Fishes in Estuaries
Edited by Michael Elliott, Krystal Hemingway
Copyright © 2002 by Blackwell Publishing Ltd
Chapter 2
Habitat Use by Fishes in Estuaries and
Other Brackish Areas
L. Pihl, A. Cattrijsse, I. Codling S. Mathieson, D.S. McLusky
and C. Roberts
2.1 Introduction
In assessing the importance of estuaries for fish and macrocrustaceans it is of note that estuaries consist of a complex mixture of many distinctive habitat types and that these habitats do
not exist in isolation. Rather, there are physical, chemical and biological links between them,
for example in their hydrology, in sediment transport, in the transfer of nutrients and in the
way that mobile animals move between them both seasonally and during single tidal cycles
(Davidson etal., 1991). The value of any habitat ‘patch’ in an estuary as a fish habitat may
depend both on its proximity and/or degree of connectivity to other habitat patches.
The definition of an estuary as adopted by the Habitats Committee on 25 April 1996 under
the Habitats &Species Directive (European Council Directive, 1992 (92/43/EEC)) isgiven as
‘ Downstream part o f a river vallej subject to the tide and extending from thelimit ofbrackish waters. River estuaries are coastalinlets where, unlike “large shallow inlets and bays”
there isgenerally a substantial freshwater influence. Themixingoffreshwater and sea water
and the reduced current flowsin the shelter ofthe estuarylead to deposition o f h esediments,
often forming extensive intertidal sand and mud flats. Where the tidal currents are faster
than flood tides, most sediments deposit to form a delta at the mouth of the estuary’ (Romao,
1996).
It is considered here that nine habitats of importance for estuarine fish can be defined
and described, and the selected habitats encompass all major environmental categories/zones
found within European estuarine waters. These classifications provide a holistic view of habitat variation together with detailed information on fish/macrocrustacean associations, both
within and between estuarine locations. Definitions of estuarine habitats have previously
not been attempted on the basis of their importance for, and use by, fish and macrocrustaceans, although the principal definition of natural habitats attempted at a European level is the
CORINE classification (Commission of the European Communities, 1991). This has been
used as the basis for defining habitats for the implementation of the European Habitats & Species Directive (European Council Directive, 1992 (92/43/EEC)). This assessment is also of
relevance for the European Water Framework Directive (European Council Directive, 2000
(2000/60/EC)), in which coastal and transitional water bodies will require to be monitored
and managed. In this context, the habitats included here are contained mostly in the term
‘transitionalwaters’, but also in ‘coastalwaters’.Although the present assessment is based on
Habitat Use byFishes in Estuaries
11
European estuarine habitats and areas, it is considered that the approach and the conclusions
will apply to other temperate areas and some tropical ones.
The definitions of marine and estuarine habitats in the CORINE list are generally inappropriate for consideration of the use of individual estuarine habitats by fish and mobile macrocrustaceans. This is partly due to the range of geographical scales (e.g. the use of ‘Estuaries’, a relatively large physiographic unit, as a CORINE habitat) and the lack of coverage in
the classification of some estuarine habitats such as seagrass beds, limited in the CORINE
classification to ‘ Posidonia beds’,which are restricted to southern Europe. Classification of
estuarine habitats for general purposes must also consider terrestrial habitats (e.g. sand dunes,
coastalgrasslands, etc.) and the maritimekerrestrial marginwhich have no direct significance
for fish and thus are not considered further here.
Following studies throughout Europe, the agreed list of fish habitats to be considered is:
Tidal freshwater;
Reed beds;
Saltmarsh (intertidal vegetated habitats) ;
Intertidal soft substratum;
Intertidal hard substratum;
Subtidal soft substratum;
Subtidal hard substratum;
Subtidal sea grass beds (subtidal vegetated habitats) ;
Biogenic reefs.
A further habitat
the pelagic part of the water column can be considered, but it is not
included here as a separate habitat but rather as a component part of each habitat listed.
In considering the above habitat types, it is necessary to assess their value for fish and
shellfish as described by associated faunal guilds within both spatial and temporal scales. The
recognition of such associations will contribute to establishing a set of criteria that may be
required for future environmental and commercial management purposes.
~
~
2.2 Habitat definitions and descriptions (including subhabitats)
The definitions described below have been created here and are only used for estuaries. Salinity values are given throughout as psu (practical salinity units).
2.2.1 Tidal freshwater
2.2.1.1 Habitat definition
In tidal estuarine areas, this is the zone upstream of saline influence and salinity values are
typically less than 0.5 psu. Water movements in this habitat are highly dynamic, with tidal
rise and fall, and tidal reversals in the direction of river water flow, resulting from river flow
backing up against saltwater incursion downstream. Subtidal and intertidal habitats here are
included together in the following analysis.
12
Fishes in Estuaries
2.2.1.2 Habitat description
This habitat is described as the tidal freshwater of rivers, downstream of the tidal limit but
upstream of the effects of saline water incursion. The extent of the zone is dependent on relative volumes of freshwater and saline water inflows and thus climatic condition, tidal range,
and geological or physiographic characteristics. Channels are typically steeper-sided and
narrower than in the downstream estuary, leading to a narrow intertidal zone. Subtidal and intertidal habitats are typically composed of mobile substrata, from fine silts to coarse gravels.
Hard substrata may be more significant in areas of limited sediment supply a n d o r scoured
areas.
2.2.1.3 Subhabitats
These include intertidal soft substrata (e.g. sediment banks and shoals), subtidal soft substrata, creeks, backwaters and gravel riffles. The soft substrata habitats are regarded as subhabitats in tidal fresh water (although also described as a habitat below) since the most significant habitat feature is the tidal nature of the freshwater. Additionally, reed beds may form
a significant intertidal subhabitat component in these regions. However, due to the potential
significance of reed beds in some estuarine areas of limited tidal movement, they are described here as a separate habitat.
2.2.2 Reed beds
2.2.2.1 Habitat definition
Reed beds are dense stands of tall herbaceous plants in low salinity zones, upstream of the turbidity maximum (typically 0- 5 p s ~ )The
. typical dominant flora includes Phragmites communities and, in tidal fresh water, Phalaris arundinacea.
2.2.2.2 Habitat description
In tidal estuarine areas, reed beds form in the upper intertidal zone, while in non-tidal estuarine areas, such as parts of the Baltic, they may form in shallow permanently subtidal areas.
The habitat may be extensive, although the linear extent on tidal estuaries is linked to the
length of the tidal freshwater zone. While subtidal reed beds in non-tidal estuarine areas are
permanently flooded and continuously accessible to fish, intertidal reed beds in tidal estuarine
areas are accessible to fish only when flooded, either periodically during the spring cycle of
high tides, or during periods of river floods.
2.2.2.3 Subhabitats
These include intertidal and subtidal reed beds. Intertidal reed beds may have open water
pools and creeks which are unvegetated.
Habitat Use byFishes in Estuaries
13
2.2.3 Sal-arsh
2.2.3.1 Habitat definition
Saltmarshes are intertidal, sediment-based, macrophyte-dominated, saline-influenced habitats. Europeansaltmarshes may develop from the mean high water level (MHWL) (Beeftink,
1977), to the upper shore where, in undisturbed systems, they may undergo transition to
brackish, freshwater or terrestrial habitats.
2.2.3.2 Habitat description
Saltmarsh develops where tidal waters are sufficiently quiescent to allow sediment to settle
out from suspension, andwhere conditions are suitable for settlement and growth of a limited
number of halophytic plant species in assemblages which undergo species successions as
the saltmarsh matures. Areas of saltmarsh often form a complex mosaic in estuaries with
other intertidal habitats such as mudflats. A wide range of typical saltmarsh types have been
described, such as estuarine fringing marsh, beach-head marsh and barrier beach marsh. The
development of linear or dendritic creeks systems is additionally a typical feature of maturing
marshes. These creeks may be of great significance to fish as the principal means of entry
to the marsh environment. Marsh surface pools are often a distinctive feature of saltmarsh
habitats.
2.2.3.3 Subhabitats
A variety of subhabitats may be present in saltmarshes, ranging from permanently subtidal
areas of creeks or pools, intertidal creeks which are largely drained at low tide, mud walls,
mud banks and mud boulders (created by undercutting or slumping of walls), to the upper
marsh surface (with permanent pools or flooded during higher tides).
2.2.4 InterfidalSOBsubstratum
2.2.4.1 Habitat definition
This includes areas of unvegetated intertidal habitats in tidal estuarine areas, lying between
the highest and lowest tides, and composed predominantly of sediments ranging from fine silt
to coarse sands or shingles.
2.2.4.2 Habitat description
These areas are sediment-based habitats between the high and low water mark, largely without higher plants. They are defined in the CORINE biotopes manual (Commission of the
European Communities, 1991) as ‘sands and muds, submerged for part of the tide, devoid
of vascular plants, but usually coated by blue-green algae and diatoms’. In many mesotidal
or macrotidal estuaries, these habitats are extensive and relatively gently sloping in nature.
The sediment composition of any particular area of intertidal soft substratum depends on a
14
Fishes in Estuaries
number of factors, including the nature and quantity of sediment supply from freshwater and
marine sources, degree of exposure to water currents or wave action, and salinity regime.
Intertidal soft substrata usually contain a high density and large biomass of macrobenthos,
which provides abundant food for estuarine fish and macrocrustaceans when the intertidal
flats are covered by the tide and may be of recognised international importance for wading
birds and waterfowl during low water periods (McLusky, 1989).
2.2.4.3 Subhabitats
In many European estuaries, intertidal soft substratum habitats form an intertidal habitat
mosaic with other habitats, such as saltmarsh. The degree to which habitats should be divided
may not be clear-cut, such as that between pioneer saltmarsh and open mud flat. The extent to
which intertidal subhabitats are subdivided here may, therefore, be dependent on the extent
and nature of habitat patches, and their use by fish, in an individual estuary. Dispersed small
patches of saltmarshvegetation on a mudflat, for example, might be considered in some cases
to be insignificant as a fish habitat, although this might not be true in other sites.
In comparison with most other habitats here, intertidal soft substrata are superficially relatively homogeneous. In areas of reduced wave energy and current speeds (such as in more
sheltered inner estuarine areas), finer sediments may be deposited to form mudflats (Elliott
etal., 1998).Where wave energies or current speeds are too high to allow settlement of finer
sediments (such as outer estuarine areas), sandflats may develop. Subhabitats may include
creeks and banks, pools of standing water (during low tide periods), and intertidal patches
of sea grasses (although not a significant structural feature by comparison with subtidal sea
grass beds).
Biogenic structures may also be a component subhabitat of intertidal soft substrata (e.g.
extensive beds of the bivalve genus Mytilus)but are considered here as a separate habitat
in creating biogenic reefs (Section 2.2.9). In estuaries, suffering from the eutrophic effects
of excessive inorganic nutrients from anthropogenic sources, dense beds of ephemeral macroalgae (e.g. from the genus Ulva) may constitute a significant seasonal subhabitat (Scott et
al., 1999).
2.2.5 Intertidal hard substratum
2.2.5.1 Habitat definition
These are areas of unvegetated or vegetated intertidal habitats in tidal estuarine areas, lying
between the highest and lowest tides, and composed predominantly of hard substrata ranging
from gravels to bedrock.
2.2.5.2Habitat description
Although European estuaries are predominantly characterised by their soft sediment habitats,
many also have areas of intertidal hard substrata, particularly where wave action or current
speeds are sufficiently high to prevent long-term settlement of sediments. Intertidal hard substrata may be dominated to a considerable degree by macrophytic algae, although domination
Habitat Use byFishes in Estuaries
15
by animal crusts (barnacles, mussels) may occur on intertidal hard substrata subjected to a
greater degree of water movement due to wave action or currents.
2.2.5.3 Subhabitats
Intertidal hard substrata may be composed of an extremely heterogeneous mixture of subhabitats, depending on a complex interaction of physical and environmental factors such as
underlying geology, degree of exposure to wave action and hydrodynamic characteristics.
Subhabitats vary from hard but potentially mobile gravel, cobble or boulder habitats, to immobile rock habitats, such as ridges and bedrock features. Boulder habitats may provide
many cryptic under-boulder niches. Subhabitats in rock-based intertidal habitats which may
have significance for fish include caves, overhangs or permanent pools. Subhabitats may
derive from biological features, such as beds of macroalgae or biogenic reefs (see Section
2.2.9).
Artificial hard substrata may also form an important component of the hard substrata (both
intertidal and subtidal) of many developed estuaries. Such subhabitats, while unlikely to support populations of estuarine species in wholly natural circumstances, may provide important
habitat heterogeneity in an otherwise degraded estuarine environment (see Sections 2.2.5 and
2.2.7).
2.2.6 Subtidal SOBsubstratum
2.2.6.1 Habitat definition
These are areas of permanently subtidal unvegetated habitats in estuarine areas, composed
predominantly of sediments ranging from fine silts to coarse sands. In tidal estuarine areas,
this habitat lies below the level of lowest tides.
2.2.6.2 Habitat description
In estuarine areas of very limited tidal range, the majority of soft substratum habitats are
likely to be permanently subtidal. In tidal areas, most estuaries have permanently subtidal
soft substratum habitats. Some estuarine basins may, however, drain almost completely of
water during the low water period and the extent of this habitat may be very limited in such
circumstances. In tidal estuaries, subtidal soft substrata typically support lower densities and
biomass of benthic invertebrates than adjacent intertidal areas of soft substrata (Elliott et
al., 1998). The Habitats Directive (92/43/EEC) has a habitat category ‘sandbankswhich are
slightly covered by sea water all the time’ which is defined as ‘Subtidal sandbanks, permanently submerged. Water depth is seldom more than 20 m below Chart Datum’.
2.2.6.3 Subhabitats
Like intertidal soft substrata, subtidal soft substrata ranges in composition from fine silts to
coarse sands, dependent on sediment supply, hydrodynamic regime and salinity characteristics. Permanently subtidal soft substrata are found in a wide range of estuarine areas, includ-
16
Fishes in Estuaries
ing creeks, main channel areas, deeper water areas, backwaters and lagoons. Clearly, other
habitats described here have, or may have, subtidal soft substrata as a component (e.g. tidal
freshwater, saltmarsh, subtidal seagrass beds), although this feature is not the main defining
component. Subhabitats may be defined according to sediment composition, whether mud or
sand-based or some intermediate composition. Coastal lagoons may have limited intertidal
soft substrata but the majority of soft substrata are likely to be subtidal.
2.2.7 Subtidal hard substratum
2.2.Z 1 Habitat definition
Permanently subtidal areas of unvegetated or vegetated habitats composed predominantly
of hard substrata ranging from gravels to bedrock. In tidal estuarine areas, these habitats lie
below the level of lowest tides.
2.2.Z 2 Habitat description
Like hard substrata in intertidal habitats, the nature of subtidal hard substrata depends on the
underlying geology and hydrodynamic regime. Hard substrata may range from potentially
mobile hard substrata such as gravels, cobbles and boulders, to rock features. Macroalgae
may also form a significant feature of subtidal hard substrata, and may add considerable
structural heterogeneity to otherwise relatively featureless rock habitats.
2.2.Z3 Subhabitats
Subtidal hard substrata may be composed of a wide range of subhabitats. These may vary
from hard but potentially mobile gravel, cobble or boulder habitats, to immobile rock habitats, such as ridges and bedrock features. Boulder habitats may provide many cryptic underboulder niches. Many subhabitats in rock-based subtidal habitats may have significance for
fish, including ridges, crevices, caves, overhangs and vertical rock faces. Subhabitats may
derive from biological features, such as beds of macro-algae or biogenic reefs (see Section
2.2.9). Beds of maerl, accumulations of twig-like or nodular unattached calcareous red algae,
may be a significant subhabitat in some northern estuarine environments, such as semiisolated saline lagoons.
Artificial hardsubstrata may also form an important component of the hardsubstrata (both
intertidal and subtidal) of many developed estuaries. Such subhabitats, while unlikely to support populations of estuarine fish species in wholly natural circumstances, may provide important habitat heterogeneity in an otherwise degraded estuarine environment (see Sections
2.2.5 and 2.2.7).
2.2.8 Subtidal seagrass beds
2.2.8.1 Habitat definition
These are subtidal vegetated habitats, based on soft substrata, and are dominated by
Habitat Use byFishes in Estuaries
17
halophytic macrophytes adapted to complete and continuous submergence in water of low to
high salinity.
2.2.8.2 Habitat description
Seagrass beds may be extensive in size and relatively stable over many years. They add
considerable vertical structure (up to 1 m in height for some species of seagrass) to the soft
substratum environments on which they develop. Typical genera which comprise European
sea grass beds include Zostera, Posidonia (in the Mediterranean) and Ruppia. Due to their
structural significance in some lagoonal and other brackish water habitats, beds of stoneworts
or charophytes (complex algae which are often encrusted with calcium carbonate deposits)
are also included.
2.2.8.3 Subhabitats
Newly developed or regenerating patches of seagrass beds (e.g. following physical disturbance from natural or anthropogenic sources) may have a different topographical nature to
that of mature seagrass beds.
2.2.9 Biogenic reefs
2.2.9.1 Habitat definition
This is an elevated structure or extensive epibenthic bed, either intertidal or subtidal, which
is built from calcareous or other concretion-forming organisms, or is formed from surfacedwelling bivalve molluscs.
2.2.9.2 Habitat description
Extensive beds of bivalve molluscs (e.g. from the genera Mytilus,Ostrea, Modiolus)may add
considerable heterogeneity to the hard or soft substrata on which they develop. Reef structures may be built by sabellid or serpulid polychaete worms. These may have considerable
structural complexity, including vertical development, and may be based on hard substrata or
on small hard features based in largely soft substrata.
2.2.9.3 Subhabitats
The unique features of each type of biogenic reef may constitute a subhabitat within this
habitat. Large bivalve beds in intertidal areas may have pools of standingwater as a subhabitat
during low-water periods. Mature bivalve reefs may have a different topography to reefs,
or parts of reefs, composed of young individuals. Serpulid and sabellarian reefs may have
cryptic niches as a sub-habitat (Holt etal., 1998).
18
Fishes in Estuaries
2.3 Quantification of fish habitats in selected European
estuarine systems
2.3.1 Introduction
This section quantifies the extent of the nine previously identified fish habitats in selected European estuarine systems by gathering information from 26 estuarine systems in 10 European
countries. These estuarine systems were selected principally on the basis of readily available
information on their fish communities.
2.3.2 European context of the selected estuarine systems
The 26 estuarine systems referred to in this chapter are distributed among three biogeographical regions within European coastal waters: BoreaVAtlantic (Atlantic and North Sea
coasts) ; BaltidSkagerrak and Mediterranean. Figure 2.1 shows the primary countries from
which data for the volume have been derived, together with the main water bodies referred to
throughout the chapters. Figures 2.2- 2.14 show selected estuaries in further detail.
2.3.2.1 Boreal/Atlantic region
The BoreaVAtlantic region covers the region from Denmark to Gibraltar, including the British Isles, and represents all the estuarine systems possessing predictable and pronounced influence from semi-diurnal tides. The estuaries of this region thus possess substantial intertidal
habitats. The total estuarine habitat of the region is approximately 18 600 km2;details of the
distribution of this total among countries is shown in Table 2.1.
The estuarine resource of most countries has not been quantified in detail. However, within
Great Britain (England, Scotland and Wales), 155 separate estuaries have been identified,
selected on the criteria of having either a tidal channel of >2 km or soft sediment shores
of >500 m at low water. These 155 estuaries have a total area of 5293 km2, incorporating
3079 km2 of intertidal area and 2214 km2 of subtidal area. The 155 estuaries have a total
shorelineof9231 k m a n d a totaltidalchannellengthof 2451 km (Davidson etal., 1991).The
Table 2.1
1991.)
The total estuarine habitat of the BorealLAtlantic region of Europe. (Modified from Davidson et al.,
Country
Area (km’)
Percentage
Britain
Denmark wadden Sea)
Germany (Wadden Sea)
Netherlands
Dutch Delta (Ems-Dollard)
Belgium
France
Spain
Portugal
Ireland
Total BorealiAtlantic
5293
1004
3790
2691
498
19
2729
1061
625
87 1
18581
28.5
5.4
20.4
14.5
2.6
0.1
14.7
5.7
3.4
4.7
100
Habitat Use byFishes in Estuaries
19
u
5
5
'5
Fishes in Estuaries
20
N
A
Fig. 2.2
The Mersey estuary, UK.
average size of a UK estuary is therefore 34 km2,with an intertidal area of 19.8 km2,a subtidal
area of 14.2 km2,and a shoreline of 59 km.
The estuaries included here have been selected on the basis of available records of fish
populations and represent only a small proportion of the total number of estuaries. It should
be recognised that they are generally larger than the mean size of UK estuaries, and indeed
represent some of the largest estuaries in Britain. Thus they represent a greater proportion of
the total estuarine resource than is apparent by simply counting estuaries.
Among the five British estuaries in the present study, four (Forth, Humber, Thames and
Mersey) were identified as estuaries for the purposes of the assessment of the extent of the
UK estuarine resource (Davidson et al., 1991), but one site (Loch Etive) was not. The four
estuaries represent 2.6% of the total number of identified estuaries, but 10.9% of the total
British estuarine area, and 9.7% of the estuarine intertidal habitat and 12.5% of the subtidal
habitat. As some of the largest British estuaries are included, the diversity and abundance of
fish assemblages in these may be greater than for a British estuary of average size.
A similar assessment of estuarine habitats on a national basis is not available for the
other countries in the BoreaVAtlantic region. Accordingly, the contributions of the selected
estuarine systems, in terms of total surface area, have been approximated (Table 2.2) using
the areas given in Table 2.1. The estuarine systems included in the present study comprise
11- 64% of the estuarine resource in the representative countries, and the role of the estuaries
with regard to commercial species should, therefore, be adequately described.
2.3.2.2 BaltidSkagerrak region
The BaltidSkagerrak region covers the area east from the interface with the North Sea
Habitat Use byFishes in Estuaries
21
N
Mattersey
Gainsborough
Trent
10 km
Dunham
Winthorpe Bridge
Fig. 2.3 The Humber estuary, UK. (Modified from National Rivers Authority, 1993.)
between Norway and Denmark and includes all of the Baltic Sea. Estuarine systems in this
region are not influenced by significant tidal movement, and both salinity and temperature
may be significantly reduced in comparison with the BoreaVAtlantic area. The seasonal occurrence of significant ice cover may modify habitats physically and may, thereby, affect fish
communities.
There has been no overall quantification of the estuarine systems of this region. Accordingly, it is not possible to describe the context of the selected estuarine systems in quantitative
terms. The five selected estuarine systems are located in three different locations within the
region: two systems in the Skagerrak (Gota River and Gullmarsfjord insweden) ; two systems
on the Baltic coast of Germany and Poland (Darss-Zingster Boden and OderhaffBtettin Lagoon) and one on NW &and, off the coast of Finland.
Fishes in Estuaries
22
Table 2.2 Approximate total surface areas of the selected 26 estuarine systems in relation to the total area national
resources in participating countries in the BorealiAtlantic region.
Participating country
Total surface area
of estuaries' (km2)
Total surface area of
selected estuarine systems (km2)
Percentage of national resource
France2
Germany3
Netherlands4
Portugal5
Spain6
UK'
2729
3790
2691
625
1061
5293
409
1713
1144.4
402.7
149.7
576.3
15
45
42
64
14
11
'
Adapted from Davidson et al. (199 1).
2Loire,Seine, Bay of Somme.
3Elbe,Weser.
4Ems-Dollard,Westerschelde, Oosterschelde
5Riade Aveiro, Tagus, Mira, Obidos.
6Bayof Cadiz, Guadalquivir.
'Forth, Humber, Thames, Mersey.
N
Beckton STW
10 km
Fig. 2.4
The Thames estuary, UK.
2.3.2.3 Mediterranean region
The Mediterranean region covers the area east from the Strait of Gibraltar, and includes
all of the Mediterranean Sea. As with the BaltidSkagerrak region, estuarine systems in the
Mediterranean are not influenced by significant tides. In contrast, however, salinity is not
significantly reduced and average temperatures are higher. Estuaries in the Mediterranean
are typically coastal lagoons with a freshwater influence. The total coastal lagoon area in
the Mediterranean has been assessed at 6500 km2 (Crivelli etal., 1995),and the distribution
between countries is shown in Table 2.3. The two estuarine systems selected in this volume
Habitat Use byFishes in Estuaries
Fig. 2.5 The Elbe estuary, Germany.
N
A
Fig. 2.6 The Waddensea, IJsselmeer and Eems estuary, Netherlands.
23
24
Fishes in Estuaries
N
GREVELINGENMEER
10 km
Fig. 2.7 The Oosterschelde, Westerschelde and Scheldt estuary, Netherlands.
represent examples of a river delta (Ebro delta, Spain) and a coastal lagoon (Messolonghi
Lagoon, Greece).
2.3.3 Diskibution and extent of fishhabitats
The information gathered from the selected areas summarises the distribution and extent of
fish habitats in the selected estuarine systems in each of the three European maritime regions
(Table 2.4). This information has been taken from published survey information, grey literature and local (unpublished) knowledge of the estuarine systems. In some cases, this is the
first opportunity to attempt to quantify the extent of particular habitats within an estuarine
Habitat Use byFishes in Estuaries
Table 2.3
25
Distribution between countries of the total area of coastal lagoons in the Mediterranean region
Country
Area (h2)
Percentage
Spain
France
Italy
Greece
Turkey
280
319
1151
618
351
24 18
72
1209
85
6503
Egypt
Algeria
Tunisia
Morocco
Total
4.3
4.9
17.7
9.5
5.4
37.2
1.1
18.6
1.3
100
N
Baie a’e la Seine
Caen
10 km
-
Fig. 2.8 The Seine estuary, France
system. The information for this latter subset represents, therefore, best estimates by workers with a knowledge of their particular system rather than definitive estimates derived from
a rigorous and consistent survey methodology. For the majority of the 26 selected estuarine
systems, it has been possible to estimate the total surface area, total intertidal area and area of
each of the nine defined fish habitats such that, in most estuarine systems, in excess of 95% of
the total estuarine area could be allocated to the nine fish habitats (Table 2.4).
The number, and hence diversity of fish habitats within an estuarine systemvaried between
1 (Guadalquivir river in Spain) and 8 (Ems-Dollard on the Dutch German border). In general,
selected estuarine systems in the BoreaVAtlantic region contained a greater diversity of fish
habitats (mean of five habitat types) than selected systems in the other regions (means of 4 in
the Skagerrak/Baltic region and 2.5 in the Mediterranean region). This is partly explained by
the influence of semi-diurnal tides in the BoreaVAtlantic region resulting in three fish habitats (tidal freshwater, intertidal hard and soft substrata) which are restricted to the selected
326.2
26 1
149.3
813
498.7
355.3
Elbe
Ems-Dollard
Westerschelde
53
53
51
611.1
900
Weser
53
56.1
Mersey
53
89
Forth
Humber
56
54
1.8
48
135.2
29.5
Loch Etive
56
NW Atlantic/
Boreal
ns
0
0
0
0
ns
nd
ns
0
1.9
0
0
15.2
74
0
0
0
0
0
161
5 0
0.7
156
708.1
6
ns
0.3 252.1 0
0.1 33
0
0
0
0
0 306.8 0.4 448.1 0
0
38.7 19
16.5 1.2 14.1 254.7 0 220.6 0.1 0
30
5.2 31.7 112.4 0 176
0
0
600
8.47 47
15
5
0.5
19.7
92.5
8
0
1 1
0.1
0
22.7
9
98
97
100
100
0 0
100
100
100
%known
habitat
100
0 100
0.4 100
0 100
0
0.1 100
0
0
1.2 0.2 ns
6.2
4
0.6
5.9
27.8
7
36 ns
168.4 0
1.5 0.3 26.6
0
0
0
0
0
1.7 43.3 ns
127
0
ns
0
0
0
0
0
0
36.8 10.8 0
ns
nd
1
0
0
0
0
0
0
Habitat type (h2)
Intertidal
1
2
3
4
5
area (h2)
84
303.6
184
NWhand
Gota River
60
Gullmarsfjord
19
Darss-Zingster 196.8
OderhaffiStettin 92.5
60
58
58
54
54
Baltic/
Skaggerak
e
Total surface
area (km2)
Estuarine
system
Latitude
N)
Geographic
region
4
4
4
4
5
No. of
habitats
present
As for Weser (above)
Dijkema (1989)
A. Cattrijsse, pers. comm.;
Van Schaik etal. (1988);
deJong & de Jonge (1995);
Soetaert & Herman (1995)
(1996)
Holt (1991); R. Gibson (pers.
comm.)
Davidson et al. (199 1)
Davidson etal. (1991); M.
Elliott (pers. comm.)
Davidson etal. (1991); I.
Codling (pers. comm.)
Baumert & Zabanski (1996);
Bundesanstalt fur
Gewasserkunde (1994); Kies
etal. (1992); Preisinger
(1991) ; Schirmer (1994) ;
Umweltbehorde Hamburg
E. Bonsdorff (pers. comm.)
L. Pihl (pers. comm.)
L. Pihl (pers. comm.)
H. Winkler (pers. comm.)
H. Winkler (pers. comm.)
References
Quantitative assessment of the extent and distribution of nine fish habitats in 26 selected estuarine systems in Europe. Habitat types: 1, Tidal freshwater; 2, Reed beds; 3,
Saltmarsh; 4, Intertidal soft substratum; 5, Intertidal hard substratum; 6, Subtidal soft substratum; 7, Subtidal hard substratum; 8, Subtidal seagrass beds; 9, Biogenic reefs.
Table 2.4
Thames
Seine
Somme
Loire
Ria de Aveiro
Mira
Obidos
Tagus
Bay of Cadiz
Guadalquivir
Ebro
Messolonghi
51
49
51
47
41
41
39
39
36
36
41
38
0, habitat not available.
ns, habitat not significant
nd. no data.
Mediterranean
Oosterschelde
51
74.1
160
47
3.7
7
344.5
99.7
50
86.8
140
182.2
99.7
350
0
0
4
0.4
1.1
160
27.2
0
86.8
41.9
44.9
60.1
84.1
0
0
0
0.1
ns
24.5
nd
0
0
0
0.8
0.5
ns
nd
ns
0
20.8
0
0
0
0
0
0
ns
1.4
ns
ns
0
53.3 0
112 40
0.4 25.8
ns
2.1
0
4.5
10 140
nd
37.8
0
5 0
0
8
12.9
0
ns
ns
14.2
0 0
0
0
0
ns ns 21.7 64.3 0.8 0
0
17.5 9.9 0.3 30.1 1.6 80.6 ns
16.1 3.3 0.1 34.6 4.1 122.5 nd
1.6 1.2
0.6 0.4
0.4 0.6
20 130
8.2 19.1
0
0
0
0.2 38.8 0
ns
4.73 56
ns
1
2.5 223.1 0.9
0
6.45 75.2
0
100
0
0
100
100
4.3 100
0.2 100
ns
99
20 100
nd
80
1 0 0
0 100
ns 100
1.4 100
ns
40.8 100
2
3
7
7
5
6
4
1
3
6
7
4
7
A. Cattrijsse (pers. comm.);
de Jong & Meulstee (1989);
deJongetal. (1994); de
Jong &van der Pluijm
(1994); Meijer & Waardenburg (1994); van Stralen &
Dijkema (1994)
Davidson etal. (1991); I.
Colclough (pers. comm.)
(EA, Thames)
S. Duhamel (pers. comm.)
S. Duhamel (pers. comm.)
J. Marchand (pers. comm.);
Anonymous (1989); Migniot
& Le Hir (1994)
J. Rebelo (pers. comm.)
P. Almada (pers. comm.)
P. Almada (pers. comm.)
P. Almada (pers. comm.)
P. Drake (pers. comm.)
C. Fernandez-Delgado (pers.
comm.)
Fishes in Estuaries
28
N
A
7
'b
Fig. 2.9 The Loire estuary, France.
Gudrande
-r.i
Nantes
Pornic
10 km
systems in this region. The diversity of fish habitats, and thus the number of niches, within
an estuarine system is one of the factors likely to determine the biodiversity of the fish community. A second factor is the area of habitat available (Wootton, 1990).
The most extensive and widely distributed of the nine identified fish habitats was habitat 6
(subtidal soft substratum) in all three biogeographic regions, accounting for over 50% of the
total surface area of the selected estuarine systems (Table 2.4). The contribution of habitat 6
in the Skagerrak/Baltic and Mediterranean regions increased to over 70% of the total because
of the reduced influence of tides and thus the relatively limited extent of intertidal areas. The
estuarine systems with the greatest areas of subtidal habitat were also the largest systems
in terms of total surface area (Weser, Elbe, Ems-Dollard, Oosterschelde, Westerschelde in
the BoreaVAtlantic; OderhaffBtettin lagoon in the Baltic and Messolonghi Lagoon in the
Mediterranean).
Habitat 4 (intertidal soft substratum) was the second most extensive habitat, accounting
for almost 30% of the total surface area of the selected estuarine systems in the Boreal/
Atlantic region. This habitat was absent from the other biogeographical regions due to the
reduced influence of tides. The greatest extent of intertidal soft sediment was found in the
large estuaries, in particular the Elbe, Weser and Ems-Dollard.
Habitat 3 (saltmarsh) accounted for more than 10% of the total surface area of the selected
systems in the Mediterranean as a result of the wide expanse of saltmarsh in the Ebro delta,
Spain. This habitat was also represented in the BoreaVAtlantic region with greatest areas in
the Oosterschelde, Westerschelde, Tagus and Ems-Dollard. Saltmarsh was not found in any
of the selected systems in the Skagerrak/Baltic region.
Habitat 7 (subtidal hard substrata) accountedfor more than 10% of the totalsurface area of
the selected systems in the Mediterranean due to the large area (40 km2) in the Messolonghi
Habitat Use byFishes in Estuaries
29
N
A
GmMes
la Tremblade
B a y of Biscay
10 km
St-Laurent-Medoc
St-Andrk-de-Cubzac
Bordeaux
Fig. 2.10 The Gironde estuary, France.
lagoon, Greece. This habitat was also present in all selected systems in the Skagerrak/Baltic
region with the greatest area (27.8 km2) in the OderhaffBtettin lagoon. Subtidal hard substrata were present at some of the selected sites in the BoreaVAtlantic region, but the areas
were very small or not significant reflecting the soft sedimentary nature of the estuaries.
Habitat 8 (subtidal seagrass) accounted for more than 10% of the total surface area of the
selected systems in the Skagerrak/Baltic region because of the large area (92.5 km2) in the
OderhaffBtettin lagoon. Subtidal seagrass was also present in selected systems in the other
regions with significant areas in the Bay of Cadiz, Spain and the Ria de Aveiro in Portugal
(BoreaVAtlantic region).
The remaining four habitat types (1, Tidal freshwater; 2, Reed beds; 5, Intertidal hard
substrata; and 9, Biogenic reefs) contributed less than 5% of the total surface area of the
selected systems in all regions.
30
Fishes in Estuaries
N
pI
A
G
U
Fig. 2.1 1 The Ria de Aveiro, Portugal (highlighting sampling stations as cited within text)
Habitat Use byFishes in Estuaries
I
Ericeira
31
i
ATLANTIC
OCEAN
10 kin
Fig. 2.12
The Tagus estuary, Portugal
Habitat 1 (Tidal freshwater) was present only in the BoreaVAtlantic region as estuarine
systems in this region are significantly influenced by semi-diurnal tides. Within this region,
the area of tidal freshwater habitat was greatest in the Weser, Elbe, Ems-Dollard, Westerschelde, Loire and Tagus estuarine systems.
Habitat 2 (Reed beds) was present in both the BoreaVAtlantic and Skagerrak/Baltic regions. In the BoreaVAtlantic region, this habitat was present in eight of the estuarine systems
as a small area although, in the Weser and Elbe systems, a significant area of reed bed (10.8
and 19 km2,respectively) was recorded. In the Skagerrak/Baltic region, significant areas were
recorded in the Darss-Zingster Boden (15.2 km2) and the OderhaffEtettin lagoon (19 km2).
The often fringing nature of this habitat dictated that it is difficult accurately to quantify its
area.
Habitat 5 (Intertidal hard substrata) was restricted to the BoreaVAtlantic region since only
estuarine systems in this region had a significant intertidal zone. This habitat was found in 12
estuarine systems, but the areas were very small.
32
Fishes in Estuaries
N
i/
Sanldcar de Barrameda
0
0
Bay of CBdiz
\
Jer6z de la Frontera
El Puerto de Santa Marid
SanFernando
10 km
Fig. 2.13
The Bay of Cadiz, Spain
Habitat 9 (Biogenic reefs) was found in the BorealIAtlantic and SkagerrakIBaltic regions.
In the BorealIAtlantic region, this habitat was found in seven of the selected systems with
the greatest areas in the Oosterschelde and in the Ria de Aveiro, Tagus and Mira systems in
Portugal. In the Oosterschelde, the biogenic reefs comprise oyster and mussel beds. In the
SkagerrakIBaltic region, this habitat was found in three of the five selected systems, with the
greatest area (22.7 km2) in the OderhaffIStettin estuary.
Habitat Use byFishes in Estuaries
33
N
A
Province of Seville
Brazo de la Torre 4
n
1
Province of Huelva
4
0
LosPalacios
0 Leinija
//
0
Trebujena
Province of Ccfdiz
ATLANTIC
OCEAN
Sanlhcar de Barrameda
20km
Fig. 2.14
The Guadalquivir estuary, Spain
2.4 Use of habitats by fish in selected European estuarine systems
2.4.1 Fish species habitat use
2.4.1.1 Data treatment
When evaluating fish habitat use in estuaries, consideration has been given to four different
functions that estuarine habitats may fulfil for fish. Habitats can serve as spawning grounds,
34
Fishes in Estuaries
nursery areas, feeding grounds, and as pathways in diadromous migrations. A further use of
habitats as refugia can be included here, although this is less straightforward (see below).
To deposit or release their eggs and to mate, fish may select specific areas because of the
environmental characteristics that optimise the survival of the eggs or of the early larval
stages. Estuarine resident species use estuarine habitats to spawn, while catadromous and
many marine species return to the sea or to polyhaline waters and anadromous species move
into freshwater habitats for spawning. In some areas, a large proportion of the freshwater
species uses the estuary for spawning.
Sediment characteristics, currents, water depth and vegetation may all have an impact on
the survival or retention of the earliest life stages in an estuary and thus determine the likelihood that a fish species will spawn in a habitat. Juveniles migrate either passively or actively
(some may use selective tidal stream transport) to estuarine nursery grounds and concentrate
in specific habitats to spend their early lives. Nurseries are defined as areas where juveniles
aggregate, are spatially or temporally separated from the adults, and where their survival
is enhanced through better feeding conditions, optimal growth a n d o r refuge opportunities.
Recruitment to the adult or subadult populations follows the emigration from these nurseries
after attaining a well-defined length class.
It is very difficult to define a refuge as a habitat usage, even though its understanding here
is straightforward and mostly seen as a refugium from predation. It is similarly not possible
to quantify the refuge value in the field especially where the fish are also feeding in a habitat.
Adult fish may also seek predation refuge, yet adult andjuvenile fish occupy different habitats
in most cases. To distinguish between these interpretations, a refuge function is defined as
an integral part of the nursery function of a habitat here and is not considered as a separate
usage.
Adult fish often perform feeding migrations into habitats where they preferably or exclusively forage. These feeding migrations can be based on a tidal, a diurnal or on a seasonal
basis. Diadromy obliges fish to migrate between marine waters and brackish or freshwater
areas for spawning (McDowall, 1988).The only path that diadromous fish can follow to reach
their spawning grounds is through the estuary, and in doing so they may use some habitats but
not others thus, the terms used may merely imply occurrence in a habitat.
The definitions used here when analysing the information on these four habitat functions
within the selected 26 estuaries, are:
~
0
0
spawning: the presence of ripe adults and the production of eggs;
nursery: concentration ofjuvenile stages which are feeding and growing;
feeding: habitats used by adults as a feeding ground; and
diadromy: use of the estuary or a habitat as a migration route for spawning.
In order to consider the importance of estuaries for fish but also in terms of human use, the
occurrence of commercial species is considered here. A commercial species is here defined as
one that is subject to a contemporary local or regional fishery at some point in its life, whether
as a target species or a bycatch which is landed.
For each estuary, a speciedhabitat list has been constructed, indicating habitat utilisation
and function (spawning, nursery, feeding and diadromy) for each fish species being recorded
in that estuary. So, for each estuary the occurrence of all species in habitats 1 to 9 is given,
Habitat Use byFishes in Estuaries
35
Examples of speciesihabitat classifications. (FW = freshwater species: ER = Estuarine resident species:
CA = Catadromous species: 1- 9 = Habitat number.)
Table 2.5
Species
Ecological guild
Habitat use
Spawning Nursery
Abrarnis brarna
FW
Agonuscataphractus ER
Anguilla anguilla
CA
4,6
Feeding
1
(4, 6)
(4, 6)
1 , 3 , 4 , 6 1,(6)
Diadromy
(1,3,4,6)
together with information on function for each habitat. Table 2.5 illustrates how this was carried out (Appendices l a - x provide habitat use data for each estuary), and the guild description is based on local or published knowledge of a species (e.g. Elliott & Dewailly, 1995).
An illustration of this is to consider a hypothetical estuary where only five habitats exist:
a tidal freshwater area; salt marshes; intertidal soft substratum; subtidal soft substratum; and
reed beds. The latter habitat has never been investigated for fish, so no data or information are
available on the occurrence of fish there. For the remaining habitats, the table was completed
as follows: Abramis brama feeds in habitat 1, the tidal freshwater part of that estuary, Agonus
cataphractusspawns in habitats 4 and 6 (intertidal and subtidal soft substratum), uses these
habitats as a nursery and the adults also feed there, and Anguillaanguillauses the salt marshes
(habitat 3) and the other habitats as pathways in the catadromous migrations, feeds in habitats
1 and 6 , and has its nursery in those four habitats. The information collected was mainly
based upon general knowledge of the ecology of the different species or on the experience
of the scientists studying the estuarine populations. However, some of the information was
reinforced by scientific data of occurrence of the different life stages of fish species in the
specified habitats, on data of the feeding ecology of the species, and on data of egg and larval
surveys. In order to distinguish between these two qualitatively different types of information, the habitat use information that was supported by hard data is given in brackets in the
table. For example, Agonus cataphractus is known to be an estuarine resident species, its
juveniles occur in the estuary and thus the species most likely spawns in this hypothetical
estuary. Despite the latter, it is possible that no data can prove the occurrence of their eggs or of
the ripe adults. For this species, spawning is assumed to occur in habitats 4 and 6. Otherwise,
stomach analyses have indicated that adult fish feed and that it is likely thatjuvenile individuals grow and feed in those habitats. The proportion of ' known data' versus the total amount of
available information was used to identify the gaps in knowledge. Table 2.6 summarises the
information of all speciedhabitat lists and the total number of species found in each habitat is
given, together with the contribution of commercial species for each estuary. The percentage
of known data for each of the selected estuarine systems is also presented. The number of
commercial species for each of the selected estuaries is shown in Table 2.7.
2.4.1.2 Proportion of known data
In general, there was a north- south gradient in research effort or knowledge about habitat use
of fish for the selected sites (Table 2.6). With a regional average of 73% of ' known data', the
estuarine sites studied in the Baltic and Skagerrak seem to be the best studied areas. On average, about 33% of the information of the Boreal/NW Atlantic region is supported by studies
Estuarine
system
NW h a n d
Gota River
Gullmarsfjord
Darss Zingster
OderhaffiStettin
Weser & Elbe
Westerschelde
Oosterschelde
Ems- Dollard
Loch Etive
Forth
Humber
Thames
Mersey
Somme
Latitude
60
58
58
54
54
53
52
52
53
56
56
54
51
53
50
Baltic/
Skagerrak
NW Atlantic/
Boreal
Geographic
region
?
?
6
8
27
24
10
3
?
5
24
?
10
13
9
2
40
18
1
13
11
5
?
5
14
?
?
~~
~~
~~
~~
~~
3
32
24
57
15
19
11
21
33
26
28
4
?
?
?
?
3
7
4
5
41
28
28
34
39
~~
61
43
65
39
47
42
63
108
57
6
?
8
12
16
29
27
14
19
7
No. of fish species found in each habitat type
2
?
21
30
27
28
32
8
4
4
3
3
?
?
?
60
?
9
78
56
74
53
50
45
85
110
69
28
44
53
50
44
50
Total no.
ofspecies
15
19
20
15
0
19
13
16
8
10
19
19
20
19
25
Total no. of
commercialspecies
50
38
13
25
65
41
67
11
74
30
90
96
96
45
40
43
36
40
43
50
19
34
27
28
0
42
15
15
12
36
% known data
% commercial
species
Table 2.6 Number of fish species in each habitat type, total number of species and commercial species, and the percentage of known data for the selected estuaries. Habitat types: 1,
Tidal freshwater: 2, Reed beds: 3, Saltmarsh: 4, Intertidal soft substratum: 5, Intertidal hard substratum: 6, Subtidal soft substratum: 7, Subtidal hard substratum: 8, Subtidal seagrass beds:
9, Biogenicreefs.
7.7
15
16
Occurrence
of habitat
, habitat not available.
?, habitat not investigated
Mean
12.1
6
Overall mean
1
2
1
~
8
3
4
?
10.7
6.7
6.0
Messolonghi
Ebro
38
41
2
12
8
10
9
?
4
Baltic
BorealiNWAtlantic 13.0
Mediterranean
1.0
Mediterranean
Seine
Loire
Bay of Cadiz
Guadalquivir
Ria de Aveiro
Obidos
Tagus
Mira
49
47
36
36
41
39
39
41
~
-
15
18
11
13
16
17
11.1 25.0
11.5 24.3
7.0
~
7
?
13
?
?
2
~~
4 2 8
? 2 3
37
39
1
16
4.8
5.5
1.0
~
15
2
?
2
?
?
47
36
25
13
12
45.4 22.8 24.0
34.0 21.0 27.6
50.8 19.0 18.0
28.5 47.0 36.0
45
12
~
~
~
4 9 ? 3 1 ?
40
?
29
30
26
18
45 26
73 30
4
44
37
15
13.0
3.5
24.0
7.0
7
10
10
16
?
?
?
57.2
48.2
61.3
43.0
62
24
79
46
53
32
55
45
84
61
20.4
20.4
19.1
31.0
46
16
20
25
36
18
26
29
51
5
38.6
42.4
33.9
70.5
74
67
25
54
68
56
47
64
61
8
39.0
73.4
32.2
13.5
0
27
20
9
25
0
11
41
24
36
38
Fishes in Estuaries
Table 2.7 Number of commercial fish speciesrecorded in each habitat type for each selected estuary. Habitat types:
1,Tidal freshwater: 2, Reedbeds: 3, Saltmarsh: 4, Intertidal softsubstratum: 5, Intertidal hardsubstratum: 6, Subtidal
soft substratum: 7, Subtidal hard substratum: 8, Subtidal seagrass beds: 9, Biogenic reefs.
Geographic
region
Baltic/
Skagerrak
Atlantic/
Boreal
Mediterranean
Mean
Overall mean
Latitude
Estuarine
system
60
58
58
54
54
NWAland
53
52
52
53
56
56
54
51
53
50
49
47
36
36
41
39
39
41
Weser & Elbe
Westerschelde
Oosterschelde
Ems- Dollard
Loch Etive
Forth
Humber
Thames
Mersey
Somme
Seine
Loire
Bay of Cadiz
Guadalquivir
Ria de Aveiro
Obidos
Tagus
Mira
38
41
Messolonghi
Ebro
Gota River
Gullmarsfjord
Darss Zingster
OderhaffiStettin
Baltic
BorealDJWAtlantic
Mediterranean
No. of commercial fish species found in each
habitat type
1
2
-
3
~~
~~
~
7
3
5
5
2
?
~~
3
?
~~
4
2
3
1
l
1
2
?
13
?
5
?
0
0
?
3
5
4
~~~
~
~~
~
~~
~~~
~
~
7
5 1 0
? 20
?
8
0
8
8
7
5 1 6
? 7
2 1 0
2 1 1
? 1 5
21 22
~
~
~
1
?
2
0
?
?
?
?
?
?
~~
2
5
1
6
2
0
8 1 0
? 1 3
6
0
1
7
0
?
-
1
3
5
1
~~
4.3
3.4 2.1
1.0 3.0
3.3 2.8
~
?
~~
6
7
8
19
18
19
15
20
10
9
9
6
12
7
9
8
11
14
15
1 8 20
?
14
1
0
0
19
13
16
8
~
~
2 0
2 4
24
18
18
2 9 1
4 9 1
5
?
0
~
~
~
~
6
9
-
0
6
-
46
16
~
~
20
?
?
~
~
~
~
14
?
?
~
?
?
?
~
~
9
9
~
~
2
0
4 6 4 0 3 0
1 6 - -
19
19
20
19
25
15
19
20
15
0
19
13
16
8
10
20
25
27
18
26
29
51
5
~
~
~
0
0
3
3
~
~
?
?
?
~
~
~
~
Total no.
ofspecies
9
5
18.2 9.2 9.8 1.5
6.4 10.0 2.0 17.9 9.0 5.0 8.5
5.0
1.0 31.0 40.0 30.0 6.0
20.4
18.3
31.0
6.2 10.1 1.8 19.3 12.2 9.5 5.1
20.0
~
~
~
~
performed in the respective sites. There also exists a clear difference between the northern
and southern sites in the BoreaVNWAtlantic region. The systems of the Iberian peninsular are
supported by data collected in the respective sites in about 23%, on average. In contrast, the
northern sites are on average confirmed in about 40%. The Mediterranean sites have the lowest (0- 27%) amount of ' known data' such that in southern Europe, further effort is required
to study the importance of estuarine areas for fish. This consideration of effort is important
particularly with regard to the number of species encountered. In an ecological study, further sampling effort will take further species, especially the increasingly rare species (see
below).
Habitat Use byFishes in Estuaries
39
2.4.2 Species richness and distribution between habitats
2.4.2.1 All fishspecies
The species richness and distribution of fish over the nine habitats, throughout each region
and estuary is displayed in Table 2.6 and Fig. 2.15. All fish recorded have been included for
each estuary. The number of fish species found within each estuarine system ranges from the
relatively poor Ebro (n=24) to the extremely richThames estuary (n= 110).
The average number of species using estuarine habitats during one or more lifestages is 57
for the 26 estuarine systems under consideration. As was also shown by Elliott and Dewailly
(1995), there was no consistent latitudinal gradient of species richness. With a regional average of 48.2 species, the BaltidSkagerrak estuarine systems appear to be less rich in species,
whilst the estuarine systems of the Boreal/NW Atlantic region hold on average 61.3 fish species. A considerable difference was seen to exist for the two Mediterranean sites. In the Ebro
delta only 24 species have been recorded, whereas 62 species were observed in the Messolonghi lagoon. However, this latter site might be more representative of Mediterranean
estuarine systems, as only one habitat was represented in the Ebro estuary. The difference
in intensity with which both sites were studied is, however, unknown. It is suggested that
any latitudinal or other trend is obscured by the influence of the intensity of study on species
richness. It must be noted that during the categorisation of regional areas (AtlantidBoreal,
Mediterranean and BaltidSkagerrak) , relatively broad generalisations and assumptions are
made when not considering differences in effort among investigations. For the selected sites,
the number of commercial species per estuary tends to increase towards the south (Table
2.7). The contribution of commercial species was generally more than 50% for southern
50
LI
l0lLhh+
n
0
Fig. 2.15
7
8
9
Average number of fish species per habitat in selected estuaries in three European regions.
40
Fishes in Estuaries
Europe, while the commercial component of the northern areas was less than 50% of the total
estuarine fish fauna. This feature may reflect the more diverse fish-based diets of southern
Europe.
There was no relationship between habitat diversity and the number of species recorded
(Fig. 2.16). The number of species recorded for any one estuary is more likely to be related to
the efforts made and the methods used to study the fauna, and also to the degree of pollution
or habitat degradation of the individual estuarine systems. Habitat diversity is only one factor determining the species richness of an estuary. The complexity of the individual habitats
(which is not accounted for here) also plays an important role in determining the number of
species occurring in a habitat. Similarly, the complexity of adjoining marine and freshwater
areas is not considered here, but will have an influence on an estuarine habitat’s diversity.
Habitat 1, the tidal freshwater habitat, occurs exclusively in the tidal estuaries of Boreal/
NW Atlantic region. For the German estuaries, the Elbe and the Weser, this habitat is extremely important, being the second most species-rich habitat type (Table 2.6). More than
50% of all species within these two estuaries use this habitat type during at least one life
stage. The remaining areas possessing tidal freshwater are generally used by 10- 30% of the
estuarine fish populations. In Messolonghi lagoon, habitats 1, 2 and 3 are virtually lacking,
but some parts of the lagoon exhibit characteristics of these habitats (e.g. freshwater springs).
Some species tend to be strongly associated with these habitats and are therefore included in
the analysis.
Reed beds (habitat 2) and saltmarsh (habitat 3) will in some cases be associated with one
another, or in other cases be subparts of habitat 1. The very nature of these habitats makes
sampling difficult (which means they are difficult to study), as is reflected in the amount of
information available. In the reed beds of the selected Baltic areas, only about 20% of the fish
120
100
8
LI
P
g
z
40
20
0
0
1
2
3
4
5
6
7
8
9
10
Number of habitats
Fig. 2.16 Relationship between the number of fish species and the number of habitats in selected European
estuaries.
Habitat Use byFishes in Estuaries
41
fauna could be found (Table 2.6), but a higher proportion of the fish fauna occurred in reed
beds of the Weser and Elbe. Where data are available from other sites it can be seen that reed
beds are relatively poor in fish species. In the BoreaVNW Atlantic region, there are regions of
tidal salt marshes, and these are important as a habitat for a number of estuarine fish (Drake
& Arias, 1991; Cattrijsse et al., 1994). The proportion of fish occurring in these intertidal
vegetated habitats was on average 18% of the total number of species, though in the Bay
of Cadiz the proportion increased to almost 60%. In the Mediterranean, a non-tidal type of
saltmarsh exists, but little information was available on its importance as a fish habitat.
As for the tidal freshwater habitat, the intertidal soft substratum (habitat 4) occurs exclusively in the tidally dominated BoreaVNW Atlantic region. Information on its use by fish has
been collected in all sites, and it displayed a high overall mean of 38.6% of the fish species
using the habitat (Table 2.6).
The subtidal soft substratum (habitat 6) had a high and constant number of fish throughout
all regions (Table 2.6). In this habitat, the overall average of 45 species was the highest in
any habitat and in all estuaries at least half of the fish fauna used the subtidal soft substratum
during at least one life stage. Due to the frequency with which this habitat type is sampled, the
probability of data bias must be considered. Indeed, the intertidal and subtidal soft substratum are the only habitats to contain data from all sites. Whilst the intertidal soft substratum
was exclusive for the Boreal/NW Atlantic region, the subtidal equivalent of this habitat was
present in all 26 estuarine systems. All other habitats occurred in only about half of the sites.
The intertidal (habitat 5) and the subtidal hard substrata (habitat 7) are habitats that impose many constraints on sampling. Based on the data presented here, the intertidal hard
substratum had the lowest species richness, with an average of only five species (Table 2.6).
In contrast, the subtidal hard substratum was one of the most species-rich habitat types with a
overall mean of 23 species that comprised approximately 38% of the present fish fauna.
Subtidal sea grass beds (habitat 8) are also species-rich environments, but are generally
absent from the selected estuaries along the North Sea and Atlantic coasts. Only in the Oosterschelde, Bay of Cadiz and Ria de Aveiro has this habitat been investigated, though subtidal
sea grass beds have largely disappeared and are still declining there. The BaltidSkagerrak
and the Mediterranean region still have extensive seagrass beds that hold many fish; in fact,
the average species richness for the two regions was estimated at 32 species, making this
habitat the second most important estuarine habitat in terms of fish diversity.
Habitat 9 (Biogenic reefs) is the least studied of the nine habitats yet, as shown by the data
of the Oosterschelde and the Ria de Aveiro, a large part of the present ichthyofauna may use
this habitat in some estuaries.
2.4.2.2 Commercialspecies
The proportion of commercial fish species utilising estuarine systems at some stage of their
life cycle accounted for approximately 38.6% (overall mean) of the total present (Table 2.6).
The average proportions of commercial species found in the BaltidSkagerrak was 42%,
while in the BoreaVNW Atlantic region it was 33%. A higher percentage of commercial fish
was recorded in the southern estuaries averaging, on the Iberian peninsular and in the Mediterranean estuaries, 48% and 70%, respectively. Among the northern systems of the BoreaV
NW Atlantic region, the Forth and the Loire estuaries had a high proportion of commercial
42
Fishes in Estuaries
species, although these estuaries had a relatively low total number of species. A similar pattern was observed for the Ebro in the Mediterranean, where few species have been recorded
but a high proportion was commercially fished.
The distribution of the commercial species over the nine habitats revealed a very similar
pattern as the distribution of the total fish communities. Table 2.7 shows that nearly all commercial fish species in all selected estuarine areas use habitat 6 during their life, while on
average more than half of the commercial species use habitat 4 during at least one life stage.
2.4.3 Habitat utilisation
The four habitat uses are ranked by average number of fish species as follows: Feeding 44.8
species >Nursery 33.6 species > Spawning 13.6 species > Diadromy 4.5 species (Table 2.8).
This ranking is evident in the three regions considered, within each individual estuarine system, and even within each of the nine habitats with only slight deviations from the above
ranking (Fig. 2.17). In the southern Boreal/NW Atlantic estuarine areas (Loire to Cadiz) ,
the ranking of feeding and nursery is reversed. When considering the percentage of the total
number of species recorded as undertaking each of the four habitat uses, the ranking order
was: Feeding 75.8 >Nursery 63.1 >Spawning 24.3 > Diadromy 8.9% of the species, respectively (Table 2.8). Since only a few species exhibit diadromy, it is not surprising that this
habitat use ranks the lowest. Also, the low percentage of species using estuaries for spawning
is not unexpected, given that only a few marine species utilise estuaries for this purpose. It
is, therefore, mainly true estuarine resident species that will spawn inside estuarine areas,
although in some areas the contribution from fresh water species could be considerable.
40
30
LI
8
P
0
.-gu8
20
%
8
OD
m
L
d
10
1
2
3
4
5
Habitat
Fig. 2.17
Lc i
7
0
6
Habitat utilisation of fish among nine habitats in 26 selected estuaries.
a
9
Habitat Use byFishes in Estuaries
43
Table 2.8 Number and percentage of species by habitat usage per selected estuary. Key to habitats: S = Spawning:
N = Nursery: F = Feeding: D = Diadromy.
No. of species recorded per estuary for each
habitat usage
S
N
NWAland
Gota River
Gullmarsfjord
Darss-Zingster
OderhaffiStettin
35
25
24
24
23
30
39
39
28
27
Weser & Elbe
Westerschelde
Oosterschelde
Ems-Dollard
Loch Etive
Forth
Humber
Thames
Mersey
Somme
Seine
Loire
Bay of Cadiz
Guadalquivir
Ria de Aveiro
Obidos
Tagus
Mira
39
14
10
9
11
9
7
7
2
3
3
7
1
6
11
11
12
20
Messolonghi
Ebro
F
Percentage of species recorded per estuary for each
habitat usage
D
Total
40
51
48
41
44
0
3
3
3
10
44
53
50
44
50
40
28
34
19
27
20
27
21
18
26
41
39
52
27
24
37
76
58
76
56
70
32
47
43
84
107
55
16
50
15
31
17
29
26
28
33
7
6
0
5
3
4
12
5
4
4
11
10
0
6
11
2
3
0
11
1
52
10
58
4
1
3
Baltic
NorthernEurope
SouthernEurope
BorealDJWAtlantic
Mediterranean
26.2
10.5
9.7
11.0
6.0
32.6
26.3
44.7
34.2
31.0
44.8 3.8
57.8 5.5
25.6 4.6
46.8 4.9
31.0 2.0
Overall mean
13.6
33.6
44.8
Mean
S
N
F
D
NWhand
GotaRiver
Gullmarsfjord
Darss-Zingster
OderhaffiStettin
79.5
47.2
48.0
54.5
46.0
68.2
73.6
78.0
63.6
54.0
90.9
96.2
96.0
93.2
88.0
0.0
5.7
6.0
6.8
20.0
78
56
74
53
50
45
85
110
69
28
79
46
53
32
55
45
84
61
Weser & Elbe
Westerschelde
Oosterschelde
Ems-Dollard
Loch Etive
Forth
Humber
Thames
Mersey
Somme
Seine
Loire
Bay of Cadiz
Guadalquivir
Ria de Aveiro
Obidos
Tagus
Mira
50.0
25.0
13.5
17.0
22.0
20.0
8.2
6.4
2.9
10.7
3.8
15.2
1.9
18.8
20.0
24.4
14.3
32.8
51.3 97.4
50.0 100
45.9 94.6
35.8 60.4
54.0 94.0
44.0 95.6
31.8 98.8
19.1 97.3
26.1 79.7
92.8 57.1
51.9 63.3
84.8 32.6
98.1 58.5
84.4 53.1
43.6 52.7
82.2 57.8
90.5 33.3
95.1 54.1
9.0
10.7
0.0
9.4
6.0
8.9
14.1
4.5
5.8
14.3
13.9
21.7
0.0
18.8
20.0
4.4
3.6
0.0
62
12
Messolonghi
Ebro
17.7
8.3
83.9
83.3
93.5
33.3
1.6
25.0
Baltic
NorthernEurope
SouthernEurope
BorealDJWAtlantic
Mediterranean
55.1
16.3
18.2
18.3
13.0
67.5
45.7
82.7
58.6
83.6
92.9
85.3
48.9
72.5
63.4
7.7
8.8
9.8
8.6
13.3
Overall mean
24.3
63.1
75.8
8.9
Mean
4.5
Since estuaries are generally less species-rich than the adjacent marine or freshwater systems, the number of species utilising estuarine areas as spawning sites will be limited. In general, more fish species are found using estuaries as adult feeding grounds than as nursery areas.
This trend is clearly reversed for the southern BorealLVWAtlantic estuarieswhere more species
are found using the available habitats as nurseries than as adult feeding grounds (Table 2.8).
2.4.4 Habitat importance
In an attempt to classify the nine habitats according to their relative importance, it is clear that
44
Fishes in Estuaries
the relative distribution of fish species between all estuarine habitat types is quite similar.
This is shown in Table 2.9, which shows the average species richness per habitat type for the
three regions, and also the overall average species number for each of the four habitat uses
in each of the nine habitats. The average species richness within habitat types throughout
the 26 selected European estuarine systems ranks as follows: Subtidal soft substratum (6) >
Intertidal soft substratum (4), Subtidal seagrass beds (8),Subtidal hard substratum (7) >Tidal
freshwater (1),Biogenic reefs (9),Saltmarsh (3) > Reed beds (2) >Intertidal hard substratum
(5).The average percentage of the total species present is similar: Subtidal soft substratum (6)
> Subtidal seagrass beds (8),Subtidal hard substratum (7), Intertidal soft substratum (4), >
Saltmarsh (3), Biogenic reefs (9),Tidal freshwater (1) > Reed beds (2) > Intertidal hard substratum (5). Only small deviations appear when making this ranking for the three regions.
The main difference exists between the micro-tidal estuarine systems of the Baltic, Skagerrak and the Mediterranean regions and the tidal estuaries of the BoreaVAtlantic region. In
the former the intertidal areas, habitats 1 , 3and 4 are largely or even completely absent. These
habitats are important in the latter region, however, especially the intertidal soft substratum
(habitat 4). Similarly, seagrass beds and subtidal hard substratum, which are less common in
the estuarine systems of the BoreaVAtlantic region, are of higher importance in the Baltic
and Mediterranean. Habitat 9 also appears to be more important in the Boreal/NW Atlantic
region than in the BaltidSkagerrak.
There is large individual variation of habitat utilisation between the individual estuarine
systems, mainly due to the extent of each habitat although lack of data for certain habitat types
and regions may seriously undermine the general conclusions on habitat use. For example,
within the Forth estuary (UK) extensive reed beds (habitat 2) exist. However, a complete
absence of data concerning the species using this habitat reduces the value of generalisations
based wholly on fish species number.
In order to evaluate further the importance of estuarine biotopes as habitats for fish, a
Habitat Utilisation Index (HUI) was developed here as a summation of the various life stages
of fishutilising a single habitat, divided by the amount of available data (sites) for that habitat
(Table 2.10). This index approximates the overlap between fish life stages and the utilisation
of each habitat type. Here, the degree of total habitat utilisation throughout the European
estuarine systems, and thus the relative importance and diverse usage of each habitat type, can
be identified. In the previous evaluation of habitat importance, merely the average number
of fish present in a particular habitat was considered. Since a fish species may use a single
habitat for several reasons (e.g. as nursery, as an adult feeding ground and in diadromous
migrations) the utilisation of the nine habitats and thus the importance may, however, differ
from the ranking based upon the average number of fish species. The HUI evaluates a habitat
on basis of an average number of uses made by all fish species.
Using this index, the ranking of the nine habitats does not differ from that given above:
Subtidal soft substratum (6) > Subtidal seagrass beds (8),Subtidal hard substratum (7), Intertidal soft substratum (4) > Tidal freshwater (1), Biogenic reefs (9), Saltmarsh (3) > Reed
beds (2) > Intertidal hard substratum (5).
To develop further the idea of habitat importance, the distribution of the commercial species over the different habitats (Table 2.7) and the utilisation of these habitats by the commercialspecies are also considered (Table 2.11). The average number of commercial species
per habitat type changes the ranking of the nine habitats: Subtidalsoft substratum (6) > Subti-
~~~
~~~
S
~
~
~
-
N
~
~
~
~~~~
~~~~
F
~
~
~
D
~
~
~
S
~
~
~
N
~
~
~
~
~
F
~
~
~
~~~~~
~~~~~
D
~
~
~
S
~
~
~
N
~
~
~
F
~
~
~
D
N
22 19
5 16
5 17
8 1 2
6 8
S
D
35 0
26 3
26 3
3 1 3
3 1 9
F
13
17
16
5
3
S
Baltic
NorthEurope
SouthEurope
BoreaV
NW Atlantic
Mediterranean
Mean
15
28
27
1 2
1 4
F
0
1
1
0
0
D
1
0
~~~~~
3
4
2
0
7
?
4
?
~
?
7
~~~~
~~
~
?
1
~
-
~
~
~
~
~~~~~
~
~~~~~
0
0
0
1
~
1
~~
3 3 4 3 0
1 1 0 4
3
~
9
-
~
~
~
~
~
~
~
?
?
?
?
~
~
?
0
9
?
~
?
?
~~
?
?
~
2
3
14
9
?
?
?
?
1
1
8
10
?
?
?
10
?
2
10
O
0
?
18
17
17
1 5
1 3
S
F
D
17
~
15
?
?
?
?
?
~
~
~~~
1.0 1.0 0
0
3.0
2.2
2.7
0
0
14.7
3.0
10.5
0
7.0
5.4
6.4
0
5.0
1.4
3.7
8.3
8.3
2.0
4.7
2.3
8.7
1.5
4.6
0
0.3
1.0
0.7
3.0 4.0 2.0 0
8.7
6.3
0.8
3.1
0
5.2
14.7
8.3
0
6.8
6.0
6.6
0
0.2
0.7
0.3
0
3.3
0.5
2.3
4.0 7.0 7.0 1.0 0
0
1.3
0.3
1.0
0
0
16.6
16.8
16.7
0
0
22.5
8.8
17.6
0
0
2.0
0.5
1.5
0
0
2.0
1.0
1.6
0
0
2.7
4.0
3.2
0
0
3.7
7.5
5.2
9.2
6.7
4.1
5.6
14.4
21.2
35.0
26.9
29.8
54.6
21.3
40.9
3.6
4.9
2.7
4.0
10.8 13.4 19.2 0.4
3.0 4.5 9.0 1.0
8.0 21.0 11.5 2.5
5.5 12.8 10.3 1.8
16.0 20.4 22.6
0
2.0 2.0
5.5 19.0 12.3
4.4 15.6 10.2
0.4
0
0.8
0.6
1.0 1.0 21.5 23.5 1.5 9.0 37.0 46.0 1.0 3.0 23.0 35.0 0
0
0
1.5
0.6
0.5
29.0
6.3
12.0
14
0 6 2
24
3 55
? 4 5
0 8 4
0 6 1
?
?
?
0
0
1.0
0.8
10.7 0.3
2.0
56.0
8.3
20.3
~
7
7
4
?
~
?
?
78
56
7 4
5 3
5 0
45
85
110
6 9
28
7 9
4 6
5 3
32
44
0 53
0 50
0 4 4
0 5 0
~
3.0 6.0 7.0 0
2.0
8.0
4.3
5.3
~
?
?
~
~
? ?
? ?
? ?
?
?
2 3 3 5 0 3 6
3
~~~
O
~~~
~~~
~~~
~
~
~
1
1
3
3
1
1
0
0
No. of
Dspecies
? 8 2 9 5 6 0
? ? ?
?
0 - -
~~~
~~~
~
~
F
4
4
0
0
N
~
S
Habitat 9
0 6
?
?
? ? ?
5 3 0 5 9
3 1 2 4 0 8 4
~
2 3 7
~~
~~
~
~~
~
~
~
~~
~
2
?
~
~
~
2
~~
?
~
~
18 13 0
25 27 1
25 27 1
1 8 2 4 0
1 6 2 2 0
N
Habitat 8
3.5 6.1 9.9 2.5 4.6 5.6 3.7 0.5 1.3 8.2 6.6 0.4 2.3 16.7 17.6 1.5 1.3 2.7 4.3 0.7 6.0 23.8 37.1 3.7 8.5 15.5 18.3 1.0 9.5 18.5 18.1 0.5 3.6 6.2
~~~
1
~~~~~~~
~~~
~~~
~~~~~
~~~
~
~
11
26
25
3
2
N
Habitat 7
3 7 4 6 1 3
~
~
~
-
S
Habitat 6
0
0
0
~
0
D
Habitat 5
Messolonghi
Ebro
~
~
1
F
10 10 3
8 1 0 3
~
5
N
Habitat 4
18
24
?
~
~
~
~
~
8
-
~~~~~~~
S
D
Habitat 3
32 31 35 6 18 18 21 0
2 12 30 2 1 1 3 0 8 12 61 7
1 4 1 8 1 ? ? ? ? 2 1 2 1 0 1 9 2 2 2 0 5 - - 1 3 2 4 4 3 6 ? ?
? ? 1 0 3 2 5 3 0 ? ? ? ? 9 2 0 6 4 0 ?
2 3 4
2 ? ? ? ? ? ?
? ? 4 1 1 1 0 1 0 1 2 0 9 1 8 2 8 4 0
3 14 18 2 5 6 6 0 7 25 44 3 6
3 5 7
4 ? ? ? ? 3 3
1 3 0 0 1
1 1 0 - - - - 7 1 7 4 1 4 ?
4 5 2 7 5 ? ? ? ? 3 3
1 1 0 0 9
2 1 0 - - - - 7 21 60 12
1 7 23 4 0 4 5 1 0 4
5 0 2 17 32 3 ? ? ? ? 7 20 1 0 5 5
0 0 8
2 ? ? ? ? ? ?
? ? 2 1 8 2 6 2 ? ? ? ? 0 1 8 5 5 4 ?
1 3 1
1 ? ? ? ? 0 5
1 0 2 2 3 1 4 4 ? ? ? ? - 1 5 9
2 1 3 0 0 0 4
1 0 2 2 4 1 2 3 ? ? ? ? 0 3 7 4 5 4 ?
2 9 0
0 3 4 0 0 ? ?
? ? 0 2 1 8
0 ? ? ? ? 2 2 7 1 4 0 ?
? ? ?
? ? ? ? ? 1 3 7 5 0 1 3 8 7
0 ? ? ? ? 1 2 9 2 6 0 ?
4 4 1 6 - - - - - 3 2 7 1 7 6 0 4 5 3 0 6
1 1 2 0 6
1 3 3 0 6 1 3 3 0 1 1 2 5 9 0 0 0
2 ? ? ? ? ? ?
? ? 2 1 4 1 0 0 2 2 2 0 1 0 3 7 2 6 2 7
0 7 8
3 0 0 0 1 ? ?
? ? 0 1 0 7
0 ? ? ? ? 6 7 1 2 0 2 9
1 7 6
0 0 0 1 0 0 1
2 0 0 1 2 8
0 ? ? ? ? 7 4 3 2 1 0 ?
~
~
~
~
F
Habitat 2
Weser & Elbe
Westerschelde
Oosterschelde
Ems-Dollard
Loch Etive
Forth EstuaIy
Humber
Thames
Mersey
Somme
Seine
Loire
Bay of Cadiz
Guadalquivir
Ria de Aveiro
Obidos
Tagus
Mira
~
~
N
Habitat 1
Number of fish species using each of the nine habitats for the four categories of habitat utilisation per selected estuary. Key to habitats: S = Spawning: N = Nursery: F = Feeding: D =
Gullmarsfjord
Darss-Zingster
OderhaffBtettin-
NWAland
GGtaRiver
Estuary
Diadromy.
Table 2.9
46
Fishes in Estuaries
Table 2.10 Habitat utilisation index (HUI)
Habitadnumber
HUI
~
Tidal freshwater (1)
Reed beds (2)
Saltmarsh (3)
Intertidal soft substratum (4)
Intertidal hard substratum (5)
Subtidal soft substratum (6)
Subtidal hard substratum (7)
Subtidal seagrass beds (8)
Biogenic reefs (9)
23.1
15.5
19.3
37.6
9.0
69.7
43.3
46.5
20.7
dal hard substratum (7) > Intertidal soft substratum (4),Subtidal seagrass beds (8),Saltmarsh
(3) > Biogenic reefs (9), Tidal freshwater (1), Reed beds (2) > Intertidal hard substratum
(5). However, when considering the percentage of the total commercial species per habitat
type the original ranking is almost restored: Subtidal soft substratum (6) > Intertidal soft
substratum (4) > Subtidal hard substratum (7), Subtidal seagrass beds (8), Saltmarsh (3) >
Biogenic reefs (9),Tidal freshwater (1),> Reed beds (2) > Intertidal hard substratum (5).The
HUI using the commercial species instead of the total species number, gives the same ranking: Subtidal soft substratum (6) > Subtidal hard substratum (7), Subtidal seagrass beds (8),
Intertidal soft substratum (4),> Saltmarsh (3), Biogenic reefs (9),Tidal freshwater (1),Reed
beds (2) > Intertidal hard substratum (5).
These analyses illustrate the fact that habitat 6 is the most widely used habitat within the
selected estuarine sites. Habitats 4, 7 and 8 take a second place in each of the rankings, then
habitats 1, 2, 3 and 9 with habitat 5 always being ranked lowest. The deviations from this
ranking are minor.
2.4.5 Ecological guilds
The occurrence of six ecological guilds in and among the selected estuarine sites is represented in Table 2.12 and Figs 2.18 (a) and (b). The six ecological guilds are: diadromous species (CA), freshwater species (FW), estuarine residents (ER), marine adventitious species
(MA), marine juveniles migrants (MJ) and marine seasonal migrants (MS) (Elliott & Dewailly, 1995).No clear patterns exist amongst the three regions. Within the selected estuarine
systems, great regional variation exists in the ecological guilds making up the estuarine fish
assemblages. For example, in the Mediterranean Messolonghi lagoon, the fish populations
are substantially influenced by an unusually high proportion (61%) of marine adventitious
species. Similarly, the fish fauna of the three Baltic systems are characterised by a strong
freshwater component. The more strongly tidal estuaries, Weser-Elbe, Ems-Dollard,Westerschelde, Humber, Thames, Mersey, Loire and Tagus all possess a relatively large tidal freshwater influence, and therefore contain a clear freshwater component. In general, the marine
seasonal migrants and the diadromous species are less well presented in the fish fauna of
the selected areas. The guilds contributing most to the fish fauna of the sites considered are
the true estuarine residents and the marine adventitious species. However, it is emphasised
that these findings relate to number of species and that abundance of each guild would show
F
D
S
N
F
D
S
N
F
D
S
N
F
D
S
N
F
D
S
N
F
D
S
N
F
0
D
Baltic
NorthEurope
SouthEurope
BoreaVNWAtl.
Mediterranean
0
0.6
0.4
0.5
0
~~~~
0
0
0
0
0
0
1
?
1
~~~~~~~~~~~~
1
~~~~~~~~
0
1.6
1.2
1.4
1.0
0
3
1
2
1
1
0
1
1
1
?
1
2
0
2.1
0.2
1.4
1.0
0
1
0
4
2
3
0
0
0
0
?
0
1
6
3
0
2.1
3.0
2.4
0
2
3
0
3
1
3
1
1
1
6
?
1
7
1
3
?
0
0
?
?
4
?
?
0
0
?
?
1
?
3
?
0
0
0
0
?
5
?
?
?
?
2
?
2
0
?
?
1
?
?
0
O
?
0
0
0
0
1.0
0
2.0
8.7
4.2
4.0
0
0.3
0.3
0.3
1.0
0
6.4
9.8
7.5
0
0
7.3
3.2
5.8
0
0
1.0
0.2
0.7
0
0
0
0
0
0
~
0
0.3
2.0
1.0
0
~
~
0
18
19
16
16
9
26
48
5
~
8
12
1 2
9
0
11
8
9
7
2.66.8
1.3 9.4
0.7 0
1.1 13.7
0 0
4
2
1
1
0
1
1
3
0
0
0
0
-0
1
2
2
0
0
0
1.5
0.6
1.0
?
0
0
?
?
?
?
?
?
3
0
?
?
0
0.7
2.5
1.4
0
~
4 7 3 0 0 1
1 0 6 2
1 2 1 9 0 ? ? ?
7 4 0 0 1 1
0 0 0 0 0 0
0 8 0 - - 0 6 0 - - 1 0 1 6 3 ? ? ?
7 7 1 ? ? ?
9 4 1 ? ? ?
1 1 3 l ? ? ?
1 4 3 O ?
? ?
2 2 2 0 ? ? ?
- - - - - 5 7 1 0 4 5
1 2 5 0 0 0 0
5 2 0 ? ? ?
1 0 0 ? ? ?
0
0.5
0
0.3
0
- 1
1 1
? 1
? 0
0
0 0
0 0
1 2
? 0
0 0
0 0
? 0
0 0
- 1 0
? 0
? 0
0 0
0
3.5
2.3
3.1
4.0
0 0 8
0 0 6
0 4 5
? ? ?
0 2 0
0 2 0
?
? ?
0 2 1 1
- - 0 5 6
? ? ?
? ? ?
0 0 0
1 0 - - ? ? 0 4 2
? ? ?
? ? ? ? ?
4.34.32.00
0.3 1.7 2.0 0.3
0 0.8 0.8 1.0
0.1 1.1 1.3 0.7
0 2.0 2.0 0
0
?
0
0
?
?
0
?
?
0
0
?
?
1
?
4
6
0
4
7
2
2
0
~~~~~~~~~~~~
I
7
2
0
3
0
3
7
~
?
~
?
-
~
?
-
? ? ? ? ?
0 1 0 0 0 0 0
? ? ? - -
-
0
7
?
1
0
-
0
2
?
0
0
?
?
?
-
0
1.0
2.0
1.8
6.0
-
1.5
19.0
2.7
6.8
7.0
? ? ?
? ? ?
? ? ?
0 4 6
? ? ?
0 2 2
0 0 0
- - - - - - - - ? ? 1 1 1 9
- - - ? ?
0 0 0 - - - - - - - - - ? ?
- - - ? ?
? ? ? ? - ? ? ? ? - ? ? ? ? 0 1 4
0 6
1 1 2 5 2 ? ?
1 1 4 4 2 0 3
? ? ? ? 0 0
?
?
0
0
?
-
-
15.03.02.64.47.80.43.67.27.80.40
13.2 2.8 0 0 0.5 0 0 0 0 0 1.0
9.3 3.0 1.0 0 4.5 2.0 0 5.8 2.0 0.5 0
11.0 2.9 0.5 6.5 2.5 1.0 0 4.6 1.6 0.4 0.3
0
1.5 6.0 0 0 1.0 2.0 0 0 0 3.0
0
5
11
6
19
8
16
9
15
18
20
7
0
19
13
14
7
0
0
1.3
1.0
0
4
?
0
0
?
?
?
?
?
2
?
-
No. of
species
15
19
0
1 5
0
19
13
1 6
8
10
2 0
2 5
1 8
32
2 6
2 9
5 1
5
0.5 1.4 1.4 2.3 1.3 2.1 1.5 0.5 0.1 4.2 3.2 0.4 0.3 7.5 5.8 0.7 0.0 0.8 1.2 0.7 1.3 13.0 12.5 2.8 2.1 7.7 8.8 0.7 1.8 6.9 6.9 0.4 0.4 1.4 4.4 0.4
-
N
Habitat 9
Mean
-
S
Habitat 8
0
0
0
4
D
Habitat 7
4
2
F
Habitat 6
4
0
N
Habitat 5
Weser & Elbe
Westerschelde
Oosterschelde
Ems-Dollard
Loch Etive
Forth EstuaIy
Humber
Thames
Mersey
Somme
Seine
Loire
Bay of Cadiz
Guadalquivir
Ria de Aveiro
Obidos
Tagus
Mira
S
Habitat 4
N
D
Habitat 3
S
F
Habitat 2
Estuary
Habitat 1
F = Feeding: D = Diadromy.
Table 2.1 1 Number of commercial fish species using each of the nine habitats for the four categories of habitat utilisation per selected estuary. Key to habitats: S = Spawning: N = Nursery:
48
Fishes in Estuaries
Table 2.12 Number of fish species in each of six ecological guilds per selected estuary. Key: CA = Diadromous
species: FW = Freshwater species: ER= Estuarineresident species: MA= Marine adventitious species: MJ = Marine
juvenile migrant species: MS = Marine seasonal migrant species.
No. of species in each ecological guild
Estuary
FW
ER
MA
MJ
3
4
4
6
9
17
3
1
20
29
16
15
13
10
8
3
17
19
3
1
3
9
9
3
2
2
5
4
2
1
44
53
50
44
50
Weser & Elbe
Westerschelde
Oosterschelde
Ems-Dollard
Loch Etive
Forth
Humber
Thames
Mersey
Somme
Seine
Loire
Bay of Cadiz
Guadalquivir
Ria de Aveiro
Obidos
Tagus
Mira
9
7
9
7
4
7
9
8
6
4
11
10
1
7
7
4
5
5
33
13
1
3
1
0
18
21
13
0
9
8
0
2
4
0
4
1
12
12
16
13
14
10
15
19
12
7
15
4
12
9
15
9
12
17
8
8
25
9
22
19
27
40
19
6
24
6
18
4
13
11
34
19
10
11
14
13
7
7
12
13
12
7
14
13
22
6
10
16
20
14
6
5
9
8
2
2
4
9
7
4
6
5
0
4
6
5
9
5
78
56
74
53
50
45
85
110
69
28
79
46
53
32
55
45
84
61
Messolonghi
Ebro
1
3
3
2
10
3
38
0
5
2
5
2
62
12
Mean
BalticiSkagerrak
BorealiN. Atlantic
Borealis. Atlantic
BorealiNW Atlantic
Mediterranean
5.2
7.4
5.6
6.7
2.0
12.4
13.1
11.1
12.4
6.5
8.6
18.8
15.0
17.3
19.0
NW h a n d
Gota River
Gullmarsfjord
Darss-Zingster
OderhaffiStettin
14
10.2
2.7
7.3
2.5
5.2
10.9
14.4
12.7
3.5
MS
Total no.
ofspecies
CA
2.8
5.6
4.9
5.9
3.5
different patterns. For example, the abundance of marine adventitious species is likely to be
very low whereas that for estuarine residents will be high.
2.5 Discussion
The analysis presented here, which may be regarded as a case study for studies of estuarine
habitats in other geographical areas, indicates the value of adequate habitat information. It
has been possible to obtain best estimates of the extent and distribution of nine fish habitats
in 26 selected estuarine systems in Europe such that, in most estuarine systems, more than
95% of the total surface area was allocated to one of the nine fish habitats. The 26 selected
estuarine systems have been categorised into three biogeographic regions (BoreaVAtlantic;
Habitat Use byFishes in Estuaries
49
100%
80%
60%
M
u
c)
40%
L
k
20%
WCA B F W OER NMA WMJ HMS
100%
90%
80%
70%
u
60%
c,
50%
k
40%
M
30%
20%
10%
0%
Baltic
(b)
1 WCA
BoreaUNW Atlantic
Mediterranean
Regions
HFW OER N M A WMJ HMSI
~
Fig. 2 18 (a) Representation of the ecological guilds in each of the selected estuaries. (b) Representation ofthe six
ecological guilds for the three geographical regions. CA, diadromous species: FW, freshwater species: ER, estuarine
residents: MA, marine adventitious species: MJ, marine juveniles migrants: MS, marine seasonal migrants.
BalticBkagerrak; Mediterranean) based on the influence of semi-diurnal tides and variations
in salinity and temperature regimes.
The diversity of the nine fish habitats within the selected estuarine systems varied between
one and eight and was on average, greater in the BoreaVAtlantic region due to the influence
50
Fishes in Estuaries
of semi-diurnal tides. Subtidal soft substratum (habitat 6) was the most extensive and widely
distributed habitat in the selected estuarine systems, accounting for more than 50% of the total
surface area. Intertidal soft substratum (habitat 4) was the next most extensive habitat, accounting for almost 30% of the total surface area in the BoreaVAtlantic region, although this
habitat was absent from the Skagerrak/Baltic and Mediterranean regions due to the restricted
tidal ranges of these regions.
It is, however, apparent that the 26 European estuaries included here all differ in the extent
and composition of their constituent habitats, with great variation both within and between
the three regions, in the extent to which the nine fish habitats have been identified and their
areas quantified. Similarly, the availability of data on the fish assemblages of these estuaries, and the use made of individual habitats by each fish species within the estuary, varies
greatly between sites. Of the three European biogeographical regions for which estuaries
are included here, only the BoreaVAtlantic has had any major quantification of the estuarine
resource.
The number of fish habitats were generally more diverse in the selected estuaries from
the BoreaVAtlantic region where, as already mentioned, they are typically subject to greater
tidal action than those of the other regions (leading generally to a greater incidence of intertidal habitats in the estuaries of this region). The relative insignificance of tides in the
BaltidSkagerrak and the Mediterranean influenced the extent to which to a single habitat,
subtidal soft substratum, became the dominant in terms of surface area in these regions, and
also meant that the second-most extensive habitat, intertidalsoft substratum, was restricted to
the selected estuaries of the BoreaVAtlantic region. Only three other habitats, i.e. saltmarsh
(habitat 3), subtidal hard substratum (habitat 7) and subtidal seagrass beds (habitat 8) accounted for more than 10% of the total surface area in any individual biogeographic region.
Tidal freshwater (habitat 1),reed beds (habitat 2), intertidal hard substratum (habitat 5) and
biogenic reefs (habitat 9) were minor habitats, accounting for less than 5% of the totalsurface
area in all biogeographic regions.
The number of fish species recorded in a single estuary varied between 24 (Ebro) and 110
(Thames), with an average of 57 species across the 26 systems. Such values are easily biased
by differences in the sampling effort and timescale of the study, making the distinction of
differences between estuaries on the basis of differences in habitat diversity from the current
datasets extremely problematic. There was great variability between regions in terms of research effort directed towards habitat use by fish in the selected estuarine systems. Similarly,
the inclusion of freshwater areas, and thus fish species varied between estuarine systems.
Such freshwater species will elevate the richness recorded, a feature shown, for example,
by the Thames information. In general, however, the BaltidSkagerrak systems were most
species-poor. Data from too few Mediterranean systems were available to permit further
meaningful comparison of total species numbers between the three regions. In addition, in
general, the proportion of commercial species per estuary increases from Northern to Southern Europe, with approximately 38.6% of species (overall mean) recorded as commercial in
Europe (Table 2.6).
In considering the use of the nine habitats by fish in the selected estuaries, four habitatuse functions were considered: (i) as spawning grounds; (ii) as nursery areas; (iii) as feeding
grounds; and (iv) as pathways in diadromous (catadromous or anadromous) migrations. The
selected BaltidSkagerrak estuaries have been subject to the most detailed investigations, and
51
Habitat Use byFishes in Estuaries
the Mediterranean least. Of the four uses, feeding was the most frequent use overall, followed
by use as a nursery and use for spawning. Only a few species were recorded as diadromous.
This ranking also holds for the three regions individually and generally, even within the individual nine habitats. A further use of an area as a refuge has not been separated but is
included as a component of the above four uses.
In terms of assessing the relative importance of the nine habitats, a HUI combined species richness and number of use functions for each species. The HUI ranked subtidal soft
substratum as the most important habitat overall, followed by subtidalseagrass beds, subtidal
hard substratum and intertidal soft substratum, to give a similar ranking, of these four habitats
as the most important, to that based on species richness. The remaining five habitats scored as
relatively unimportant, with intertidal hard substratum, recorded as only a minor constituent
habitat of most of the selected 26 estuaries, scoring lowest in terms of habitat utilisation.
These analyses could be improved by obtaining more consistent data between sites, or
by the inclusion of comparisons of abundance of each species. Furthermore, account should
be taken of the total or relative areas of each habitat available at each site. In an attempt to
compare the number and extent of habitats at each site with the number of species present,
Fig. 2.19 shows the number of fish species recordedversus the percentage occurrence of each
of the nine habitat types, for the AtlantidBoreal and BaltidSkagerrak regions. Too few data
were available to include the Mediterranean region in the analysis. From this figure it is clear
that the subtidal soft substrata habitat (number 6) is dominant in terms both of area and species diversity for bothregions. For theAtlantic/Boreal, the number of species decreases as the
percentage occurrence of the habitat type diminishes, but for the BaltidSkagerrak other areas
such as subtidal seagrass beds (habitat 8) or subtidal hard substrata (habitat 7), which may be
present in relatively small areas, may have an equally high species diversity as the subtidal
soft substrata habitat.
~
Q2
~
250
.*E
2 200
4
c
9$
150
..-g 100
e
m
R
.
*-
L
0 4
0 9
6.
8.
50
z
0
0
20
40
60
Percentage occurrence of the 9 habitat types
80
Fig. 2.19 Number of fish species recorded in each of the nine habitat types versus the percentage of occurrence of
each of the nine habitat types in the AtlanticiBoreal and BalticiSkagerrak.
52
Fishes in Estuaries
2.6 References
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Cattrijsse, A,, E.S. Makwaia, H.R. Dankwa, 0. Hamerlynck & M.A. Hemminga (1994) Nekton communities of an intertidal creek of a European estuarine brackish marsh. Marine EcologyProgress
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Crivelli, A.J., M.C. Ximenes, B. Gout, G. Lasserre, P. Freaon & T. Do Chi (1995) Causes and effects
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Davidson, N.C., D.d’A. Laffoley, J.P. Doody, J.S. Way, J. Gordon, R. Key, M.W. Pienkowski, R. Mitchell & K.L. Duff (1991) Nature conservation andestuariesin GreatBritain. Joint Nature Conservation Committee, Peterborough.
de Jong, D.J. & V.N. de Jonge (1995) Dynamics and distribution of microphytobenthic chlorophyll-a
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de Jong, D.J. & C. Meulstee (1989) Weren en weiden in de Oosterschelde. Rijkswaterstaat, Dienst
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de Jong, D.J. &A.M. van der Pluijm (1994) Consequences of a tidal reduction for the salt-marshvegetation in the Oosterschelde estuary (The Netherlands). Hydrobiologia,283,317- 333.
de Jong, D.J., Z. de Jong & J.P.M. Mulder (1994) Changes in area, geomorphology and sediment nature
of salt marshes in the Oosterschelde estuary (SW Netherlands) due to tidal changes. Hydrobiologia,
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Dijkema, K.S. (1989) Habitats of the Netherlands, German andDanish WaddenSea, 1:100,000. Research Institute for Nature Management, Texel, Veth Foundation, Leiden.
Drake, P. &A.M. Arias (1991) Ichthyoplankton of a shallow coastal inlet in southwest Spain: factors
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Elliott, M. & F. Dewailly (1995) Structure and components of European estuarine fish assemblages.
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Elliott, M., S. Nedwell, N.V. Jones, S.J. Read, N.D. Cutts & K.L. Hemingway (1998) blume I I ~
Intertidal sand and mudflats & subtidal mobile sandbanks: an ovemiewof dynamic and sensitivitycharacteristics for consemationmanagement ofmarine SACs.Institute of Estuarine & Coastal
Studies, University of Hull, (report for and prepared by Scottish Association for Marine Science
(SAMS) for the UK Marine SACs Project).
~
Habitat Use byFishes in Estuaries
53
European Council Directive (1992) Council Directive 92/43EEC of 21 May 1992 on the conservation ofnaturalhabitats and wildfauna andflora. The ‘Habitats & Species Directive’, O.J. L206,
22.07.92.
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O.J. L327,22.12.2000.
Holt, R. (1991) Marine Nature ConservationReview SurveysofScottishSea Lochs: LochEtive.Vol. 1.
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Holt, T.J., E.I. Rees, S.J. Hawkins & R. Seed (1998) blume I X - Biogenic Reefs: an overview of
dpamic and sensitivitycharacteristics for conservation management of marine SACs. Port Erin
Marine Laboratory, University of Liverpool, (report for and prepared by Scottish Association for
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Kies, L., L. Neugebohrn, H. Braker, T. Fast, G. Gatje & A. Seelig (1992) Primarproduzenten und
Primarproduktion imElbe-Astuar.Berichte aus dem Zentrum fur Meeres- und Klimaforschung der
Universitat Hamburg, 19, 137- 168.
McDowall, R.M. (1988) Diadromyin Fishes. Croom-Helm, London.
McLusky, D.S. (1989) TheEstuarine Ecosystem.2nd edn. Blackie A & P, Glasgow.
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