Studies conclude that not only European eel,

but also Japanese (Anguilla japonica) and North American eel the most important

environmental aspects that influence glass eel migration is temperature of water, time of the

day, lunar phase, water discharge, tidal cycle, water conductivity, salinity and water

(6)(16)(18)(16). In addition, it is also important to consider that all mentioned factors differ

according to location, estuary characteristics and physiological status of the eels. (15) (16)

Figure 3 shows the time trends for European eel among others. At first glance there is clear

evidence that since the 1980s, the abundance of the European eel has declined throughout its

distribution range. Hypothesis of what have led to it will be discussed below. (17)

Recently European water masses experienced negative effect of the invasive nematode

species Anguilla Crassus. Firstly it was discovered in Germany in the early eighties, however at

the moment it has reached enormous abundance and present almost in each eel stock in

Europe. Variety of studies which were focused on biological effect of nematode on host and

colonized species conclude that European eel is much more vulnerable to these parasites, and it

might be one of the main reason of continues population decline. (15)


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According to the report (16), the major causes for the decline are considered to be

habitat destruction and obstruction of migration routes by dams and other chemical or physical

obstacles. Also factors such as exploitation of glass eel in estuaries, increased predation,

barriers to upstream migration, exploitation of yellow and silver eel, impediments to

downstream migration and habitat loss needed to be taken under consideration in order to

restore the depleted stocks, included all biological stages. (15) (16) (17)

1.3. Habitat and behaviour

Distribution of European eel indicates that it can live in a wide range of temperatures

and it is extremely tolerant to low oxygen environments and poor water quality generally. (20)

(29)

As it is mentioned above, habitat loss and degradation played a major role in the decline

of the eels in the second half of the 20th century. Vegetated costal habitats on rocky bottoms

provide shelter from predation, protect against currents and rich food conditions for European

eel, both adults and juveniles. Reduction of predation risk may be especially important to eels,

because they have lower burst swimming speed compared to other fishes with fusiform

shape.(20)

From Tesch’s book we know that Yellow-stage anguillia eels generally forage

nocturnally, but spend the day in the substrate. It might both burrow soft sediments such as

sand, or burrows excavated in spaces among rocks and gravel. (6) (19)

Figure 3 Time trends in juvenile abundance of the major eel stocks of the world. The average trend of the four longest data series is shown for the European eel. (From Dekker 2004)


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1.4. Problem description

Sediment grain size is one of the most important environmental variables determining

the distribution of other fish, e.g. flatfish (24), however there is little known about substrate

choice and eels. Loss and destruction of important habitats can have a negative effect on the

eel population, which can lead to extinction of the European eel. Studies have provided

evidence, that up to 80 % of the eel population complete their growth phase by staying in salt

or brackish water along the coast, without ever reaching fresh water. (10) (13) (18) (22) (36)

In additional investigation of feeding behaviour of glass eel by using stable isotopes in

estuarine proved the fact that such intermediate zones are important feeding habitat and not

just a place for immigration. Moreover studies conducted in French river showed preference for

brackish water. (20)

This element of population dynamics is particularly poorly understood; therefore further studies

are needed to provide a better understanding of the life condition during coastal residence time,

habitat selection of juvenile eels in this important life stage. (20)

In order to fill this gap, the project was set up by DTU Aqua for availing more

information on habitat preference and use by the elvers. The present experiments are set up to

investigate one aspect of their habitat ecology: eel juvenile preference for specific coarseness of

sediment. The goal of this study was to provide new information for the protection of eel

habitat and migration corridors, as well as development of restoration plans for eels.

To address these needs we developed specific objectives:

1. What kind of sediment do juvenile eels prefer?

2. Do juvenile eels always bury in the sediment?

3. How much time it takes for eels to choose the substrate, and how do they interact with

the water column?


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2. Method

We conducted a research on marine species, which is a member of fish order that

consists of 19 families and around 400 species, who have many common features and are

distributed from temporal to tropic aquatic environment.

There are 15 species within the genus Anguilla, most of which originate from tropical

areas in the Pacific. Our research is primarily focused on one of the most exploited temperate

species: European eel - Anguilla anguilla. There is a variety of different studies on ecology and

behaviour of other species such as American eel - A. rostrata, Japanese eel - A. japonica,

Australian and New Zealand shortfin eel - A. australis, and longfin eel - A. dieffenbachia.

Particularly European eel Anguilla anguilla have long lifespan. Its life cycle involves several

morphological shifts, which end with spawning migration of mature Silver eel back to the

Sargasso Sea. The life cycle has still not been completely revealed. European eel tend to appear

in all aquatic environment and being caught as a food source in many different parts of the

World, and particularly along the coastline of almost all of the Europe. They have been noted as

highly adaptive and environment resistant for different abiotic and biotic factors such as

temperature, salinity and many others. Also, need to take into account their environment

importance and what role do they play into whole aquatic ecosystem, being a vital food source

of food source for many other aquatic species. (6)(17)(29)

2.1. Specimen collection and husbandry

During the experiment period all eels were kept in a flow-through holding tank with

differently-sized plastic tubes that could be used as hiding places, until they were needed for

experiments. (Figures 4 and 5) They were fed with dry food once every second day, starting

three weeks before the conduction experiments. Approximately 70 eels came from a small

Danish stream from the northern part of Zealand, called Hellebækken, and additional 200 were

bought from Jupiter eel, a Danish eel hatchery. The juvenile eels were originally from France.

Water temperature was kept constantly at 17°C and salinity level was approximately 14 PSU to

simulate salinity level found in a typical Danish fjord system. (23) All experiments were

conducted indoors with a constant light and air temperature in The Blue Planet.


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2.2. Sediment preference experiments

In order to observe eel subtract preference, experiments were conducted in two

identical holding tanks, the tanks were 75 cm long, 15 cm wide and 40 cm high (total volume

45 litres), specially constructed for this experiment. Each tank was divided into four equal

quadrants and separated by 15 cm high Plexiglas walls in order to avoid mixing of habitat. The

establishment of boxes with different substrates can be seen on the Figure 6. Quadrants were

proposed for fixed habitat along the bottom, each containing one of the following substrata:

sand with artificial vegetation, small gravel (GS 0-8 mm), medium gravel (GM 12-32 mm) and

large gravel size (GL 32-64 mm). Substratum size composition is presented in Table 1.

Figure 5 - Differently sized tubes used in experiment

Figure 5 - Flow –through holding tank

Figure 6 - Establishing boxes with different substrates


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The types of substrate were chosen from earlier observations and experiments (Pers.

Comm. Christoffersen, M), the set-up of the distribution of different substrate types can be seen

on the Figure 7. Sand was collected from Amager Strand, gravels were collected from a mineral

resources company and vegetation was simulated with bundles of plastic ribbon-shaped to

imitate shoots of macro algae or eelgrass.

Table 1 - Sediment size (mm) used in the experiment

Sediment type Sediment size(mm)

Sand and Vegetation(SV) Half area of sand and half area of

artificial eelgrass

Small Gravel (GS) 0 - 8 mm

Medium Gravel (GM) 12 - 32 mm

Large Gravel (GL) 32 - 64 mm

Figure 7 - Distribution of different habitats into their permanent place in both tanks


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During the experiments artificial vegetation were above sand. To improve techniques of

handling substrate and for taking it out of the experimental tank to count the eel after each

experiment, eight handmade boxes were made from two type of nets, one with small fraction

spaces for sand with vegetation and small gravel, and another for medium and large gravel, the

boxes can be seen on the Figure 9. During experiment both tanks were held in one big plastic

box which was provided by The Blue Planet (Figure 8). Aquariums were 30 cm deep . There

was also water in the plastic box (25 cm) cm to reduce water pressure on aquarium walls.

2.3. Experiment design

The plan for the experiment consisted of different parts: three weeks of experiments,

including six trials of releasing 20 randomly chosen elvers into the two aquariums (10 in each);

observations of eel behaviour in substrate, week of preparation for the experiment (getting

elvers from South of Zealand, changing water salinity and cleaning aquariums, making boxes

for different substrates), and week of calculation of the results, planning and discussion

experiment method, cleaning and maintaining. Trial days were held scheduled and followed by

feeding of elvers.

Each eel was released from 10 cm above the aquarium into random habitat with its

head facing towards the tank. While releasing the eels we tried to randomly distribute them

approximately equally to exclude bias on habitat selection. Some of the eels chose to rest on

the sides of the boxes in the water column. After the end of the experiment they were also

included in the analysis (all were in the small gravel box).

To let the eels acclimate from handling it were decided to include first 30 minutes for

the juvenile eels before starting of the experiment. The example of scheduled work sheet which

were used at each experiment day could be observed in Appendix (Table 2).

Figure 9 - Handmade boxes which we tried for our experiment

Figure 9 - Yellow plastic box for both

experiment tanks


Page 19 of 52

2.4. Observation strategy

Each of the six trials was done using the similar technique. Before beginning of the

experiment, salinity and water temperature were measured. Then a group of 20 elvers was

collected randomly from the large holding tank, and then the first 10

individuals were released into the Tank 1. Then, after 30 minutes of

acclimatization, first observation was made and counting of elvers

that present in water column or bury into the sediment. The next l 10

elvers were again randomly released, but now into the Tank 2 (Figure

10). Next step included observation and counting elvers and their

behaviour features such as presence in a specific water column,

change of substrate during the experiment or bury into sediment. It

was done every 30 minutes during the next two hours. After each

short observation, plastic box were covered in order to increase their

activity, knowing their nocturnal lifestyle. (6) (19)

The term “bury activity” (burial activity) means that the

individual is partly submerged in the substrate while partly present in the water column. To be

present in the water column means that 100% of the body is outside of any substrate type and

the individual is freely swimming between different habitats. While being in habitat means that

the body of the individual is completely submerged in the substrate.

Figure 10 - Releasing

the eels into random

habitat in each tank

with 30 minutes interval in between

Image 1 – Behaviour Explanation


Page 20 of 52

2.5. Technique of summarizing sediment preference result

After the experiments, the quadrants of Tank 1 were separated with additional Plexiglas

walls to eliminate the probability that juveniles could change their substrate due to disturbance.

To calculate how many elvers choose particular habitat, each box with juveniles and sediment

was taken outside the experiment tank and placed into a plastic bucket. Then each eel was

counted and removed from the box with substratum. (Figure 10) It was made as fast as

possible in order reduce the stress of elvers and to fit into the time frame of 30 minutes to start

with tank 2. At the end, when all calculations and observations were done, all eels were

released into another holding tank, which was separated from the main holding tank, and were

fed right after experiment. Each eel was used only once in the experiment in order to avoid any

behaviour acquired.

Experiments were conducted to gain knowledge of elvers habitat preference and to

observe their behaviour in the sediment during the experiment.

Figure 11 - The method used for

collecting and counting eels after the

experiment


Page 21 of 52

3. Results

3.1. Morphological measurements

Size and weight of each individual were measured in millimetres and grams respectively,

the results of experiments are presented in Table 3 and Table 14 in the appendix. In total, 127

eels were measured. Eels’ sizes vary from 7 cm to 14 cm with an average of 10.9 cm. Table 3

and Figure 12 show that the most abundant group of individuals - with 69 eels - is with length

slightly above average, length varied from 11cm to 13cm. Followed by 44 juvenile eels which

compose the group of individuals between 9cm and 11cm. Least populated group contains eels

who are much smaller than average, lower than 9 cm with the number of 8 individuals, as well

as the adults who grew up to 13 cm and longer, with only 6 individuals.

Weight of observed individuals varies from 0.3 g to 3.13 g, with an average of 1.6 g.

From Table 4, Table 14 from the Appendix and Figure 13 you can distinguish that most

abundant group is within range of weight from 1.5 g o 2 g with 55 individuals, followed by

group of juvenile eels weighting from 1 g to 1.5 g. Both groups are close to the average value

and together compose 75% from all measured individuals. Next group consists of eels that had

a body length within the range from 2 g to 3 g. Last group of eels were who had highest

weight, 3 g and more, with just 4 eels.

0

10

20

30

40

50

60

70

80

l<9 9<l<11 11<l<13 13<l

N o

f e

els

Lenght fraction

Figure 12 - Results of measurements of eel lenght


Page 22 of 52

3.2. Behaviour observation of presence in water column and bury

activity

A total of 129 European eels were used in experiments. Results at the end of all

experiments showed that the majority of the juvenile eels preferred to stay totally or almost

covered in the habitat; (Table 13 in Appendix) with only 4 juvenile eels who remained in the

water column, therefore we will not include their subtract type preference due to insignificance.

(Table 5) (Table 13) However, the observations at different time showed that at the start of

experiment there were 7 eel in total that stayed in water column.

After, it followed an increase and reached its peak with 11 eels at 30 minutes after the

start of the experiment (Figure 14) (Table 6 and Table 7). After that, at 1 hour after eels where

released, we observed the same number off eels in the water column, as it was at the start of

0

10

20

30

40

50

60

w<1 1<w<1.5 1.5<w<2 2<w<3 3<w

N o

f e

els

Weight fraction

Series1

Figure 13 - Results of measurements of eel weight

0

2

4

6

8

10

12

Start 30 min Hour Hour and 30

min

Two Hours

N o

f e

els

Time

Sum of N eels in water

column

Figure 14 - Summarized number of eels which presented in different time frame their presence into water column and do not entering any chosen habitat


Page 23 of 52

experiment. The number continued decreasing to just 4 juvenile eels at the end of experiment.

The numbers were summarized after conduction of all experiments.

From the total number of individual, more than 10%, which is 14 individuals, showed

bury activity (Table 8) (Table 13). Those individuals showed that among all sediments the most

popular was medium gravel (GM) and large gravel (GL), with 5 eels in each at the end of

experiment days. The observations at different time showed that at the start of experiment

there were 7 eels that were present partly in water column and partly in sediment. It followed

with a slight increase and sharply reached its peak with 17 eels in an hour after the start of

experiment (Figure 15) (Table 9) (Table 10). Then it decreased to just 11 buried eels, and at

the end of experiment we observed intermediate number with 14 buried eels in a different

subtracts. All numbers were summarized after all the experiments.

3.3. Subtract preference of juvenile eels

Table 11 presents data from experiment including trial dates, and numbers of eels

presented in different habitat by the end of the experiment. Summarized data, which is a part

of the bootstrapped confidence intervals (to include the extra uncertainty); it is shown in Figure

16. Counts were normalized and plotted into bar charts function (GM+GL/GS+SV). Distribution

can be visually compared in the Figure 16. We observe that there are two groups (GL + GM)

and (GS+SV) and those two groups are significantly different from each other with GL+GM

dominance, with 72% from total eel juvenile number (Table 12). Tendency of GM is slightly

higher than GL; however more data is needed to conclude that statement.

0

2

4

6

8

10

12

14

16

18

20

Start After 30 min Hour 90 min Two hours

N o

f e

els

Time

N of eels bury sediment

Figure 15 - Summarized number of eels which presented in different time frame

their bury behaviour (means partly presented in substrate and in the water

column)


Page 24 of 52

Table 12 Results of eel habitat Preference

Habitat Significance Number of eels observed in habitat

GL 0.302326 39 (30%)

GM 0.426357 55 (42%)

GS 0.124031 16 (12 %)

SV 0.147287 19 (15%)

Total: 129 (100%)

Figure 16 - Comparing observed and theoretical proportions (P- proportions in each habitat with 95% ci) Normalized counts (GM+GL/GS+SV)


Page 25 of 52

4. Discussion

4.1. Behaviour observation of presence in water column and bury

activity

Burial activity is one of the most important aspects of eel lifestyle. Despite the fact, that

in our experiment most of eels decided to stay in substrate, 14 eels at the end of trials were

partly present outside of preferred habitat. (Table 8) Most of them give preference to medium

and large size gravel, both with 5 eels respectively. Suitable size of holes between gravel might

be a reasonable explanation why eels choose especially this type of habitat.

It is reasonable to assume that we did not observe any of burying eel in small gravel

because of subtract tight composition, which did not allow eels to bury in it. Explanation why

there is such as small number of eels burying eels in sand and vegetation habitat can happen

due to the fact that artificial vegetation was spread among all boxes with subtract, therefore

when we tried to observe them and count they, hided or tried to escape and bury completely.

Also artificial vegetation might be not as good and qualitative as natural, which also might be

an explanation. (20)

If we take a look on time frame (Figure 15) we observe that highest burying activity was

shown after one hour from the start of the experiment. That might be due to their

acclimatization and time for exploration for suitable habitat. After this peak, the number of eels

who were actively burying in subtract decreased continuously supposedly because of the fact

that eels have made their choice of habitat and just bury into it completely.

During the experiments every 30 minutes eels were checked for their presence in the

water column. Results showed insignificant number of eels at the end of experiment, probably

due to fact that there had not been enough space to swim between habitats.

Another explanation might be because of too short time of experiment and too much

disturbance during opening and closing of the aquarium. The time observation diagram 2

showed that the highest number off eels that were present in the water column after 30

minutes of the experiment (Figure 14). Reasonably to presume this is the time for them to

decide, so they swam around and tried to find most preferable habitat. Experiment proves fact

that elvers like to hide into subtract or at least bury in it partly, therefore a very small number,

only 4 individuals, were present in a water column during observation and at the end of the all

experiments.(Table 5)


Page 26 of 52

4.2. Subtract preference of eel juveniles

Numerous studies on flatfish proved that there is a direct link between burial capability

and fish defence mechanism. Sediment might increase the survivorship of vulnerable flatfish

juveniles. In addition, good habitat choice is advantage to have higher prey density. Therefore,

in nature those two mechanisms are taken first into the consideration when it comes for choice

the habitat. (24)

In contrast with halibut and flounder, no studies of substratum preference of eel

juveniles were conducted. Some studies used field observations for adult eels, but their habitats

and life is very poorly understood.

Nevertheless, at the end of each experiment we had a chance to see the results of our

main project objective - what substrate young elvers favoured the most. We could clearly see

that medium gravel and large gravel highly outperform small gravel and sand vegetation, with

94 and 35 elvers respectively. (Table 12) Our results fully match our expectations which were

based on various researches on eel subtract preference all over the world. According to

research of New Zealand short-finned eel habitat preference, juvenile eels were mainly found

on a gravel and/or mud substrate, but larger eels (>300 mm) preferred sandy substrates.(25)

Another study was conducted on Anguilla japonica eels by Tsukamomo in 2003 which

results also support the idea of preferred tube-design habitat to sand, and can be explained

that in tubes it is easier to bury than in sand. (19)

In addition to this, in the book which studied habitat off eels and two other species, it is

mentioned that eels mostly preferred coarse and gravel substrata among others. (26)

Moreover, according to the results of research on Anguilla Reinhardtii adult eels were

most abundant in rocks, fine gravel, and sand; whereas juveniles and sub-adults were occurring

in cobbles and gravel and rocky subtract. Another eel species - short-finned eel (Anguilla

australis) mostly preferred coarse and fine gravel and sand. (27)

Despite the positive results, those researchers made field observations of influence of

biotic and abiotic factors. Absence of biological factors such as food, predation and conspecifics,

in the laboratory experiments, the habitat choice of eels may be more strongly determined by

the physical attributes of available habitats. Therefore if observation would be conducted at the

field, data may vary.

We believe that habitat choice was based on different aspects of eel behaviour. As

claimed by different researches the main tactic for eel is to find safe place where it is able stay

and hide from disturbances and predators. (6)


Page 27 of 52

Good substrate, with fine coarse holes is a perfect habitat which helps young eels be

less vulnerable and increase their chances for surviving. That also might explain why we

observed that juvenile eels had a lower preference for sediment substrata with grain sizes. It is

because they simply could not bury in it (GS). However, in our results there are some

individuals who were found in this habitat, and it might confuse the reader. Those eels were

found between the box with substrate and plastic quadrant. Especially small eels showed this

behaviour due to their tiny size, where they were probably hiding from larger elvers.

Least chosen habitat was sand and vegetation. There are possible few reasonable

explanations why this type of substrate was less popular compared to medium and large gravel.

In our experiment we used artificial vegetation, which probably might have affected choice of

habitat. Also, as it is claimed in mentioned studies, smaller eels prefer coarse, rough stones and

gravel, while larger eel prefer mud and fine sand. Larger eels seem stronger and powerful,

while elvers need a hard substrate such as gravel to feed and spend time at their migration

corridors .Above all, we suggest that the ability to bury and be fully covered by substrate was a

primary (leading) factor influencing substratum preference.

4.3. Sampling techniques

Various factors had affected our results on observation of eel presence in water column,

burring behaviour, and habitat choice.

Firstly, it is fact that it was an indoor experiment in a controlled environment. Predation,

prey availability, noise and other disturbances might effect on eel presence in water, burring

activity or preferred habitat in natural environment. (6) (8)

Secondly, our observation time might be too short for eels to find most suitable habitat

for living. Each 30 minutes we had to open the large aquarium tank and disturb them with light,

and noise. We know that they are nocturnal species, meaning they are more active during the

night; however our experiment were carried during the day, which also might have affected on

their performance. Although, during experiment we carry them in a closed tank, and tried to be

accurate and quiet while making time observations and notes. (6) (8) (19)

Even though, at the end of experiment, when we checked our first holding tank we had

taken all boxes with substrate out to count eels who preferred to stay in this habitat. During

this we unintentionally disturbed other eels in the second holding tank because both of them

were in the same aquarium. Therefore, experiment techniques could be improved if we had two

separate tanks where we could place holding glass tanks.


Page 28 of 52

Subtract were collected accurately and no eel suffered any injury. However artificial

vegetation, which was handmade cut and designed, can be too sharp or dark, meaning the eels

just did not like it. Finally, when experiments were finished we placed all eels in one holding

tank and results of observation showed that 129 eels were sampled, however when after the

experiment we measured them we counted only 127, which means two small eels were eaten

by larger sized eels. (6) (28) (29)

4.4. Implication on management

A significant part of population is economically dependent on eel fisheries. Only in

Europe about 25000 tons are consumed annually. However, since early eighties eels faced a

dramatic decline not only the European eel population, but also all eel species around the

world. (Figure 3) (30)

Due to the strong human influence, such as: overfishing, water pollution, degradation

and habitat loss through engineering works, hydro-electric power stations there is a negative

effect on the whole aquatic system, as well as there is species decline. (15)

Spawning migration is crucial driving mechanism in eel life cycle, but unfortunately due

to wide range of threats there is no longer an easy access between inland waters and the

ocean. Anoxic condition in brackish water, tidal currents, physical barriers like dams, port

activity such as dredging and extraction of substrate could be some of possible reasons of eel

natural occupancy destruction. Of course, there are other biological factors such as predation

and diseases caused by viruses and invasive species, they have a great impact on population

decline, however we do not discuss them in this report. (15) (31)

4.5. Habitat destruction

Marine aggregate extraction operation can potentially have a negative influence on sand

eel, making them vulnerable. It is due to the fact that sand eels possess loyal attention to their

habitat therefore loss of resources targeted sands and gravels could be one of key factor

affecting their survivability. (32)

There was one research which activity was conducted on 3 exploration areas and 3

extraction areas in Øresund. Results of mapping showed that sand, gravel and course to fine

Holocene sand were major type of coastal substrate. The environmental effects of extraction of

raw materials showed direct effects on flora and fauna caused by suctions hopper dredging and

trailer dredging in the areas. (33)

Another one, The INDICANG1 project, highlighted that most preferred habitats for eels

were vastly ruined. (34)


Page 29 of 52

According to OSPAR overview, it probably is the main reason of recent eel population

decline and switching the sex ratio which negatively affects the population growth rate. They

suggest that mortality which is caused by habitat destruction has the same level, as mortality

level caused by fishing or turbines. (34)

Report from FISRWG also showed that eels prefer pools with deep waters and large

substrate, making them highly vulnerable to human activities which lead to habitat degradation

such as logging and land development. It is strongly recommended to avoid that activation to

protect stream environment for eel natural habitat. (35)

Thus, in light of the results of this study, it can be assumed that excavation and habitat

destruction observed in recent years has probably reduced the quantity and quality of suitable

habitats for juvenile eel, which has negative effect on both ecology and economy.

4.6. Further studies

Various observations of European eel life history can describe eels not as completely

catadromous, but "facultative or semi-catadromous". Due to the fact that the European eels

were found in different water types, such as freshwater, brackish, and coastal area. Therefore,

the fact that freshwater phase is not an essential part of their life history, makes them not

absolute catadromous species. (10) (13) (18) (22) (36)

This is the only experimental study until now, as far as we know, that contribute to the

understanding the habitat choice of eel juveniles in brackish salinity water. Therefore further

studies should be taken in order to contribute to a better understanding of substrate selection

of all Anguillae group.


Page 30 of 52

5. Evaluation of the Project

Aim of this project was to find answer for our research question from experimental

studies related to overall problem formulation "Is there a preference choice for a specific type

of substrate for elvers"?

The project has been a challenge due to the fact that problem has a very specific

formulation; there were very limited amount of information sources, combined with not very

favourable and suitable laboratory conditions. Theory was based on biological knowledge, which

includes topics such as life cycle analysis and general eel behaviour, mainly provided by Tesch

2003 book (6) and Dekeer (17) and Deeler (8) reports, those sources fully covered all our basic

knowledge topics of eel ecology. Another topic of our theoretical background was focused on

conservation status of particular species, and was considered by Red List as critically

endangered (14). By this and other researches we could highlight factors which have most

negative effect on European eel population, who recently experience a rapid decline.

However, there is a lack of information about such important life style aspect like

European silver eel habitat preference; neither there is any information about elver habitat

preference. It might be due to the fact that this specific field is still very novel, because just few

decades ago scientist have noticed that not all eels perform catadromous behaviour. Therefore

our expectations were based mainly on biological knowledge from above-mentioned books,

researchers report from other eel species, such as A. japonica, A. australis, and A. rostrata,

means we have not been fully confident in our expectations.

In addition to that, it was quite difficult to find an acceptable information about habitat

destruction and it effect on population decline. Some information was performed in various

countries; however we did not find nearly descriptive enough research or report about Danish

coastal habitat destruction, which highlight the issues of marine excavation and it negative

effect on flora and fauna, focused on eel habitat destruction.

Despite on the fact, that various researcher has proved of unnecessary part of

freshwater in ell life cycle, there is no any report from Danish board on how many of eel

juveniles enter Danish estuarine annually. Also, there is no studies which observe their further

migration to fresh water, or how many decide to stay in brackish water, as well as what factors

might affect that behaviour.

Taken into consideration and reviewing project we need to agree that six experiment

trial were enough to find that there is a preference for subtract. In order to achieve results of

sufficiently high significance level, in order to confirm confident answer for our problem

formulation. To provide more significant data on the habitat preference, we strongly


Page 31 of 52

recommend increasing the number of trials. In addition, observation with time frame has

showed that two hours, which were given to elvers to select an a appropriate habitat among

given substrates is not enough. This is based on observation of eels that were present in water

column, and on their burying activity. If elvers show such behaviour pattern that means that

they did not find a right habitat to stay in. Moreover, due to strict time frame we could not use

another box to separate holding tanks during the experiment. That lack lead to continues eel

distribution during the experiment trials, especially at the end of the trial when first holding tank

has to be cleaned up for eel counting, while another holding tank was stayed too close, so eels

from this holding tank were continuously interrupted. This fact, probably, might have an effect

on eel behaviour and preference of particular subtract. All this experimental concerns might be

avoided and improved in further studies.

Regardless of all this points of question and challenges, it should be mentioned the

advantages of the project in terms of handling real experiment and participation of highly

experienced scientists, relative literature search, evaluation and analysis. Overall, by doing this

particular project author have not only improved her qualifications within the fields of fish

biology, ecology and environmental conservation, but also strongly confirmed her desire of

professional career in marine conservation ecology as a future career choice.


Page 32 of 52

6. Conclusion

According to the results of this research project, we confirm that there is a pattern of

elvers substrate preference. Major part (72% of all sampled elvers) had favoured gravel with

medium and large coarseness. Whereas the number of eels that preferred to stay in gravel with

small coarseness, and sand and vegetation was very small: only 12% and 14 %, respectively.

During the observation, eels showed moderate burial activity, with only 14 individuals observed

by the end of experiment. Such behaviour feature like swimming in the water column was

insignificant, with just 4 eels at the end of all experiment trials. This study is part of ongoing

effort to fully understand the population dynamics and life cycle of eel species, and European

eel in particular, therefore the research will continue in the field or in laboratory, but with more

trials and number of individuals, this will expand our knowledge and confidence about juvenile

eel subtract preference, and will have a significant positive improvement on conservation

techniques of all European eel population.


Page 33 of 52

7. Glossary

Catadromous species - fish live in fresh water, breed in the sea.

Amphihaline - Aquatic species which pass periodically, at well-defined stages of their

life cycle, from salt to fresh water and vice versa

Leptocephalus - the leaf-shaped marine larval stage

Glass eel - marine larvae when they reach brackish water, before developing into the

pigmented, growth phase

Elver - referred to a migration stage when during feeding in the coastal environments,

the glass eels become pigmented.

Yellow eel - major growing stage, which starts after eels' migration to fresh water, it

continues for 2-20 years.

Silver eel – last stage of yellow eel maturation, when body fat and summer water

temperature promotes European eel maturation and get it ready for migration to the sea.

Major difference between freshwater, brackish and marine water - from

ecological point of view for eel life cycle there is different salinity level, with highest at marine

water, followed estuarine waters and with the lowest salinity level at freshwater ecosystem

Marine snow - small particles of organic material that fall from upper waters down to

the sea bed. It includes dead animals and plants, fecal matter, sand, soot, and other inorganic

dust.

The Kattegat (in Danish, English) or Kattegatt (in Swedish) is a sea which is

surrounded by Denmark and Sweden.The sea area is a continuation of the Skagerrak.

The Skagerrak - is a strait running between the three Scandinavian countires, which

also connect the North Sea and the Kattegat sea area, finally leads to Baltic sea.

Nocturnal life style - active mainly during the night.

Sex ratio - the relative number of males and females in a population.


Page 34 of 52

8. Reference list

1) Schmidtt J. 1906. Contributions to the life of the eel (Anguilla(Anguilla vulgaris FLEM).

Rapports et Proces-Verbaux dess Reunions du Conseil International pour1'Explorationn

de la Mer 5:137-264.

2) Schmidt, J. (1922-1922) The breeding places of the eel. Philosophical Transactions of

the Royal Society of London Series B 211: 179–208.

3) Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic

eels. / Munk, Peter; Hansen, Michael Møller; Maas, Gregory E.; Nielsen, Torkel Gissel;

Castonguay, Martin; Riemann, Lasse; Sparholt, Henrik; Als, Thomas Damm; Aarestrup,

Kim; Andersen, Nikolaj G.; Bachler, Mirjam.

In: Royal Society of London. Proceedings. Biological Sciences, Vol. 277, No. 1700, 2010,

p. 3593-3599.

4) Harden Jones FR (1968) Fish migration. Edward Arnold Ltd, London, p 325

Rev Fish Biol Fisheries (2005) 15:367–398 DOI 10.1007/s11160-006-0005-8 123

RESEARCH ARTICLE The European eel (Anguilla anguilla, Linnaeus), its lifecycle,

evolution and reproduction: a literature review

5) Tsukamoto, K., Aoyama, J. and Miller, M.J. 2002. Migration, speciation, and the

evolution of diadromy in anguillid eels. . Canadian Journal of Fisheries and Aquatic

Sciences 59: 1989-1989)

6) The eel: biology and management of anguillid eels. Chapman and Hall, London. Tesch,

F.W. (2003). The Eel, 5th ed. Blackwell Publishing, Oxford 408pp

7) Otake T., Nogami K., Maruyama K.Dissolved and particulate organic matter as possible

food sources for eel leptocephali. Marine Ecology Progress Series1993;92:27-34.

8) Deelder, C. 1970. Synopsis of biological data of the eel Anguilla anguilla (Linnaeus,

1758). FAO Fish. Synop., 80: 68.

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10) Nature 396, 635-636 (17 December 1998) | doi:10.1038/25264 Do all freshwater eels

migrate?Katsumi Tsukamoto1, Izumi Nakai2 & W-.V. Tesch3

11) Swedish Board of Fisheries. Fakta om svenskt fiske [Facts about Swedish fisheries].

Fiskeriverket, Göteborg. [In Swedish] 2006.

12) Coastal habitat support to fish and fisheries on the Swedish west coast Johan Ståla ,

Sandra Paulsenb , Leif Pihla , Patrik Rönnbäckc , Tore Söderqvistd and Håkan

Wennhagea

13) Daverat F, Limburg KE, Thibault I, Shiao JC and others (2006) Phenotypic plasticity of

habitat use by 3 temperate eel species, Anguilla anguilla, A. japonica and A. rostrata.

Mar Ecol Prog Ser 308:231–241

14) Jacoby, D. & Gollock, M. 2014. Anguilla anguilla. The IUCN Red List of Threatened

Species 2014:e.T60344A45833138. http://dx.doi.org/10.2305/IUCN.UK.2014-

1.RLTS.T60344A45833138.en.)

15) Laffaille, Pascal and Caraguel, Jean-Marie and Legault, Antoine Temporal patterns in the

upstream migration of European eels (Anguilla anguilla) at the Couesnon estuarine dam.

(2007) Estuarine, Coastal and Shelf Science, Vol. 73 (n° 1-2). pp. 81-90. ISSN 0272-

7714

16) ICESS 2002. International Council for the Exploration of the Sea.. Report of the

ICES/EIFAC Working Group on Eels.. ICES CM. 2002/ ACFM: 03

17) Willemm Dekker [2004] Slipping through our hands - Population dynamics of the

European eel

18) Tzeng WN, Wang CH, Wickstrom H, Reizenstein M. Occurrence of the semicatadromous

European eel Anguilla anguilla in the Baltic Sea. Marine Biology 2000;137(1):93-9

19) Dou SZ, Tsukamoto K (2003) Observations on the nocturnal activity and feeding

behavior of Anguilla japonica glass eels under laboratory conditions. Environ Biol Fish

67: 389−395


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20) How American eels Anguilla rostrata construct and respire in burrows J. P. N. Tomie1 ,

D. K. Cairns2,*, S. C. Courtenay1

21) A Review of the Population Dynamics, Migrations and Management of the European eel

(Anguilla anguilla) James Barry University of Glasgow, Scottish Centre for Ecology and

the Natural Environment

22) Tsukamoto K, Arai T (2001) Facultative catadromy of the eel Anguilla japonica between

freshwater and seawater 215 Aquat Biol 1: 205–216, 2008 habitats. Mar Ecol Prog Ser

220:265–276

23) Naturen i Danmark, bd. 1- Havet by Tom Fenchel & Kaj Sand Jensen (in Danish)

24) Relationships between size-specific sediment preferences and burial capabilities in

juveniles of two Alaska flatfishes Journal of Experimental Marine Biology and Ecology,

Vol. 282, No. 1-2. (2003), pp. 85-101, doi:10.1016/S0022-0981(02)00447-1 by Allan W.

Stoner, Michele L. Ottmar

25) Habitat preferences of shortfinned eels (Anguilla australis), in two New Zealand lowland

lakes. N. Z. J. Freshw. Res. 33, 233–248. Jellyman, D.J., Chisnall, B.L., 1999

26) Environmental Hydraulics, Two Volume Set: Proceedings of the 6th International

Symposium on Enviornmental Hydraulics, Athens, Greece, 23-25 June 2010 June 9,

2010 by CRC Press George C. Christodoulou, Anastasios I. Stamou; June 9, 2010 by

CRC Press

27) Freshwater Fishes of North-Eastern Australia; Brad Pusey;Mak Kennard;Angela

Arthington ; Publisher: CSIRO Publishing (2004)

28) Jessop, B. M. 2000. Estimates of population size and instream mortality rate of American

eel elvers in a Nova Scotia river. Transactions of the American Fisheries Society 129:

514- 526.


Page 37 of 52

29) Scientific Opinion of the Panel on Animal Health and Welfare on a request from the

European Commission on Animal Welfare Aspects of Husbandry Systems for Farmed

European Eel. The EFSA Journal (2008) 809, 1-18

30) The European eel (Anguilla anguilla, Linnaeus), its Lifecycle, Evolution and

Reproduction: A Literature Review;Reviews in Fish Biology and Fisheries, 2006, Volume

15, Number 4, Page 367 Vincent J. T. Ginneken, Gregory E. Maes

31) Emde S, Rueckert S, Kochmann J, Knopf K, Sures B, Klimpel S. Nematode eel parasite

found inside acanthocephalan cysts – a “Trojan horse” strategy?Parasites & Vectors.

2014;7:504. doi:10.1186/s13071-014-0504-8.

32) ABPmer, (2013). Summary of the 1995 Sandeel Survey on Nash Banks by ABP Research

& Consultancy Ltd. ABP Marine Environmental Research Ltd. Report No. R2188TN.

Produced for BMAPA.

33) Marin råstofkortlægning og miljøundersøgelser i Øresund : Steen Lomholt”, Bo

Riemann*, Karsten Dahl*, Niels N. Pedersen”,Jørgen O. Leth”, Cordula Göke*, Michael

Bo Rasmussen*,Sara Skar” & Ole G.N. Andersen (GEUS)(in Danish)

34) OSPAR 2010. Background Document for European eel Anguilla Anguilla. Biodiversity

Series.

35) FISRWG- Federal Interagency Stream Re storation Working Group. October

1998.Disturbance affecting stream corridors. Pages 3.1-3.28 in Stream Corridor

Restoration: Principles, Processes, and Practices.

36) Occurrence of the semi-catadromous European eel Anguilla anguilla (2000)in the Baltic

Sea. Marine Biology 137: 93-98) (Tzeng, W. N., Wang, C. H., Wickstrom, H. &

Reizenstein, M.

____________________________________________________________________


Page 38 of 52

Frontpage Source : “Glass Eel” Chris Bowser NYSDEC

Figure 1 Distribution patterns of eel larvae with the size of the larvae in mm (source: Schmidt

1923)

Figure 2 Life Cycle of European Eel from Rob Slaupkaukas (proposal FOR THE INCLUSION OF

THE European EEL (Anguilla anguilla) On CMS Appendix II)

Figure 3 Figure 3 Time trends in juvenile abundance of the major eel stocks of the world. The

average trend of the four longest data series is shown for the European eel. (From Dekker

2004)


Page 39 of 52

9. Appendices

Table 2 Sample of worksheet from one of experiment day

Date

Salinity of water in Big holding aquarium

N of experiment

Holding Tank 1

Presence in water column Time N of eels in water

column SV GS GM GL

Releasing time

Start of experiment(after 30 minutes of acclimatization)*

30 minutes after start of experiment

One hour later

90 minutes after

End of experiment ( two hours long )

Substrate preference by the end of experiment

*time to release elvers to Holding tank 2

Bury activity Time N of eels who bury** SV GS GM GL

Releasing time

Start of experiment(after 30 minutes of acclimatization)


One hour later

90 minutes after



**can see a part of eel present out of substract


Page 41 of 52

Holding Tank 2

Presence in water column Time N of eels in water

column SV GS GM GL

Releasing time



One hour later

90 minutes after

End of experiment ( two hours long )*


*should be after 30 minutes of the end of experiment in holding

Tank 1

Bury activity Time N of eels who bury** SV GS GM GL

Releasing time



One hour later

90 minutes after


**can see a part of eel present out of substract

Table 3 Results of measurements of eel length

Table 4 Results of measurements of eel weight

Table 5 Summarized results of observation on eel presence in water column after two hours versus

staying in a preferred subtract type

Table 6 Summarized results of observation on eel presence in water column during all experiment days

versus a each time scale

Length fraction

(cm) N of eels

% from

total

So

rte

d L

en

gth

[cm

]

<9 8 6

9,1-11 44 357

11,1-13 69 54

13,1< 6 5

Total 127 100

Weight Fraction (g) N of eels % from total

So

rte

d W

eig

ht

[g]

<1 11 9

1.1-1.5 39 31

1.6-2 55 43

2-3 18 14

3< 4 3

Total: 127 100

Type of substrate Amount of replicates

Sand & Vegetation 1

Gravel S 0

Gravel M 1

Gravel L 2

Time Amount of replicates

Start 7

30 min 11

Hour 7

After 90 minutes 6

Two Hours 4


Page 43 of 52

Table 7 Results from all experiment days with detailed observation on eel presence in a water column.

Each section is referred to time scale from start of experiment and till the end.

Start of experiment

Type of substrate N of eels

Sand & Vegetation 3

Gravel S 0

Gravel M 2

Gravel L 2

After 30 minutes


Sand & Vegetation 2

Gravel S 2

Gravel M 5

Gravel L 2

After one hour


Sand & Vegetation 1

Gravel S 1

Gravel M 2

Gravel L 3

After 90 minutes

Type of substrate Amount of eels

Sand& Vegetation 1

Gravel S 0

Gravel M 2

Gravel L 3

After two hours

Type of substrate Amount of eels

Sand & Vegetation 1

Gravel S 0

Gravel M 1

Gravel L 2


Page 44 of 52

Table 8 Summarized results of observation on eel bury activity at the end of experiment versus a

preferred subtract type

Type of substrate Amount of Eels

Sand & Vegetation 2

Gravel S 2

Gravel M 5

Gravel L 5

Table 9 Summarized results of observation on eel bury behaviour during all experiment versus each

Table 10 Results from all experiment days with detailed observation on eel bury behaviour. Each

section is referred to time scale from start of experiment and till the end

Start of the experiment


Sand & Vegetation 2

Gravel S 0

Gravel M 3

Gravel L 2

After 30 minutes


Sand & Vegetation 1

Gravel S 0

Gravel M 6

Gravel L 3

After one hour


Sand & Vegetation 3

Gravel S 0

Gravel M 12

Gravel L 4

Time N of eels bury in

sediment

Start 7

After 30 min 10

Hour 19

90 min 11

Two hours 14


Page 45 of 52

After 90 minutes


Sand & Vegetation 1

Gravel S 0

Gravel M 7

Gravel L 3

After two hours


Sand & Vegetation 2

Gravel S 2

Gravel M 5

Gravel L 5

Table 11 Results from all experiment days with detailed observation on eel subtract preference

Date of

experiment Tank # Habitat

Experiment

# Replicants at

the end

29/09/2015 I SV 1 1

29/09/2015 I Gr S 1 3

29/09/2015 I Gr M 1 5

29/09/2015 I Gr L 1 0

29/09/2015 II SV 2 6

29/09/2015 II Gr S 2 1

29/09/2015 II Gr M 2 5

29/09/2015 II Gr L 2 1

30/09/2015 I SV 3 2

30/09/2015 I Gr S 3 0

30/09/2015 I Gr M 3 3

30/09/2015 I Gr L 3 5

30/09/2015 II SV 4 3

30/09/2015 II Gr S 4 1

30/09/2015 II Gr M 4 2

30/09/2015 II Gr L 4 5

2/10/2015 I SV 5 2

2/10/2015 I Gr S 5 1


Page 46 of 52

2/10/2015 I Gr M 5 4

2/10/2015 I Gr L 5 3

2/10/2015 II SV 6 0

2/10/2015 II Gr S 6 2

2/10/2015 II Gr M 6 4

2/10/2015 II Gr L 6 4

5/10/2015 I SV 7 1

5/10/2015 I Gr S 7 0

5/10/2015 I Gr M 7 6

5/10/2015 I Gr L 7 5

5/10/2015 II SV 8 0

5/10/2015 II Gr S 8 0

5/10/2015 II Gr M 8 5

5/10/2015 II Gr L 8 5

6/10/2015 I SV 9 0

6/10/2015 I Gr S 9 2

6/10/2015 I Gr M 9 5

6/10/2015 I Gr L 9 3

6/10/2015 II SV 10 0

6/10/2015 II Gr S 10 0

6/10/2015 II Gr M 10 8

6/10/2015 II Gr L 10 2

7/10/2015 I SV 11 0

7/10/2015 I Gr S 11 2

7/10/2015 I Gr M 11 6

7/10/2015 I Gr L 11 3

7/10/2015 II SV 12 4

7/10/2015 II Gr S 12 4

7/10/2015 II Gr M 12 2

7/10/2015 II Gr L 12 3


Page 47 of 52

Table 13 Results of experiment observation on number of eels present in water column at the end of experiment

compared to eels who were fully covered in substrate and who present bury activity.

At the end of experiment Amount of replicates % from total N

Stayed in a water column 4 3.10%

Bury activity in sediment 14 10.85%

Present in substrate 111 86%

Total : 129(100%)


Page 48 of 52

‘Table 14 Eel Length and Weight Measurements

Weight [g] Length [S cm] Length [cm] AVG W [g] AVG L [Scm] AVG L [cm]

1.00 18 9.0 1.60 21.87 10.94

2.20 25 12.5 Weight [g] Length [S cm] Length [cm]

1.76 22 11.0 2.41 24 12.0

1.24 20 10.0 1.75 22 11.0

1.95 24 12.0 0.33 15 7.5

1.88 24 12.0 1.50 22 11.0

1.50 21 10.5 1.62 23 11.5

1.55 23 11.5 2.24 25 12.5

1.76 24 12.0 2.29 25 12.5

2.20 25 12.5 1.15 20 10.0

1.29 21 10.5 2.10 25 12.5

1.35 20 10.0 2.33 24 12.0

0.76 17 8.5 1.60 22 11.0

1.62 23 11.5 1.58 22 11.0

1.20 19 9.5 1.30 20 10.0

1.29 20 10.0 1.60 22 11.0

1.42 23 11.5 2.05 23 11.5

0.80 17 8.5 3.11 27 13.5

2.10 25 12.5 1.22 20 10.0

2.33 25 12.5 3.03 27 13.5

3.13 28 14.0 1.65 22 11.0

1.49 23 11.5 1.44 23 11.5

1.83 20 10.0 1.31 21 10.5

0.99 19 9.5 2.42 25 12.5

1.38 21 10.5 0.77 16 8.0

1.88 24 12.0 1.50 21 10.5

1.73 23 11.5 1.74 23 11.5

1.91 24 12.0 1.76 24 12.0

1.50 20 10.0 1.20 20 10.0

1.20 21 10.5 2.48 26 13.0

1.45 21 10.5 0.67 17 8.5

1.64 23 11.5 1.75 23 11.5

1.52 23 11.5 1.53 22 11.0

1.47 21 10.5 1.69 22 11.0

1.72 23 11.5 1.82 24 12.0

1.32 20 10.0 1.07 18 9.0

1.27 21 10.5 2.82 27 13.5

1.42 22 11.0 1.74 22 11.0

1.82 23 11.5 1.24 20 10.0

1.58 24 12.0 1.79 24 12.0

1.34 20 10.0 1.55 22 11.0

1.69 24 12.0 1.95 22 11.0

1.37 21 10.5 1.58 22 11.0


Page 49 of 52

1.27 22 11.0 0.45 16 8.0

1.35 21 10.5

Weight [g] Length [S cm] Length [cm]

2.13 24 12.0

1.54 21 10.5

1.70 23 11.5

1.50 22 11.0

1.07 19 9.5

0.20 14 7.0

1.14 19 9.5

0.73 17 8.5

3.10 27 13.5

2.08 25 12.5

1.30 20 10.0

2.26 25 12.5

1.74 23 11.5

1.30 19 9.5

1.63 22 11.0

1.94 23 11.5

1.20 20 10.0

1.03 19 9.5

0.58 19 9.5

1.68 22 11.0

1.80 23 11.5

1.90 23 11.5

1.32 21 10.5

1.73 23 11.5

2.74 25 12.5

1.17 20 10.0

1.75 23 11.5

1.40 20 10.0

1.24 20 10.0

2.00 24 12.0

1.63 21 10.5

1.28 20 10.0

1.16 19 9.5

1.85 24 12.0

1.69 23 11.5

0.94 19 9.5

1.52 22 11.0

1.74 23 11.5


Page 50 of 52

Sorted Weight

0.20 0..1 11

0.33 1..1.5 38

0.45 1.5..2 52

0.58 2..3 17

0.67 3.. 4

0.73 1.42 1.75

0.76 1.44 1.75

0.77 1.45 1.76

0.80 1.47 1.76

0.94 1.49 1.76

0.99 1.50 1.79

1.00 1.50 1.80

1.03 1.50 1.82

1.07 1.50 1.82

1.07 1.50 1.83

1.14 1.52 1.85

1.15 1.52 1.88

1.16 1.53 1.88

1.17 1.54 1.90

1.20 1.55 1.91

1.20 1.55 1.94

1.20 1.58 1.95

1.20 1.58 1.95

1.22 1.58 2.00

1.24 1.60 2.05

1.24 1.60 2.08

1.24 1.62 2.10

1.27 1.62 2.10

1.27 1.63 2.13

1.28 1.63 2.20

1.29 1.64 2.20

1.29 1.65 2.24

1.30 1.68 2.26

1.30 1.69 2.29

1.30 1.69 2.33

1.31 1.69 2.33

1.32 1.70 2.41

1.32 1.72 2.42

1.34 1.73 2.48

1.35 1.73 2.74

1.35 1.74 2.82

1.37 1.74 3.03

1.38 1.74 3.10

1.40 1.74 3.11

1.42 1.75 3.13


Page 51 of 52

Sorted Length [cm]

7.0 7..9 7

7.5 9..11 42

8.0 11..13 67

8.0 13.. 6

8.5 10.5 11.5

8.5 10.5 11.5

8.5 10.5 11.5

8.5 10.5 11.5

9.0 10.5 11.5

9.0 10.5 11.5

9.5 11.0 11.5

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.5

10.0 11.0 12.5

10.0 11.0 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.5 11.5 13.0

10.5 11.5 13.5

10.5 11.5 13.5

10.5 11.5 13.5

10.5 11.5 13.5

10.5 11.5 14.0

10.5 11.5

10.5 11.5

Sorted Length [cm]


Page 52 of 52

7.0 7..9 7

7.5 9..11 42

8.0 11..13 67

8.0 13.. 6

8.5 10.5 11.5

8.5 10.5 11.5

8.5 10.5 11.5

8.5 10.5 11.5

9.0 10.5 11.5

9.0 10.5 11.5

9.5 11.0 11.5

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

9.5 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.0

10.0 11.0 12.5

10.0 11.0 12.5

10.0 11.0 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.0 11.5 12.5

10.5 11.5 13.0

10.5 11.5 13.5

10.5 11.5 13.5

10.5 11.5 13.5

10.5 11.5 13.5

10.5 11.5 14.0

10.5 11.5

10.5 11.5

anastasija martjanova bachelor project report 2015

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