harbasins report: water management strategies for estuarine and

HARBASINS Report:

Water management strategies for estuarine and

transitional waters in the North Sea Region.

Prepared by: Research Institute for Nature and Forest

INBO.R.2008.3

HARBASINS is a project funded under the European Regional Development

Fund INTERREG IIIB North Sea Region Program – A European Community

Initiative concerning Trans National Co-operation on Spatial Development 2000-

2006.

HARBASINS – Conservation goals 2

HARBASINS: Harmonised River Basin Strategies North Sea

Estuarine Ecosystem Functioning and Health

Habitat needs to realise conservation goals for fish in estuaries:

case study of the tidal Schelde

Jan Breine, Joachim Maes, Maarten Stevens, Ilse Simoens, Mike Elliott, Krystal Hemingway

and Erika Van den Bergh

INBO.R.2008.3


Colofon

Jan Breine, Ilse Simoens

Instituut voor Natuur- en Bosononderzoek

Wetenschappelijke Instelling van de Vlaamse Gemeenschap

Duboislaan 14, 1560 Groenendaal

www.inbo.be

email: [email protected]

Joachim Maes

VITO, Vlaams Instituut voor Technologisch Onderzoek

Integrale Milieustudies

Boeretang 200

B-2400 Mol

Mike Elliott, Krystal Hemingway

Institute of Estuarine & Coastal Studies, University of Hull

Cottingham Road Hull

HU6 7RX

UK

Maarten Stevens, Erika Van den Bergh

Instituut voor Natuur- en Bosononderzoek

Kliniekstraat 25

1070 Brussel

wijze van citeren: Breine, J., Maes, J., Stevens, M., Simoens, I., Elliott, M., Hemingway, K. & E. Van

den Bergh, 2008. Habitat needs to realise conservation goals for fish in estuaries: case study of the

tidal Schelde. INBO.R.2008.3

Rapportnummer: INBO.R.2008.3

Depotnummer: D/2008/3241/009

Druk: Ministerie van de Vlaamse Gemeenschap, Departement L.I.N. A.A.D. Afd. Logistiek – Digitale

Drukkerij

Trefwoorden:Keywords: habitat needs, conservation goals, fish, estuaries, Schelde


Index

1. Introduction

2. Conservation goals

3. Guilds for estuarine fishes with relevance to the formulation of conservation goals 3.1. Introduction

3.2. Salinity preference guilds

3.3. Estuarine use guilds

3.3.1. Freshwater resident species

3.3.2. Estuarine resident species

3.3.3. Diadromous fish species

3.3.4. Marine adventitious species

3.3.5. Marine juvenile migrants

3.3.6. Marine seasonal migrants

3.4. Flow preference guilds

3.5. Reproductive special demands guild

3.6. Trophic guilds

3.7. Stratum guilds

3.8. Tolerance guilds

4. Conservation goals for fish in estuaries

4.1. Introduction

4.2. General quality needs within the estuary

4.3. Guild-specific goals and associated habitat needs

4.3.1. Introduction

4.3.2. Salinity preference guilds

4.3.3. Estuarine use guilds

4.3.3.1. Conservation goals and associated habitat needs for freshwater

resident species

4.3.3.2. Conservation goals and associated habitat needs for estuarine

resident species and marine juvenile migrants

4.3.3.3. Conservation goals and associated habitat needs for diadromous

fish species

4.3.3.4. Conservation goals and associated habitat needs for marine

adventitious species and marine seasonal migrants

4.3.4. Flow preference guilds

4.3.5. Reproductive special demands guild

4.3.6. Trophic guilds

4.3.7. Stratum guilds

4.3.8. Tolerance guilds

4.4. Habitat status

4.4.1. Introduction

4.4.2. Habitat loss in the Schelde

4.5. Mitigating measures

4.5.1. Introduction

4.5.2. Mitigating processes in the Schelde

5. Conclusion

Annex

Table A List of fish species found in the Zeeschelde and their guilds

Table B Essential fish habitats in relation to ecological guilds


1. Introduction

The protection of nature values and functioning of aquatic ecosystems is the subject of several legal

commitments and international agreements. European legislation includes the Water Framework

Directive (WFD) as well as the Wild Bird and Habitat Directives (BHD). In addition, different

competences derive from local, regional, national or multilateral and international initiatives, each

with its own objectives and targets. Clearly these commitments apply on different spatial scales

(Europe, North Sea area, country, river basin). Consequently, evaluations of present regulations that

aim to protect environments optimally adopt an integrated, hierarchical structure (Fig. 1). Under this

approach, particular commitments aim at sustainable and integrated management but may focus on a

different spatial level of the ecosystem and its functioning. Accordingly, objectives at each level aim

at proper functioning of the ecosystem in a way that the commitments are respected. Each level feeds

back to the higher level in order to avoid contradictions. Once the objectives are set, quantitative

indicators or measurement endpoints have to be defined to measure the actual status and to compare

the ecosystem state with references set by the conservation goals. Depending on the scale, indicators

are either based on integrated data or represent an explicit measure of the state of the ecosystem.

Therefore, any monitoring scheme should provide a wide range of information so that for each level of

assessment the necessary information can be deduced. Restoration measures will then aim at restoring

the processes that generate the required habitats and species populations. The potential for recovery

remains since most species and functional groups persist, albeit in greatly reduced numbers.

This proposal describes habitat needs for fish in estuaries, defined as parts of a river under tidal

influence (Fairbridge, 1980), to assure a good status of fish populations as defined by the Water

Framework Directive (EU Water Framework Directive, WFD, 2000). This good status is obtained

when the conservation goals are fulfilled. A good habitat quality, a proper ecosystem functioning and

a good carrying capacity will assure that conservation goals are achieved. Conservation goals for fish

described by Adriaensen et al. (2005) are refined. The tidal Schelde is taken as a case study but this

approach can be applied to all North Sea estuaries. As such, this report informs to the objectives of the

HARBASINS project. In particular, the HARBASINS project addresses the question as to how

different approaches by regions in Europe, with respect to legal implementation of European law and

site specific management experiences, result in improved estuarine functioning and health as reflected

by the structure of the estuarine fish community. In this context, estuarine fish are used as a top

component in the ecosystems and one which has a high public and political resonance, hence often

being (with wading birds) the end point for the management of the systems. An accompanying report

(Cutts et al., in prep.) will consider similar aspects for estuarine wading birds.


In this report, and in order to make the method widely applicable and thus not reliant on the particular

species inhabiting any single estuary, we firstly classify fishes into guilds with relevance for the

formulation of conservation goals. We describe general and guild-specific conservation goals for fish.

Based on literature, habitat requirements are defined and then this information is used to define habitat

needs allowing a general functioning of the estuarine ecosystem and provide some suggestions to

achieve these needs. In addition we describe habitat needs at a guild level. A fish-based tool to assess

the present status relative to the conservation goals will be developed and reported in another

document.

Fig. 1. Hierarchical integration of conservation objectives depending on the spatial scale

(adapted from Wolfstein & Van den Bergh, 2004)

2. Conservation goals

Van den Bergh et al. (2005) describe quality goals for key attributes in the Schelde estuary in a

hierarchical way, starting from ecosystem physical and chemical processes down to the level of

specific habitat types and species (groups). Conservation goals for fish are defined as: achieving the

good ecological status as required by the Water Framework Directive. This ecological status can be

assessed or quantified with the classification tools that were developed for this purpose (e.g. fish-based

index of biotic integrity (EBI, Breine et al., 2007a). Fish population attributes included are: total

number of species, percentage of smelt (Osmerus eperlanus) individuals, percentage marine migrating

juveniles, percentage omnivorous and piscivorous individuals. In other words: the different ecological

guilds or functional groups that are typically present in an estuary should be able to complete their

North sea region Environmental quality OSPAR, WFD, others

River basin Environmental quality WFD, others

Estuary

Habitat

Conditions for ecological processes WFD, Natura 2000, LTV

Environmental quality Type/species: conservation goals


lifecycles and establish sustainable populations. Hence we apply the guild approach to define the

habitat needs to meet these conservation goals.

3. Guilds for estuarine fishes with relevance for the formulation of

conservation goals

3.1. Introduction

Fish can be aggregated into functional groups (guilds) according to different characteristics or along

different gradients. Fish species are adapted to the different ecological gradients in rivers, transitional

waters and coastal areas. As such fish assemblages along a river course are structured according to the

slope, discharge, temperature and oxygen content of the corresponding river section (Léger, 1945).

The longitudinal distribution of fishes occurs according to their tolerances, in a manner that lets some

progressively displace others (Roule, 1927). Therefore, fish species diversity, trophic structure as well

as reference conditions change along a river gradient in a predictable way (RCC, River Continuum

Concept, Vanote et al., 1980) as is the case in estuaries (Maes, 2000). This concept corresponds with

the distinction of different zones in rivers using type species, wetted river width and slope (Huet,

1949). The RCC is a longitudinal concept and ignores interactions between river channel, riparian

zone and floodplain. Particularly in estuaries, the exchange of water along a transversal gradient is of

crucial importance providing temporary essential fish spawning and nursery habitats in periods of

flooding either by tides (Tide Pulse) or by peak discharges (Flood Pulse, Junk et al., 1989). In terms of

energy and nutrient exchange, this lateral interaction is locally probably more important than axial

processes. It follows that in particular estuaries, river landscapes under tidal influence, support a high

biological production in both the fresh and brackish water parts. We can use these theoretical concepts

to classify estuarine fish faunas according to their ecological guilds. Root (1967) defined a guild as a

group of species that exploit the same class of environmental resources in a similar way. Species

within a guild are defined by the similarities in the use of ecological niches. In this report we group

species along environmental variables into guilds. Different categories within one guild are not always

clearly separated i.e. depending on their life cycle, species can belong to different categories (Gerking,

1994). Table A in annex is a summary of a literature review classifying each species into guilds. Only

species occurring in the presence absence list for the tidal Schelde and its tributaries are considered

(Breine et al., 2007b). These are species that should be in the Schelde when it reaches its Maximal

Ecological Potential (MEP) or its Good Ecological Potential (GEP).

The guild structure of an ecosystem is often more stable in time than its species composition and

therefore guilds have been used to assess the biological integrity of estuaries (Borja et al., 2004;

Breine et al., 2007a; Coates et al., 2004; Deegan et al., 1997; Harrison et al., 2000; Harrison &

Whitfield, 2004; Hughes et al., 2002; Jager & Kranenbarg, 2004; Moy, 2004; Salas et al., 2004;


Whitfield & Elliott, 2002). We have defined the following guilds: salinity preference, estuarine use,

flow preference, reproduction, stratum use (adults), tolerance guilds and trophic guilds. These guilds

contain relevant ecological and functional information of the estuary. Guilds describe the main

features of the fishes’ biology and the way in which they use the estuary (Elliott et al., 2007).

Therefore conservation goals and the related habitat needs can be defined guild-specific.

3.2. Salinity preference guilds

In estuaries a clear salinity gradient is present and therefore the first guild in our classification defines

groups according to their salinity preference (following a longitudinal gradient). Indeed fish can

occupy different habitats during particular periods of their life history. Salinity is an important factor

influencing the dispersal of fish and other organisms (Elliott et al., 2007). For classification of the

estuarine ichthyofauna into salinity preference classes we combined information obtained from fish

surveys and from literature (Bulger et al., 1993; Elliott & Dewailly, 1995; Schiemer & Waidbacher,

1999; Pihl et al., 2002; Quak, 1994; van Emmerik, 2003). We distinguish different classes or salinity

zones in the Schelde (Table 1). Based on the salinity preference fish species can be attributed to these

classes (Table A in annex).

Table 1: Salinity classes in the Schelde

Salt guild Salinity range (Venice system, 1959)

Freshwater <0.5

Oligohaline 0.5-5

Mesohaline 5-18

Polyhaline 18-30

In the Schelde we defined five different zones based on the Venice system. We did not differentiate

between the freshwater zone with short and long retention time (Fig. 1).


Figure 1: Salinity zones and Omes segments (numbers, Hoffmann & Meire, 1979) in the Schelde

Information about the presence of each species, in a particular salinity zone, according to its age is

used to classify species into the salinity preference guild. E.g. a species can only be considered as a

freshwater species if it occurs as a juvenile and adult in freshwater while diadromous species should

occur in all zones according to their life stage. Age is taken into account because some species show

an ontogenetic shift of habitat e.g. adult cod (Gadus morhua) live in the sea but juveniles are found in

the mesohaline zone of the estuary. The habitat requirements for juveniles are different then for adults

and should be taken into account while defining conservation goals. The results are represented in the

table A in annex.

3.3. Estuarine use guilds

In this group we have guilds that correspond largely with the ‘estuarine use functional group’ defined

by Elliott et al. (2007). According to the use of habitat which is related to the life history strategy, we

distinguish between different guilds described below (Elliott & Dewailly, 1995; Mathieson et al.,

2000; Pihl et al., 2002; Thiel & Potter, 2001).


3.3.1. Freshwater resident species

The freshwater resident species occur in the freshwater part of the estuary during their complete life

cycle. They reproduce, grow up and feed in freshwater, but can also be found in the oligohaline zone.

However, there is a difference between fish assemblages in the mainstream and in the tributaries. The

freshwater fish fauna of lowland rivers under tidal influence is normally dominated by eurytopic

species. These are typical for the potamon, the river stretch with slow running water. This area

includes the bream and ruffe/flounder zone (Huet, 19491). From survey data in the Zeeschelde for a

period from 1995 to date, we observe that the freshwater part of the mainstream is dominated by roach

(Rutilus rutilus), pikeperch (Sander lucioperca), bream (Abramis brama) and white bream (Blicca

bjoerkna). Other species such as rudd (Scardinius erythrophthalmus), gibel carp (Carassius gibelio),

ruffe (Gymnocephalus cernuus), three-spined stickleback (Gasterosteus aculeatus), perch (Perca

fluviatilis) and other cyprinids frequent this river zone. In the tributaries typical rheophilic species

including dace (Leuciscus leuciscus) and chub (Leuciscus cephalus) may dominate further upstream

where the current velocity is higher. We differentiate between Rheophilic A and B. Rheophilic A

species are confined tot the main river channel or all life stages are dependant on running water.

Rheophilic B have stages in backwaters or tributaries that are well connected with the main stream and

prefer running water or part of their life stage depends on it. In the Schelde tributaries we also have a

dominance of eurytopic species that occur in all kind of stream conditions. In our survey data we have

a dominance of roach followed by three-spined stickleback. Further upstream rheophilic species were

collected such as bleak (Alburnus alburnus) and gudgeon (Gobio gobio). The freshwater migrants

defined by Elliott et al. (2007) are included in this group.

3.3.2. Estuarine resident species

These species complete their complete life cycle within the estuary (excluding the freshwater zone).

They are sensitive to the disappearance of specific habitat such as intertidal mudflats, creeks and

marshes, accumulation of toxic substances and often show specialised parental care (e.g.

Pomatoschistus microps and P. minutus).

1 Huet (1949) described for each river zone a typical fish assemblage. Stating that water temperature and stream

velocity are important factors influencing the distribution of fishes. Both factors are dependant on the slope. He concluded that in a given geographical area, rivers of similar width, depth and slope have identical properties and similar fish populations. He distinguished 5 river types depending on the width and for each of these different zones. In the trout zone salmonid fish occur (trout and salmon). In the grayling zone we have a mixed population dominated by salmonids. In a barbel zone we expect a mixed population dominated by cyprinoids and in the bream zone we find a cyprinid fish population with piscivores.


3.3.3. Diadromous fish species

Diadromous species form an ecological group in the migration guild. Diadromous migrant species

necessarily pass estuaries when migrating between the ocean and upstream fresh water areas. The

osmotic regulation is known to be adapted in the mesohaline zone. Some species are diadromous on a

facultative basis: i.e. they may complete their life cycle in the environment where they hatch or start

migrating. We differentiate between anadromous and catadromous species since conservation goals

will be different especially concerning their spawning habitat. Anadromous species spend the adult

life history stage at sea and subsequently migrate to fresh or brackish water to reproduce either

pelagically or on gravel or sand beds e.g. twaite shad (Alosa fallax) and mullet (Liza ramado).

Catadromous species live in freshwater and reproduce at sea e.g. eel (Anguilla anguilla).

3.3.4. Marine adventitious species

Marine adventitious species or marine stragglers (Elliott et al., 2007) coincidently occur in the

estuaries and only in small numbers e.g. brill (Scophthalmus rhombus). Estuaries are not considered

crucial habitats for these species. Although there are marine adventitious species that each high tide

visit the estuary to feed (Zander et al., 1999). Their presence in estuaries reflects what happens in the

coastal zone (Jager & Kranenbarg, 2004).

3.3.5. Marine juvenile migrants

These use estuaries as a nursery area within the first year of life. It is assumed that this estuarine

habitat use results in either increased survival or increased growth and hence, enhances recruitment

success. Some species in this group are intensively fished in coastal zones (e.g. herring Clupea

harengus, Sole Solea solea, plaice Pleuronectes platessa, Cod Gadus morhua and sea bass

Dicentrarchus labrax).

3.3.6. Marine seasonal migrants

Marine seasonal migrants use the estuary when conditions are favourable, usually during post juvenile

stages and on a seasonal basis. We can compare this group with the marine estuarine –opportunistic

group described by Elliott et al. (2007). They enter estuaries in substantial numbers. A typical marine

seasonal migrant in the Schelde is sprat (Sprattus sprattus).

3.4. Flow preference guilds

To define conservation goals for the fish species in the freshwater estuary we divide the fish fauna

according to their flow preference along a lateral gradient. This especially concerns habitat criteria for

freshwater species but also diadromous and some estuarine resident species. We use literature to


define species preferences (Aarts & Nienhuis, 2003; Elliott & Dewailly, 1995; Elliott & Hemingway,

2002; Huet, 1949; Kroes & Monden, 2005; van Emmerik, 2000; Quak, 1994; Schouten & Quak,

1994). In the Annex table A we have defined rheophilic species (obligate and partial) which need lotic

water, eurytopic species and limnophylic species which prefer rivers with a semi lentic conditions or

standing waters with macrophytes (stagnophilic). Species from the latter guild are not common in the

mainstream of estuaries.

3.5. Reproductive special demands guild

Spawning involves the presence of ripe adults and the production of eggs. It is a functional element of

the adult period in fish ontogeny. Therefore the functional status of an ecosystem should be reflected

in the composition of its reproducing fish assemblage (Copp, 1989). Although fishes will accept some

variation in spawning conditions, the known reproductive tolerances of various fishes or group of

fishes can be used to describe a functional change in an ecosystem’s development. If an ecosystem

changes it may no longer offer the conditions necessary for one species’ functional state and

processes, although the new conditions may be acceptable for a previously absent species. Several

attempts were made to develop a classification based on reproductive styles. The reproductive guild

concept is difficult especially due to insufficient information. Knowledge of preferred spawning

grounds and features of reproductive behaviour are essential to define conservation goals. We used

information from Aarts & Nienhuis (2003), Balon (1975, 1981), Costa et al. (2002), Elliott et al.

(2007) and van Emmerik (2003) to determine special spawning requirements. The special demands we

investigated are: special habitat demands (gravel, stone, plants and floodplains), parental care and nest

building. Gravel spawners depend on the presence of gravel to deposit their eggs (e.g. grayling,

Thymallus thymallus). Diadromous fish can be gravel bed spawners with benthic larvae e.g. river

lamprey (Lampetra fluviatilis) spawns on gravel or sand and the European sturgeon (Acipenser sturio)

females spawn above gravel and stones while the sea lamprey (Petromyzon marinus) need gravel and

build a nest high upstream. Stone spawners depend on the presence of rocks or stones to deposit their

eggs (e.g. Spirlin, Alburnoides bipunctatus). Sand spawners need the presence of a sand soil to deposit

their eggs (e.g. Burbot, Lota lota). Ostracophilic spawners deposit their eggs in living shells or houses

of other organisms (e.g. Bitterling, Rhodeus sericeus) while speleophilic spawners deposit eggs in

cavities (e.g. Bullhead, Cottus gobio). Soil spawners that deposit eggs on the bottom are not

specialised (e.g. White bream, Abramis bjoerkna) which applies also for diadromous species that are

open water spawners and are not included in the list. The results are presented in the Annex Table A.


3.6. Trophic guilds

Feeding guilds may be used to assess the structure and functioning of estuarine fish communities as a

stable trophic network is reflected by a healthy trophic composition (Breine et al., 2007). As Elliott et

al. (2007) mention, it is essential to understand the relations within the food webs so that the

conservation goals can ensure that the trophic network is optimal. The feeding guilds and feeding

preferences of dominant types are present in most estuaries (Costa & Elliott, 1991). Many estuarine

fish are either generalist feeders or opportunists. Therefore absolute separation is difficult and

combinations should be allowed (Elliott & Dewailly, 1995). To define the trophic guilds one is faced

with some problems:

1. The diet of many fish species varies with the availability of food.

2. Many species undergo an ontogenetic shift.

3. Some guilds overlap, e.g. piscivores are part of the vertivores and the planktivores are part of

the invertivores.

From literature we define the diet for juveniles and adult specimens (last step in the food chain) based

on the diet preference of the species. We use information from Batzer et al. (2000), Belpaire et al.

(2000), Breine et al. (2001), Breine et al. (2004), Breine et al. (2007a), Bruslé & Quignard (2001), De

Nie (1996), Elliott & Dewailly (1995), Elliott et al. (2002), Gerking (1994), Gerstmeier & Romig

(1998), Jager & Kranenbarg (2004), Maitland (2000), Mathieson et al. (2000), Muus et al. (1999),

OVB (1988) and van Emmerik (2003) to assign species to food classes. We have following groups

(Table A in annex):

Planktivores (P): feed mainly on zooplankton and phytoplankton

Benthivores (B): feed mainly on benthic invertebrates

Planktivores and benthivorous (PB): feed on both plankton and benthic invertebrates

Piscivores (F) feed mainly or only fish of the same or different species

Benthivorous and piscivorous (BF): feed on both fish and benthic invertebrates

Omnivores (O): feed on animals and plants

Benthivorous and herbivorous (BP): feed on benthic invertebrates and plants

Herbivores and planktivores (PPB): feed on plants, zooplankton and zoobenthos

Herbivores and phytoplankton (PP): feed on plants and phytoplankton

Vertivores and piscivores (VF): feed on fish and vertebrates

The feeding mode functional group presented by Elliott et al. (2007) takes some of these groups

together and considers also additional groups. However we think that the above groups explain

sufficiently the functioning of the estuary.


3.7. Stratum guilds

For the stratum guild or vertical distribution guild the use of the water column by adult species is taken

into account. We distinguish three groups. The benthic fishes or bottom dwellers live in or on the

bottom (e.g. flounder Platichthys flesus). Benthic species are sensitive to the degradation of benthic

habitats (dredging, sedimentation etc…). A second group consists of demersal fish that occur nearby

the bottom (e.g. bream). Pelagic fish occur in the open water sometimes close to the surface (e.g.

Twaite shad). Stratum guilds in the table A (annex) are defined according to Breine et al. (2004),

Breine et al. (2007a), Elliott & Hemingway (2002), Hughes & Oberdorff (1999), Mathieson et al.

(2000) and van Emmerik (2003).

3.8. Tolerance guilds

In Belgium the tolerance guilds have been used by Belpaire et al. (2000), Goffaux et al. (2001), Breine

et al. (2004) and Breine et al. (2007a). Within these guilds we differentiate between habitat sensitivity

and pollution intolerance. This includes information about sensitivity to canalisation, dredging and

fragmentation on the one hand and tolerance to oxygen deficiency and pollution sensitivity (organic

pollution) in general on the other hand. Oxygen demands are an important issue for conservation goals

(Maes et al., 2007a,b). In table A (annex) we compiled information from different authors: Belpaire et

al., 2000; Berrebi dit Thomas et al., 1998; Breine et al., 2001; Breine et al., 2007a; De Nie, 1996;

Grandmottet, 1983; Kruk, 2007; Mann, 1996; Muus et al., 1999; Nijsen & De Groot, 1987; OVB,

1997; OVB, 1988; Phillipart & Vranken, 1983a; Phillipart & Vranken, 1983b; Reitsma, 1992;

Riemersma, 2000; Turnpenny et al., 2004; Van Aelbroeck, 1910; Vandelannoote et al., 1998; Vanden

Auweele, 1995; van Emmerik, 2003; Verneaux, 1981 and Williot, 1991. We define each species as

tolerant or intolerant to pollution i.e. low oxygen and organic pollution. Habitat sensitivity is

expressed as fragmentation sensitivity and the need for undisturbed shelter since some species have

special shelter demands or strategies (van Emmerik, 2003).

4. Conservation goals for fish in estuaries

4.1. Introduction

Estuaries serve several ecological functions at different spatial and temporal scales. Conservation

goals and associated indicators are therefore appropriate only if they target a specific scale. Here, we

focus on four geographical scales. On a regional scale (i.e. North Sea), it is hypothesized, though yet

to be assessed in an analytical way, that the various North Sea estuaries contribute significantly to fish

recruitment to adult populations of especially marine fish but also of estuarine, diadromous and

freshwater fish species. Elliott et al. (1990), for example, indicate the way that estuaries support


nursery stages which become the adult populations. For the purpose of this exercise, fish recruitment

is defined here as the ingress of 0-group fish to the 1-group old, one year after hatching. Basin-wide,

the estuary (mesohaline and polyhaline) is per unit surface area the most productive part of the

watershed. The elevated estuarine productivity relates to the nursery function that estuaries perform

for juveniles of offshore spawned marine, estuarine and catadromous fishes and for upstream spawned

freshwater and anadromous fishes. Within the estuary different salinity zones can be identified.

Finally, within different salinity zones, species are adapted to particular habitats. Here we also

consider the essential estuarine habitats for fish. Essential habitats are habitats (waters and substrata)

that are necessary to spawn, breed, feed and grow to maturity and that act as pathways in diadromous

migration. In other words habitat demands depend on biological aspects. Table 2 illustrates the relation

between the biological aspect and habitat demand. Estuaries are subject to pronounced seasonal cycles

in species occupancy. Wintering waterfowl probably provide the best example but also fish

communities - as integral part of estuarine ecosystems - undergo seasonal changes in species

composition. We hypothesise that seasonality in estuarine fish community reflects the year-round

fulfilment of the nursery function.

Table 2: Biological aspect versus habitat demands, adapted from Milner (1984)

Biological aspect Habitat demand

Reproduction

Migration to spawning place Sufficient water depth, appropriate flow velocity

and absence of physical and chemical (water

quality) barriers

Spawning Appropriate substratum if not pelagic

Incubation Substratum stability, appropriate water

temperature and salinity, sufficient oxygen and

water movement

Feeding and growth

Availability of food Bank and aquatic vegetation, substratum allows

macro-fauna production, transport of organic

material

Optimal use of energy for movement and to seek

food

Shelter and protection (obstacles, tree roots)

diverse flow pattern, diverse substratum and

vegetation and an appropriate water temperature

Protection

Against physical displacement

Against predation

Against inter- and intraspecific competition

Habitat diversity: variable substratum (protection

and hiding places), flooded banks, tree roots,

branches, trees, debris, vegetation, pools,

turbidity etc…

The essential fish habitat concept derives from US legislation. In a European context it may be

interpreted as habitat directive area. Here, we identify essential fish habitats for the different


ecological guilds (Table B in annex). This list differs from the one proposed by Pihl et al. (2002) in

that we give more weight to tidal freshwater and freshwater fishes.

Conservation goals and restoration measures often target specific habitats or single species. However,

they have influence on larger spatial and temporal scales as well, so we argue that tools to evaluate the

effectiveness of restoration practices must comply with these scales. We will first set up general

functional needs for fishes and then, by considering the different functional groups, more guild-

specific goals and associated habitat needs.

4.2. General quality needs within the estuary

Fish need an essential level of estuarine water quality. This essential level will allow that the estuary

contains a healthy sustainable fish population. Van den Bergh et al. (2005) set quality goals to

evaluate the rehabilitation process for essential habitats within the Schelde estuary. From those we

select water temperature, turbidity, discharge flow rate, salinity and dissolved oxygen (DO) as key

ecological quality factors for the fish population (Maes et al., 2007a). The physical and chemical

conditions set up and reflect the main fundamental niches - for the substratum and the water column

(see figure in Borja & Elliott, 2007).

In addition different types of migration should be possible in one estuary.

Temperature directly or indirectly controls physiological rates, hatching success (Maitland & Hatton-

Ellis, 2003), juvenile growth rate (Aprahamian, 1988), year class strength (Aprahamian &

Aprahamian, 2001), juvenile seaward migration (Limburg, 1996), adult upstream movements

(Aprahamian, 1988). More generally, temperature also relates to natural mortality of teleost fishes

(Pauly, 1982) as well as to prey abundance. It is of note that with a temperature increase a double

challenge with respect to hypoxia is created: (1) the oxygen demand increases because of increased

metabolism at higher temperatures and (2) the solubility of oxygen is negatively correlated with the

water temperature. At present we are faced with a global warming resulting in a steady increase of the

water temperature. This is a worldwide effect and should be treated as such. On a basin-wide level we

should be aware that this temperature increase affects the DO. We should therefore implement

mitigating measures that increase the DO.

Turbidity decreases the light penetration which has a direct effect on the planktonic primary

production such as the development of diatoms. In addition photosynthesis will be reduced hence

lower DO concentrations. The high load of suspended matter can clog the gills although in some

estuaries, some species are adapted to it (Blaber & Blaber, 1980). It results also in sediment


accumulation which can have a negative effect on the benthic organisms although estuaries are

affected by natural erosion-deposition cycles and so the benthos is adapted to the elevated turbidity.

As many other estuaries the Schelde is turbid with poor light penetration. This is a natural

phenomenon limiting the production of estuarine phytoplankton and thus limiting the food availability

for zooplankton. As such much microplankton primary production may be the result of resuspended

microphytobenthos. Zooplankton is well adapted to high suspended sediment levels, together with

attached and free-living bacteria which form their food source. The high turbidity reduces visibility

and has an impact on visual predators since it reduces the most important visual characteristics of

prey: contrast with background, size, movement, shape, colour and unusual form (Wootton, 1992).

Dredging causes a resuspension of the bottom sediments and has an effect on the turbidity. Both

temperature and turbidity should be such that they do not hamper the life of fish occurring in

the estuary.

Another important factor influencing the habitat suitability for fish is the freshwater discharge. This

habitat suitability versus discharge for different stages of development of species is defined by depth,

velocity and substratum (Angermeier & Karr, 1983; Capra et al., 1995) as well as its influence on

salinity regime. In the downstream part of the estuary, where the cross-sectional area is large

compared to the cross-sectional area of the Schelde, the river discharge can be disregarded

(Horrevoets et al., 2004). Upstream, where the cross-sectional area approaches that of the river, the

fresh water discharge gains importance over the tidal flow and affects the tidal range (Horrevoets et

al., 2004). In the upstream area discharge should not create floods.

Salinity is influenced by water discharge and tidal intrusion and its importance has been described

above. The balance between freshwater (riverine) flows and the tide should assure the salinity

gradient. Any disrupter in salinity will have an impact on the distribution of fish. Salinity determines

the distance that a species is capable of penetrating the estuary (McLusky & Elliott, 2004). Its effects

on organisms is complex and is influenced by other variables e.g. temperature and discharge

(McLusky & Elliott, 2004).

The DO has an effect on the distribution of fish (Reincke et al., 1992) and too low values may result in

fish mortality (Alabaster et al., 1979; Erichsen Jones, 1964; Scholz, 1986; Turnpenny et al., 2004). Of

course certain fish can adapt to hypoxia or avoid it and move upwards or laterally but this reduces the

habitat availability. Fish larvae and juvenile fish are less successful at leaving regions with low DO

concentrations while adults mostly escape from those areas and form typical distributions along the

oxygen gradient. Young life stages of fish may be more susceptible to low DO effects (Turnpenny et


al., 2004). Adriaensen et al. (2005) propose a dissolved oxygen (DO) concentration of 5 mg l-1

as a

lower limit for essential water quality with respect to estuarine fish. Maes et al., (2007b) state this

threshold of 5 mg l-1

as the DO minimum for surface waters in Flanders. This limit is based on DO

criteria for US estuaries as outlined by USEPA and on empirical models describing the response of

estuarine fish to different oxygen concentrations (Maes et al., 2005, 2007a,b). Experiments to measure

the DO requirements of fish in the Thames estuary produced, however, limits that are significantly

lower than 5 mg l-1

(Turnpenny et al., 2004). Table 3 shows DO standards developed by the Thames

Tideway Strategy Group (TTSG) aimed specifically at the Thames Tideway but having more general

application in other British transitional waters (Turnpenny et al., 2006). However, fish populations in

the Tideway were likely to be sustainable (<10% mortality per annum) under this DO standards

regime. We propose to apply the same standards for the Schelde estuary. The objectives are directly

linked to these standards (Table 3).

Table 3: DO standards proposed by the TTSG (Turnpenny et al., 2006)

DO (mg l-1

) Return Period (years) Duration (# of 6 hour tides)

4 1 29

3 3 3

2 5 1

1.5 10 1

Note: the objectives apply to any continuous length of river ≥ 3 km.

Duration means that the DO must not fall below the limit for the

stated number of tides.

A tide is a single ebb or flood.

The bases for these standards are:

- The one week standard (4 mg l-1, 1 yrRP, >29 tides) was selected to ensure protection against chronic

effects such as depression of growth and avoidance of hypoxic areas.

- The 24 h standard (3 mg l-1, 3 yrRP, >3 tides) and the 6 h standard (2 mg l-1, 5 yrRP, > 1 tide) were

selected to provide protection to stocks.

- The lowest standard (1.5 mg l-1) was included to ensure protection from mass mortalities.

A migration possibility within the estuary and between the estuary and the ocean is an important

factor for the sustainability of migrating species. These species use the estuary as a migrating route

especially for spawning. According to Lucas and Barras (2001) besides horizontal and vertical

migrations we have other types towards essential habitats: feeding migrations, refuge-seeking

migrations, spawning migrations and post-displacement movements, recolonisation and exploratory


migrations. Physical and chemical (water quality) barriers can be present in different places along the

estuary and should therefore be alleviated, removed or made passable.

4.3. Guild-specific goals and associated habitat needs

4.3.1. Introduction

Sediment type, degree of vegetation cover, tidal elevation, slope of intertidal area and depth of subtidal

area determine the suitability of a habitat for a species. Water currents influence fish abundance and

distribution. Average current velocities determine the sediment characteristics and consequently the

resultant benthic communities. Dietary preferences of fish will in turn influence their occurrence on

specific sediments. The habitat complexity, productivity and the area of a single habitat type are major

contributors to determine the fish diversity of an area (Wootton, 1990). The width of the intertidal

zone will qualify its potential as a feeding ground or a nursery area. The depth of the subtidal area may

affect its suitability as a habitat for fish. Rocky shores and bottoms offer more niches than the uniform

habitat of soft sediments which is the case for the Schelde estuary. Estuaries are very dynamic systems

and geomorphological processes such as erosion and sedimentation affect the habitat value of areas for

fish. We take these criteria into consideration when defining habitat needs.

4.3.2. Salinity preference guilds

We defined five guilds of which three occur in the Zeeschelde: freshwater species, oligohaline and

mesohaline species. The conservation goals and associated habitat needs for these species are identical

to those defined for the estuary use guilds.

4.3.3. Estuary use guilds

4.3.3.1. Conservation goals and associated habitat needs for freshwater resident species

The presence of freshwater species is restricted to the freshwater, oligohaline and mesohaline parts of

the estuary. Conservation goals for this group specifically target the freshwater tidal part of the

estuary. For this group we define two conservation goals (CG):

CG1: Presence of a sustainable population of freshwater species (Annex Table A)

CG2: Presence of core populations of type specific species (for Schelde HD annex II and red list

species)

CG1 is the most essential goal since CG2 comprises species from lists that to our opinion should be

revised. In order to realise these goals particular conditions in the freshwater habitat should be present.

Tidal freshwater rivers and wetlands constitute rare habitats (Pihl et al., 2002). They are situated above

the saltwater boundary but still experience tides, which distinguish them from the more common


bream zone. The tidal amplitude creates a cross sectional gradient with a subtidal riverbed, intertidal

mudflats and higher located freshwater tidal marshes which are, in North Western Europe, typically

dominated by reed and willow species. Species compositions of freshwater tidal areas tend to be

influenced by landscape patterns just above the tidal limit and can thus be predicted using the zonation

concept. Through colonisation from upstream areas, tidal freshwater rivers may have fish faunas from

the adjacent barbel zone (as in the Thames) or from the bream zone (as in the Elbe or the Schelde).

The freshwater tidal area of the Thames is dominated by dace, a rheophilic A species. The freshwater

tidal area of the Schelde is dominated by representatives of eurytopic fishes such as roach, pikeperch

and bream, this as a result of the flat landscape and a river system that is categorized as bream zone.

Rheophilic B, species that need well connected backwaters, may lack as these habitats are absent and

replaced by intertidal marsh. According to Ward (1989) connectivity, succession and ecotones define

the functional and structural elements of the four-dimensional nature of running water. Increased

habitat and shoreline complexity is beneficially for most of the fish species (Shlosser, 1982; Lobb &

Orth, 1991; Grenouillet et al., 2000; Pollux et al., 2006, Kruk, 2007). Eurytopic fish benefits from a

good hydrological connection between the different components that together constitute the river

corridor (channel, marshes, floodplain) and from the presence of tributaries (Pollux et al., 2006).

Especially supratidal floodplains are considered essential habitats as they provide suitable spawning

and juvenile habitats (Junk et al., 1989, Ward et al., 1999, Scholten et al., 2003). In the Schelde the

rheophilic A species are all diadromous species except one (dace).

Given differences in life history characteristics and habitat use, habitat needs for rheophilic species

probably take into account ecological gradients along a longitudinal section. For eurytopic species, on

the other hand, the possibility of lateral exchanges between floodplains, marshes and the river channel

are crucial factors. As fish grow their habitat requirements change. Therefore, habitat preferences

often switch in the course of the development.

Limnophylic species may occur but estuarine habitats are not essential for them. Based on habitat

preferences, efforts should focus on rheophilic species as well as eurytopic fishes.

Using present knowledge of rheophilic A and eurytopic fishes, we suggest three habitat needs (HN):

HN1: On a basin-wide scale, the ecological connectivity along longitudinal and transversal river

gradients permits the development of a sustainable fresh water population of rheophilic A and

eurytopic fish species in the estuary. This includes an absence of physical and chemical (water quality

e.g. good DO) barriers.


HN2: On an ecosystem scale, the presence of floodplains along the tidal freshwater part of the estuary

assures the annual recruitment of freshwater eurytopic fishes. This goal relates to the hypothesis that

floodplains represent a critical factor in life history of eurytopic fishes.

HN3: The availability of sheltered diversified intertidal habitat surfaces and subtidal areas with a

diverse food supply in the freshwater estuary are essential as nursery and feeding grounds for

eurytopic and rheophilic species. Surface roughness is a measure of diversity for upstream parts of the

river (Chow, 1973) and the higher its value the more natural the river.

4.3.3.2. Conservation goals and associated habitat needs for estuarine resident species

and marine juvenile migrants

Marine juvenile migrants and estuarine resident species respectively use estuaries often during a

particular part or all of their life cycle. We consider two resources of importance: space and food and

the former can be subdivided into amount of area used and amount of time the area is used (extent

versus duration). The main difference between the guilds is the time spend in the estuary. Marine

species strictly spawn at sea while estuarine species may extend their spawning habitat to estuaries

sensu strictu. However, for the purpose of this exercise, marine juvenile migrants and estuarine fishes

are grouped together as they share many biological and ecological properties. Typical estuarine

species, such as common goby (Pomatoschistus microps), pipefishes (Syngnathidae) and eelpout

(Zoarcus vivparus) are therefore able to complete their life cycle both in the brackish water parts of

estuaries as well as outside estuaries in a fully marine environment. Similar conservation targets may

thus apply on both marine and estuarine fishes. For estuarine resident fishes, estuaries can be regarded

as essential habitats providing habitats for spawning, feeding and growing. It is situated in the salinity

range from the oligohaline to the marine. Marine juvenile fishes were defined as those species that use

estuaries as a nursery area as 0-group individuals. In particular, shallow areas in the marine and

brackish part of estuaries that are either turbid or vegetated may qualify as fish nurseries. The same

habitats as listed for estuarine resident species apply as essential habitats for marine juvenile migrants

and need preferential protection (Table B annex) another reason why these two groups are combined.

These habitats are essential and probably contribute significantly to fish recruitment or the survival of

recruits which have migrated into the estuary. The relationship between nursery size and fish

recruitment is postulated by Rijnsdorp et al. (1992). Under this hypothesis, we accept that increasing

total estuarine fish nursery habitat has a positive effect of the recruitment of marine juveniles and can

act as a mitigating measure. Estuaries are characterised by seasonal patterns in species composition as

a result of successive “migration waves” of estuarine resident and marine juvenile migrant individuals.

These migrations relate to species specific life history strategies and are highly influenced by


processes occurring in the sea. Complete seasonal niche partitioning of the particular estuarine

ecosystems suggests optimal functioning of the fish nurseries.

Based on the present knowledge of estuarine resident species and marine juvenile migrants, we

suggest following conservation goals:

CG4: On a regional and basin-wide scale marine juvenile migrants (0-group individuals) should be

present in accordance to the season as well as a sustainable population estuarine species.

CG5: Preserving the seasonal dynamics of the estuarine fish communities and marine juvenile

migrants in the estuary is a prior goal.

CG6: On habitat level different life stages of estuarine species should be present according to the

habitat type and young marine individuals are present according to the season.

In order to achieve these goals we propose following habitat needs:

HN4: On regional and basin-wide scales we aim at an estuarine nursery size that is sufficiently large

(temporal and spatial) such that it contributes significantly to the recruitment of young marine and

estuarine fish populations or at least have several small nursery areas within a geographic area which

are connected. Size is only one criterion, the estuary should have an appropriate water depth, its shape

should be convenient for the larvae, connectivity should allow the larvae to move to adult habitats and

the physical-chemical conditions (DO and salinity) should not be restrictive. Further research is

needed to quantify the nursery function of estuaries. An alternative is provided by the estimation of

carrying capacity of estuaries for fish i.e. the number of individuals an estuary can support per unit of

surface area. Elliott et al. (2007) indicate the methods of defining carrying capacity but as yet these

have not been tested for estuarine fishes. In order to examine the nursery function of the estuary for

juveniles of a particular species we should assess whether its contribution per unit area to the

production of individuals that recruit to adult populations is greater, on average, than production from

other habitats in which juveniles occur (Beck et al., 2001). According to these authors we should

assess different biotic and abiotic factors including landscape factors.

HN5: Seasonal migrations can only occur if conditions in the estuary are favourable for the fish i.e.

temperature and DO conditions should be optimal and food should be available. Physical and chemical

(water quality) barriers should be absent.

HN6: The presence of shallow, low dynamic (as opposed to high tidal energetic areas) habitats

(sheltered mud flats, salt marches; reed beds,…) that provide protection and a high and continuous

supply of food should be insured. We should therefore define the characteristics of mud flats and salt


marches with a good quality and define their dimensions allowing the development of a dynamic and

diversified habitat. The mudflats should have an optimal sediment and tidal position for the

maintenance of suitable prey biomass (Elliott et al., 1998 and McLusky & Elliott, 2004).

4.3.3.3. Conservation goals and associated habitat needs for diadromous fish species

The (sub)tidal transition zone between rivers and oceans is a vital habitat for diadromous fish linking

spawning grounds with adult habitat. Some anadromous fish species, for instance twaite shad (Alosa

fallax), use the freshwater intertidals as spawning habitat. Shallow areas or vegetated habitats

throughout the estuary serve as essential nurseries for 0-group anadromous and catadromous fish (see

intertidal creeks and marches Table B).

Based on the present knowledge of diadromous species, we suggest:

CG7: On a regional scale, endangered anadromous populations of in particular Atlantic salmon,

Atlantic sturgeon, houting (Coregonus oxyrhynchus) and allis shad (Alosa alosa) should have self

sustainable populations. At present eel is also a species that can be considered as being at risk (Robinet

& Feunteun, 2002; ICES, 2006). For the Convention on International Trade in Endangered Species of

Wild Flora and Fauna (CITES) a work document has been released in which it is proposed to

reconsider the status of eel in Europe (CITES, 2006).

CG8: Basin-wide, self sustaining populations of geographically-relevant diadromous species (E.g.

twaite shad, river lamprey, smelt, thinlip mullet and flounder for the Schelde) should frequent the

estuary.

CG9: Within the estuary diadromous individuals from the > 0-group should be present.

We suggest following habitat needs:

HN7: On a regional scale fishing activities should be reduced and controlled.

HN8: Within a basin-wide level and within the estuary a good water quality (DO concentration) and

ecological connectivity permit unconstrained movements of diadromous fish between spawning and

nursery grounds and the adult habitats.

HN9: To assure the nursery function of the estuary the necessary amount of space and the proper

morphology to provide shelter and foot for > 0-group diadromous individuals should be present. This

part should also have a good water quality. We need to quantify the concept of a fish nursery (Beck et

al., 2001).


HN10: The presence of upstream situated spawning habitats and sufficient intertidal habitat are

required for the success of the rehabilitation programs of diadromous populations. The upstream

habitat should have clear and oxygenated water. No dredging activities should occur but mud should

be absent so that spawning can occur on gravel or sand.

4.3.3.4. Conservation goals and associated habitat needs for marine adventitious and

marine seasonal migrant species

We will not further consider conservation goals for these two ecological guilds since estuaries are not

considered as essential habitats for these ecological groups although it is of note that if the above

Conservation Goals are achieved then these groups will also be supported in the estuaries. Marine

seasonal migrants facultative exploit estuaries if conditions are favourable. Sprat is a good example of

such a life history strategy. Their presence in estuaries, however, reflects a good hydrological

connection between estuarine habitats and the ocean. Ecological connectivity requires suitable water

quality and quantity between habitats. Connectivity issues include poor water quality for transient

fishes, temporal (un)availability of tidal creeks due to marsh rising as a result of channel deepening or

strong tidal currents preventing upstream migration. These issues return for other ecological groups

and in particular for diadromous fish and are treated in this chapter.

4.3.4. Flow preference guilds

The conservation goals for these guilds are incorporated in freshwater resident species conservation

goals. Indeed freshwater species are rheophilic, eurytopic or limnophylic. We consider the latter group

less relevant for the estuary. In addition most op rheophilic A species are diadromous. As a summary

we repeat the main goal and essential habitat needs:

The main goal is the presence of a sustainable population of freshwater species including rheophilic A,

B and eurytopic species.

Rheophilic and eurytopic species need the ecological connectivity along longitudinal and transversal

river gradients which is translated in a good DO concentration and absence of barriers.

Accessible floodplains assure recruitment possibilities for eurytopic species.

Sheltered diversified intertidal habitat surfaces and subtidal areas with a diverse food supply in the

freshwater estuary are essential as nursery and feeding grounds for eurytopic and rheophilic species.

4.3.5. Reproductive special demands guild

We define special demands for the different groups within the salinity preference guild. The

importance of estuaries for recruitment of marine species decreases upstream along the decreasing

salinity gradient (Elliott et al., 1990). Reproductive habitats of marine fishes are generally located


offshore so estuarine conservation goals will specifically target younger life history stages. And those

will be similar to estuarine resident species demands. Most estuarine resident species, including partial

residents that live intertidally as adults during the breeding season, spawn within the intertidal zone.

Estuarine residents often produce demersal eggs since pelagic eggs and larvae cannot withstand the

occurring currents and wash-out events. For example, smelt (Osmerus eperlanus) will produce its eggs

in areas less liable to wash-out thus ensuring its young are retained in the estuary. Other species

migrate into the intertidal to spawn during high water levels. The benefits of intertidal spawning

follow from the increase of the embryonic development rate when emergent due to the food

availability and from temporal and spatial refuge for adult spawners and embryos. Success of

recruitment is affected by habitat factors while many larvae behave as habitat generalists and so

shelter and associated food availability will still be important. Sheltered microhabitats (e.g. tidepools)

will provide refuge from predators or competitive species but are subjected to high DO, salinity and

temperature fluctuations. Thus environmental variation will result in different species having

successful recruitment at different times. This is important for the coexistence and persistence of

assemblages. The upstream spawners consist of freshwater and diadromous species and will have

specific demands.

Based on the present knowledge of special reproductive demands we suggest:

CG10: On an estuary and habitat level offspring from diadromous, estuarine resident and freshwater

species should be present.

This presence is assured thanks to following habitat needs:

HN11: On a regional and basin-wide scale good water quality (DO concentration), a lack of physical

barriers and ecological connectivity should permit unconstrained movements for diadromous species

between the spawning grounds and adult habitat.

HN12: Within the estuary a variety of undisturbed macrohabitats should be present along the

longitudinal and transversal gradient. This includes in the mainstream sheltered habitats and

floodplains while upstream accessible spawning habitats such as sand beds, habitats with gravel and/or

stone bottom with clear and oxygenated water should be present.


4.3.6. Trophic guilds

Hydrographic regime and site specificity/substratum are factors controlling feeding controlling (Elliott

et al., 2002). Tidal elevation influences population size in fish. There is a relationship between the size

of the fish and the depth: as the body length increases, the depth of the water that the fish inhabits

increases (Gibson et al., 1995). Changes in tidal elevation by sea level rise or dredging activities may

increase the time for feeding but reduce the area and hence the carrying capacity. Salinity and DO

concentrations may also affect the feeding behaviour of fish. Intertidal areas are well defined as

juvenile fish feeding areas (Costa & Elliott, 1991). Mud and sandflats are important nursery areas for

plaice (Lockwood, 1972) as well as feeding areas for sea bass and flounder (Elliott & Taylor, 1989a,

b). The importance of mud and sandflats has been described above (see Elliott et al., 1998). Though in

general fish in estuaries are opportunistic (Breine et al., 2007b) food should be available and different

levels in the food web should be present. It may be assumed that the abundance and distribution of fish

feeding in the estuary is related to the quantity of food available in the intertidal and subtidal areas.

Though in general smaller in surface the intertidal area is the most important food source. Due to land-

claim the benthos community decreases and the fish communities in terms of species richness,

abundance and biomass reduce. Not only has the reduced the food supply diminished the estuaries’

value as nursery area, but also the loss of habitat.

We suggest following goals:

CG11: Basin-wide and within the estuary all species forming an undisturbed trophic web should be

present. This includes individuals from the groups represented in Table A.

CG12: According to the habitat type different species representing different links within the trophic

chain are present.

We therefore suggest following habitat needs:

HN13: Basin-wide and a variety of site substratum should be present along a longitudinal gradient.

HN14: The estuaries should have an undisturbed hydrographic regime.

HN15: The presence of shallow, low dynamic habitats providing a high and continuous supply of food

should be protected for estuarine resident and marine juvenile migrants. The availability of sheltered

diverse intertidal habitat surfaces and subtidal areas with a diverse food supply in the freshwater

estuary are essential feeding grounds for the freshwater resident species.


4.3.7. Stratum guilds

The stratum in which a fish species occur can be more or less sensitive to human impact. The most

widespread mechanism employed in the maintenance of navigable channels is physical dredging of

sediments from the bed of the channel for subsequent disposal elsewhere in the estuary, in coastal

waters or to land. At the disposal sites, fine-grained sediment may smother the bed and kill or cause a

change in the benthic community affecting benthic and demersal species. Clay particles will remain in

suspension for a longer time and high loads of suspended solids are harmful to pelagic fish (Bruton,

1985).

The goals we propose are corresponding to the more general goals or were already mentioned in one

of the described guilds. In general an undisturbed stratum should be considered as the ultimate habitat

need. As already mentioned dredging can not be stopped and therefore we suggest the following:

HN16: Within the ecosystem fish the ecological connectivity should allow fish to avoid effects.

HN17: Undisturbed habitat should be present.

These two habitat needs are also generic CG.

4.3.8. Tolerance guilds

Conservation goals for species tolerance to oxygen deficiency have been treated within the general

goals. Habitat sensitivity concerns the impact of fragmentation on species which depends on their

tolerance, the needed habitat, their dispersion ability (swim and jump capacities) and reproduction

capacities. Species adapted to a specific habitat type are generally less adaptive and therefore more

impacted by fragmentation e.g. rheophilic species (obligate). Eurytopic species (e.g. bream and rudd)

are less sensitive to fragmentation, while those needing a large habitat (eel, bleak, wells catfish,…) are

more sensitive. Species that do not migrate (brook lamprey, spined loach,…) are more sensitive than

species with a higher dispersion capacity (eel). Species having a high fecundity can easily re-colonize

new habitats. Species with parental care or nest builders having less offspring are more sensitive to

fragmentation Kroes et al. (2006). Habitat needs to correct fragmentation are similar as those for

diadromous species i.e. longitudinal and lateral migration should be possible within the estuary.

Habitat sensitivity deals also with special criteria such as presence of water vegetation for phytophilic

species (obligate or partial) or sensitivity to dredging. Conservation goals dealing with these criteria

have already been described and concern the presence of upstream undisturbed vegetated habitats and

floodplains.


4.4. Habitat status

4.4.1. Introduction

The progressive loss of estuarine areas remains a serious threat to the preservation of estuarine

biotopes and the integrity of the estuaries as a whole. Canalisation has resulted in an inhibition of

natural dynamics of sediment transport and water movements (Cattrijsse et al., 2002).

Observed loss of estuarine areas is most pronounced in intertidal areas, and the effects of that loss are

disproportionally high (Elliott & Taylor, 1989a, McLusky et al., 1992). Davidson et al. (1992) referred

to the process of gradual loss of shallow habitats as the estuarine squeeze. In the upper part of the

estuarine squeeze the high water line has shifted downshore through land-claim, building of sea

defences and the construction of docks. The driving forces of the lower part of the estuarine squeeze

are the extraction of sediments and the construction of barrages as well as sea level rise are. The

combined effort of these processes results in the loss of intertidal areas, the relative increase in deeper

subtidal areas and the consequent narrowing of the estuarine channel. Information on 14 European

estuaries showed that land-claim, channel management and barrages/impoundments are the most

important mechanisms resulting in disturbances on the estuarine fish communities ranging from loss

of species diversity over complete change from estuarine to a freshwater assemblage to the

disappearance of whole communities (Cattrijsse et al., 2002). Under natural conditions, erosion and

sedimentation are more or less in balance, and the natural loss of a habitat is likely to be mitigated by

sedimentation at a distant site. Even floods or storm-surges that may destroy complete habitats are

likely to be remedied. There is a natural drift of the main tidal channel, but in the Schelde it has ceased

due to consolidation of its banks. Due to these natural changes in geomorphology fish communities

will slowly change to a species composition that is better adapted to the new situation created by the

geomorphological changes. However, land-claim will decrease the benthic community and, if any of

the resources required by fish (space and food) are limiting, as a result also the fish communities in

terms of species number, abundance and biomass will reduce. Reduction of the food supply and the

loss of habitat reduce the estuaries’ value as nursery area and thus its carrying capacity. McLusky et

al. (1992) indicate the effects of loss of intertidal area on bird and fish populations. A similar effect is

observed with dredging although few reported quantitative effects on fish assemblage are available.

Due to the presence of a port and other artificial structures such as channel stabilisation with dykes an

increased flushing effect is created. This leads to segregation of the tidal currents.In turn an increase in

the ebb-related effects in the main channel and the flood current predominates in the adjacent

channels. The building of docks, wharves and jetties may induce loss of intertidal area or soft sediment

although they may create an artificial hard substratum which attracts a rocky shore community and the

associated fish fauna.


4.4.2. Habitat loss in the Schelde

Van Braeckel et al. (2006) assessed three human impact types on the River Schelde estuary. The

ecological changes during the past century were analysed using a hierarchical ecotope approach.

Changes in hydrodynamic and morphological parameters were analysed over the past century while an

overview of anthropogenic interventions and natural changes with potential effects on habitat acreage

and quality in the Zeeschelde and its tidal tributaries was given for past two centuries.

For the tidal regime Van Braeckel et al. (2006) analysed characteristics of the ten year mean high

water (MHW), mean low water (MLW), tidal asymmetry and the tidal amplitude. Temporal and

spatial patterns of these parameters were examined within different salinity zones in the Schelde, the

Durme and the Rupel (See Fig. 3). A gradual increase was observed for MHW in the whole Schelde

estuary while the MLW and tidal asymmetry showed a more irregular pattern. These are also more

sensitive to anthropogenic activities such as dredging and deepening in the navigation channel. MHW

and MLW changes resulted in a high increase of tidal amplitude in the Schelde and Rupel. The

observed changes can be explained by an analysis of the morphological evolutions in the estuary. Here

the authors indicated channel straightening, reduced flooding area and upstream shifts of the main

flood zones as the most important changes. Canalisation has a direct impact on the river ecosystem

e.g. by reducing its connectivity with its valley. In the fresh water zone with short retention time the

channel was shortened with 22% (10,5km). Consequently sinuosity declined and the river is now

classified as straight instead of meandering. The accompanying dike normalisations induced a loss of

95% or 826 ha of the former flooding area in this zone. In the tributaries canalisations equally resulted

in a significant reduction of connectivity with the surrounding valley. The Durme shortened with 12%

(2,5km) of its length compared to 1850. The disruption between the Rupel and its tidal brook the Vliet

caused the loss of one third of its tidal area. Fish migration between river and valley was hampered by

loss of natural flooding zones by blocking many tidal brooks. The largest reduction of storage width in

the Zeeschelde occurred in the mesohaline zone by the embankment of some vast polders (Fig. 3

Prosperpolder, Hedwigepolder, Groot Buitenschoor and Ketenisseschor). As mentioned, huge alluvial

and flooding areas such as ‘Kalkense Meersen’ were lost in the fresh water zone with short retention

time. A reduction of storage width was also found near Tielrode and St. Amands. The decline of

flooding area along the Durme from 1955 till 1990 resulted in an overall loss of 87,4% or 730 ha of

the adjacent flood zones compared to 1850. Figure 2 illustrates for each Omes segment the percentage

intertidal area loss (marches and mudflats) due to land claim for the last two centuries.


0

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

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Figure 2. Intertidal habitat loss for each OMES segment in the Schelde estuary between 1850 and

2001. The percentage habitat loss (right axis) refers to the combined losses of marsh and mudflat area

here indicated as eulittoral.

Finally, we observe a loss of ecological important shallow waters due to an increasing section of the

estuary having a high dynamic macrotidal regime. Shallow waters and weak slopes form a natural

protection against destructive erosion for tidal mud flats and marshes. This sheltering border is almost

completely lost near Doel, Lillo and the Ballastplaat. The littoral and tidal mud flat area decreased

strongly since 1850. Supralitoral zones (tidal marshes and flood systems) are most susceptible to

human pressure. In the 19th and the first half of the 20th century embankments and dike enforcements

caused direct loss of tidal marshes. In the last decennia indirect loss through increased tidal amplitude

and energy are a bigger threat. This historical analysis leads to the basic hypothesis that under the

present geometric and abiotic conditions the shallow waters and intertidal mudflats and marshes along

the Schelde can not persist in a sustainable way. In order to compare the habitat status of the different

estuaries in the North Sea Region, there should be a possibility to translate available existing habitat

maps and typologies for each estuary into GIS maps according to one common typology system.

These maps could then be used to compare the habitat status and historical habitat losses and gains in

the different estuaries in a quantified way. A first attempt whereby habitats are classified as

physiotopes and as ecotopes is explained in Stevens et al. (2008).


Figure 3. The Schelde estuary and sublittoral zones 1: Buitenschoor; 2: Hedwigepolder; 3:

Prosperpolder; 4: Doel & Ballastplaat; 5: Ketenissepolder; 6: Lillo; 7: Tielrode; 8: Sint-Amands; 9:

Kalkense Meersen

4.5. Mitigating measures

4.5.1. Introduction

The implementation of mitigating measures has to ensure the presence of a diversified fish population

as stipulated by the WFD. In particular, by 2015 the fish community should be comparable to that of

an estuary in a good status. For the Schelde being heavily modified this means that the fish community

should be that of an estuary having a good ecological potential. A fish-based index should score at

least good in each habitat of the estuary.

Essential habitats were described and the general quality needs were defined. For the latter we

highlighted that DO may be the bottleneck for a good functional ecosystem. Migrations to the essential

habitats were defined as a second bottleneck. Three scenario’s for the rehabilitation for the Zeeschelde

are described by Van den Bergh et al. (1999). In 2003 Van den Bergh et al. proposed nature

restoration scenario’s in the framework of the Nature aspect within the Long Term Vision of the

Schelde estuary. Mitigating processes are only meaningful when the water quality does not hamper a

sustainable fish population.


4.5.2. Mitigating processes in the Schelde

To reduce the tidal energy Van den Bergh et al. (2003) suggest to increase the stream profile by

adding shallow intertidal habitats downstream Dendermonde Temse. To avoid strong discharge

flushes retention areas upstream should be created. These restoration practices may result in an

increased DO in the estuary. Intertidal, shallow habitats, inland water treatment and floodplains are

essential to reinstate physical and chemical processes and hence increase the estuary’s self-purification

capacity and filtering capacity that sustains water quality (Van den Bergh et al., 2005, Lotze et al.,

2006). Aeration is the most important oxygen source in an estuary and can be improved by the

creation of tidal areas since their surface-to-volume area is high. Thus, estuarine functions and

carrying capacity are probably enhanced by increasing the surface of estuarine habitat. Adriaensen et

al. (2005) and Couderé et al. (2005) suggest that for the Zeeschelde an additional mudflat area of at

least 500ha will ensure a sustainable fish population. The same authors state that there is a need of

1500ha extra marsh surface to avoid silicon limitations which hamper the diatoms. As a second effect,

the creation of space will reduce the velocity and the associated effects such as erosion, turbidity and

sedimentation. Pas et al. (1998) state that Tielrodebroek, a flood controlling area of about 90ha nearby

the mouth of the River Durme, functions as a spawning and nursery area for certain freshwater

species. The Lippenbroek is a recently created flood control area situated in the freshwater part of the

Schelde-estuary. This pilot project assesses the possibility to combine the concept of a flood control

area (FCA) with nature development and the functions of intertidal areas, by use of simple sluice

constructions to introduce a controlled reduced tide (CRT). The creation of shallow area’s will be

beneficial for fish since these can be used as nursery and spawning places, shelter and resting areas

and feeding grounds. Floodplains have a higher productivity than the main channel and they provide

habitat complexity and habitat quality (Grift, 2001). Grift (2001) observed that fish rapidly colonized

newly created floodplain water bodies in the river Rhine. As such reconnected oxbow lakes have a

beneficial value for the riverine fish community since they provide a habitat that is better suitable for

0-group fish than the main stream. In addition they form important spawning and nursing areas for the

rheophilic species. Restoring interaction between the river and its floodplains is essential.

Physical and chemical barriers preventing this migration should be removed or bypassed. Solving fish

migration problems may not be a formal fish passage, but a natural solution such as restoration of old

meanders or the creation of a nature-like bypass channel. The basic steps to create a fish passages are

explained comprehensively by Kroes et al. (2006).

Upstream river rehabilitation and mitigation measures should improve the lateral connectivity of main

channels to ecotone complexity beneficial for fish (Angermeier & Karr, 1983; Sindilariu et al., 2006).

Turbidity should be reduced by construction of sedimentation areas such as wetlands where the flow is


reduced and this will also reduce sedimentation on the spawning grounds and increase their

availability. These wetlands increase the retention time in the Schelde.

5. Conclusions

An extensive literature study has allowed the definition of essential habitat requirements for fish

species both at general and guild levels. Some of the described conservation goals are applicable in

different guilds hence the importance of having CG which go across the different guilds. The proposed

conservation goals are qualitative and so further research is needed to provide quantitative

conservation goals. A crucial step constitutes the definition of stressors or impacts for each particular

estuary and the subsequent assessment of these impacts. Impacts comprise land-claim, recreation,

fishery, industrial and harbour activities including navigation and dredging, presence of barriers to

migration, aquaculture, coastal defences and so forth. Possible threats to estuarine habitats are listed

by McLusky & Elliott (2005) or Kennish (2004). Also the DPSIR approach is considered as a useful

tool to classify impacts. A report on impacts is in progress. Impacts were assessed according to Aubry

& Elliott (2006). In addition the carrying capacity of the different habitat types within the different

estuary zones should be calculated. Finally a fish-based evaluation system should be developed to

assess the status of the estuary and to evaluate the impact of the implementation of mitigating

measures.

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Annexes

Table

A L

ist

of

fish

sp

ecie

s f

ou

nd

in

th

e Z

eesch

eld

e a

nd

th

eir

gu

ild

s

SP

: S

tratu

m p

refe

rence; E

U:

Estu

arine u

se;

FP

: F

low

pre

fere

nce; T

G:

Tro

phic

guild

; R

SD

: R

epro

duction s

pecia

l dem

ands; S

: S

tratu

m

(adults);

HS

: H

abitat sensitiv

ity;

P:

pollu

tion into

lera

nce

Scie

ntific n

am

e

Com

mon n

am

e

SP

E

U

FP

T

G

RS

D

S

HS

P

Scie

ntific n

am

e

Com

mon n

am

e

Freshwater

Oligohaline

Mesohaline

Polihaline

Marine

Freshwater resident species

Diadromous species

Estuarine resident species

Marine adventitious species

Marine juveniles migrants

Marine seasonal migrants

Rheophilic (A)

Rheophilic (B)

Eurytopic

Limnophilic

Juvenile

Adult

Reproduction score freshwater

Reproduction score freshwater tributaries

Reproduction score oligohaline

Reproduction score mesohaline

pelagic fishes

Demersal fishes

Benthic fishes

Need undisturbed habitat for shelter

Fragmentation sensitivity

Intolerant

Abra

mis

bra

ma

Bre

am

1

1

0

0

0

1

0

0

0

0

0

0

0

1

0

P

B

0

0

0

0

0

1

0

0

0

0

Agonus c

ata

phra

ctu

s

Hook-n

ose

0

0

1

1

1

0

0

1

0

0

0

0

0

0

0

B

B

0

0

0

0

0

1

0

1

1

Alb

urn

us a

lburn

us

Ble

ak

1

1

0

0

0

1

0

0

0

0

0

0

1

0

0

P

O

0

1

0

0

1

0

0

0

0

0

Alo

sa f

alla

x

Tw

aite s

had

1

1

1

1

1

0

1

0

0

0

0

1

0

0

0

P

BF

1

1

0

0

1

0

0

0

1

1

Am

modyte

s tobia

nus

Sand-e

el

0

0

1

1

1

0

0

1

0

0

0

0

0

0

0

P

P

0

0

0

0

0

0

1

1

0

Anguill

a a

nguill

a

Eel

1

1

1

1

1

0

1

0

0

0

0

0

0

1

0

O

O

0

0

0

0

0

0

1

0

1

0

Aphia

min

uta

T

ranspare

nt goby

0

0

1

1

1

0

0

1

0

0

0

0

0

0

0

P

P

0

0

0

1

1

0

0

0

0

Ath

erina p

resbyt

er

Sand s

melt

0

0

1

1

1

0

0

0

0

1

0

0

0

0

0

P

B

0

0

0

0

1

0

0

0

0

Belo

ne b

elo

ne

Garf

ish

0

0

1

1

1

0

0

0

0

0

1

0

0

0

0

B

F

0

0

0

0

1

0

0

0

1

Blic

ca b

joerk

na

White b

ream

1

1

0

0

0

1

0

0

0

0

0

0

0

1

0

P

O

0

0

0

0

0

1

0

0

0

0

Cara

ssiu

s c

ara

ssiu

s

Cru

cia

n c

arp

1

0

0

0

0

1

0

0

0

0

0

0

0

0

1

PB

O

0

1

0

0

1

0

0

1

0

0

Chelid

onic

hth

ys lucern

us

Tub g

urn

ard

0

0

1

1

1

0

0

0

0

1

0

0

0

0

0

PB

B

F

0

0

0

0

0

1

0

1

Chelo

n labro

sus

Thic

k-lip

ped m

ulle

t 0

0

1

1

1

0

0

0

0

0

1

0

0

0

0

B

O

0

0

0

0

0

1

0

0

0

Cili

ata

muste

la

Fiv

e b

eard

ed r

ocklin

g

0

0

1

1

1

0

0

0

0

0

1

0

0

0

0

PB

B

0

0

0

0

0

0

1

1

0

Clu

pea h

are

ngus

Herr

ing

0

1

1

1

1

0

0

0

0

1

0

0

0

0

0

P

P

0

0

0

0

1

0

0

0

0

Cobitis

taenia

S

pin

ed loach

1

0

0

0

0

1

0

0

0

0

0

0

0

1

0

B

B

0

1

0

0

0

0

1

1

1

1

Cyclo

pte

rus lum

pus

Lum

psucker

0

0

1

1

1

0

0

0

0

0

1

0

0

0

0

B

BF

0

0

0

0

0

0

1

0

0

Scie

ntific n

am

e

Com

mon n

am

e

SP

E

U

FP

T

G

RS

D

S

HS

P

HA

RB

AS

INS

– C

onserv

ation g

oals

44

Scie

ntific n

am

e

Com

mon n

am

e

Freshwater

Oligohaline

Mesohaline

Polihaline

Marine


Diadromous species





Rheophilic (A)

Rheophilic (B)

Eurytopic

Limnophilic

Juvenile

Adult





pelagic fishes

Demersal fishes

Benthic fishes



Intolerant

Dic

entr

arc

hus labra

x

Sea B

ass

0

1

1

1

1

0

0

0

0

1

0

0

0

0

0

PB

B

F

0

0

0

0

0

1

0

0

0

Engra

ulis

encra

sic

olu

s

Anchovy

0

0

1

1

1

0

0

0

0

0

1

0

0

0

0

PB

P

B

0

0

0

0

0

1

0

0

0

Esox luciu

s

Pik

e

1

1

0

0

0

1

0

0

0

0

0

0

0

1

0

PB

V

F

1

1

0

0

0

1

0

1

1

1

Gadus m

orh

ua

Cod

0

0

1

1

1

0

0

0

0

1

0

0

0

0

0

PB

O

0

0

0

0

0

1

0

0

1

Gaste

roste

us a

cule

atu

s

Thre

e-s

pin

ed s

tickle

back

1

1

1

1

1

1

1

0

0

0

0

0

0

1

0

PB

B

1

1

0

0

1

0

0

1

1

0

Gobio

gobio

G

udgeon

1

0

0

0

0

1

0

0

0

0

0

0

1

0

0

B

B

0

0

0

0

0

0

1

0

0

0

Gym

nocephalu

s c

ern

uus

Ruff

e

1

1

1

1

0

1

0

0

0

0

0

0

0

1

0

PB

B

0

0

0

0

0

0

1

1

0

0

Lam

petr

a f

luvia

tilis

R

iver

lam

pre

y

1

1

1

1

1

0

1

0

0

0

0

1

0

0

0

B

F

1

1

0

0

0

0

1

1

1

1

Leucis

cus c

ephalu

s

Euro

pean c

hub

1

1

0

0

0

1

0

0

0

0

0

0

1

0

0

PB

O

0

1

0

0

1

0

0

0

1

0

Leucis

cus idus

Ide

1

1

0

0

0

1

0

0

0

0

0

0

1

0

0

PB

B

F

0

1

0

0

1

0

0

0

1

1

Leucis

cus leucis

cus

Dace

1

1

0

0

0

1

0

0

0

0

0

1

0

0

0

B

B

1

1

0

0

1

0

0

0

1

1

Lim

anda lim

anda

Dab

0

0

1

1

1

0

0

0

0

1

0

0

0

0

0

B

BF

0

0

0

0

0

0

1

1

1

Lip

aris lip

aris

Sea s

nail

0

0

1

1

1

0

0

1

0

0

0

0

0

0

0

B

B

0

0

0

0

0

0

1

1

1

Liz

a r

am

ado

Thin

lip m

ulle

t 1

1

1

1

1

0

1

0

0

0

0

0

1

0

0

PP

B

O

0

0

0

0

1

0

0

0

1

1

Lota

lota

B

urb

ot

1

1

1

0

0

1

0

0

0

0

0

0

1

0

0

B

F

0

1

0

0

0

0

1

0

1

1

Merlangiu

s m

erlangus

Whitin

g

0

0

1

1

1

0

0

0

0

1

0

0

0

0

0

B

BF

0

0

0

0

0

1

0

1

0

Mis

gurn

us f

ossili

s

Wheath

erf

ish

1

1

0

0

0

1

0

0

0

0

0

0

0

0

1

B

B

0

1

0

0

0

0

1

1

0

0

Myoxocephalu

s s

corp

ius

Bull

rout

0

1

1

1

1

0

0

1

0

0

0

0

0

0

0

B

BF

0

0

0

0

0

0

1

0

0

Osm

eru

s e

perlanus

Sm

elt

1

1

1

1

1

0

1

0

0

0

0

0

1

0

0

B

BF

1

1

0

0

1

0

0

0

1

1

Perc

a flu

via

tilis

P

erc

h

1

1

1

0

0

1

0

0

0

0

0

0

0

1

0

B

BF

0

0

0

0

1

0

0

0

0

0

Petr

om

yzo

n m

arinus

Sea lam

pre

y

1

1

1

1

1

0

1

0

0

0

0

1

0

0

0

B

F

1

1

1

1

0

1

0

1

1

1

Pholis

gunnellu

s

Butt

erf

ish

0

0

1

1

1

0

0

1

0

0

0

0

0

0

0

PB

B

0

0

0

0

0

0

1

1

Scie

ntific n

am

e

Com

mon n

am

e

SP

E

U

FP

T

G

RS

D

S

HS

P

HA

RB

AS

INS

– C

onserv

ation g

oals

45

Scie

ntific n

am

e

Com

mon n

am

e

Freshwater

Oligohaline

Mesohaline

Polihaline

Marine


Diadromous species





Rheophilic (A)

Rheophilic (B)

Eurytopic

Limnophilic

Juvenile

Adult





pelagic fishes

Demersal fishes

Benthic fishes



Intolerant

Pla

tichth

ys fle

sus

Flo

under

1

1

1

1

1

0

0

1

0

0

0

0

0

1

0

PB

B

F

0

0

0

0

0

0

1

1

1

0

Ple

uro

necte

s p

late

ssa

Pla

ice

0

0

1

1

1

0

0

0

0

1

0

0

0

0

0

B

B

0

0

0

0

0

0

1

1

Pom

ato

schis

tus lozanoi

Lozano's

goby

0

0

1

1

1

0

0

0

1

0

0

0

0

0

0

PB

B

0

0

0

0

0

0

1

0

Pom

ato

schis

tus m

icro

ps

Com

mon g

oby

0

1

1

1

1

0

0

1

0

0

0

0

1

0

0

PB

B

0

0

1

1

0

0

1

1

Pom

ato

schis

tus m

inutu

s

Sand g

oby

0

1

1

1

1

0

0

1

0

0

0

0

1

0

0

PB

B

0

0

1

1

0

0

1

1

Psett

a m

axim

a

Turb

ot

0

0

1

1

1

0

0

0

0

1

0

0

0

0

0

PB

F

0

0

0

0

0

0

1

1

Pungitiu

s p

ungitiu

s

Nin

e-s

pin

ed s

tickle

back

1

1

1

0

0

1

0

0

0

0

0

0

0

1

0

PB

B

1

1

0

0

0

1

0

1

1

0

Rhodeus s

ericeus

Bitte

rlin

g

1

1

0

0

0

1

0

0

0

0

0

0

0

0

1

PP

B

P

0

1

0

0

0

0

1

1

1

0

Rutilu

s r

utilu

s

Roach

1

1

1

0

0

1

0

0

0

0

0

0

0

1

0

O

O

0

0

0

0

1

0

0

0

0

0

Salm

o tru

tta

Bro

wn tro

ut

1

1

1

1

1

0

1

0

0

0

0

1

0

0

0

B

BF

0

1

0

0

1

0

0

0

1

1

Sander

lucio

perc

a

Zander

1

1

1

0

0

1

0

0

0

0

0

0

0

1

0

PB

F

0

0

0

0

0

1

0

0

0

0

Scard

iniu

s e

ryth

rophth

alm

us

Rudd

1

1

0

0

0

1

0

0

0

0

0

0

0

0

1

O

O

0

0

0

0

1

0

0

1

0

0

Scophth

alm

us r

hom

bus

Brill

0

0

1

1

1

0

0

0

1

0

0

0

0

0

0

PB

B

F

0

0

0

0

0

0

1

1

Silu

rus g

lanis

W

els

catfis

h

1

1

0

0

0

1

0

0

0

0

0

0

0

1

0

PB

V

F

1

1

0

0

0

0

1

0

1

0

Sole

a s

ole

a

Sole

0

1

1

1

1

0

0

0

0

1

0

0

0

0

0

PB

B

0

0

0

0

0

0

1

1

1

Spra

ttus s

pra

ttus

Spra

t 0

1

1

1

1

0

0

0

0

0

1

0

0

0

0

P

P

0

0

0

0

1

0

0

0

Syngnath

us a

cus

Gre

ate

r pip

efish

0

1

1

1

1

0

0

1

0

0

0

0

0

0

0

PB

B

F

0

0

0

1

0

0

1

1

1

Syngnath

us r

oste

llatu

s

Nils

son's

pip

efish

0

1

1

1

1

0

0

1

0

0

0

0

0

0

0

P

B

0

0

0

1

0

0

1

1

1

Tin

ca t

inca

Tench

1

0

0

0

0

1

0

0

0

0

0

0

0

0

1

PP

B

0

1

0

0

0

1

0

1

0

0

Trisopte

rus luscus

Pouting

0

1

1

1

1

0

0

0

0

1

0

0

0

0

0

B

BF

0

0

0

0

0

1

0

0

Zoarc

es v

ivip

aru

s

Viv

iparo

us b

lenny

0

1

1

1

1

0

0

1

0

0

0

0

0

0

0

PB

B

0

0

0

1

0

0

1

1

Table B: Essential fish habitats in relation to ecological guilds

MJ ER AN CA RA RB EU LIM

FRESHWATER TIDAL AREA

subtidal channel X X X X X

intertidal sandbanks X X

intertidal mudflat X X X

intertidal creeks X X X X X

intertidal marsh X X X X X

floodplain X X X X

BRACKISHWATER TIDAL AREA

subtidal channel X X X X


intertidal mudflat X X

intertidal creeks X X X X

intertidal marsh X X X X

MARINE TIDAL AREA

subtidal channel X X X X


intertidal mudflat X X

intertidal creeks X X X X

intertidal marsh X X X X

Mussel beds X X

Eelgrass beds X X

MJ: marine juvenile migrants; MA: marine adventitious species; MS: marine seasonal migrants; ER:

estuarine resident species; AN: anadromous species; CA: catadromous species; RA: rheophilic A

(obligate) species; RB: rheophilic B; EU: eurytopic species and LIM: limnophilic species.

harbasins report: water management strategies for estuarine and

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