Effect of Stocking Density on the Resistance to Fasting, Growth and Survival ...
Coates et al
1. Efficacy of a multi-metric fish index as an analysis tool for
the transitional fish component of the Water Framework Directive
Steve Coates *, Adam Waugh, Alice Anwar, Matthew Robson
Environment Agency, Rivers House, Crossness Works, Belvedere Road, Abbeywood, London SE2 9AQ, UK
Abstract
The WFD has introduced an international commitment to assess the ecological status of transitional waters (TWs), within which fish
communities are a key biological monitoring component. The Transitional Fish Classification Index (TFCI) outlined in this paper uses
10 ecological measures to analyse fish populations caught from various ecological niches using a variety of gear types within the Thames
estuary. These reach and method-specific communities are then compared to a reference population created from a ‘healthy’ population
from TWs of a similar type. The results indicate a progressive downstream increase the quality of fish communities, consistent with pre-
vious work; variation between methods can be accounted for by gear selectivity. Overall, the TFCI is an effective communication tool for
converting ecological information into an easily understood format for managers, policy makers and the general public.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: WFD; Thames estuary; IBI; Biological indicators; Ecosystem health; Fish
1. Introduction
The Water Framework Directive (WFD) has introduced
an international commitment to assess the ecological status
of transitional waters (estuaries), within which fish commu-
nities are a key biological monitoring component (Euro-
pean Council Directive, 2000). Fish communities can be
described according to a variety of characteristics such as
composition, trophic structure and diversity of the assem-
blage, as well as abundance and biomass of the individuals
(Harrison et al., 2000; Lobry et al., 2003; Coates et al.,
2004; Harrison and Whitfield, 2004). Trends in one or more
of these community attributes can be used to monitor the
ecological functioning and ‘health’ of a particular ecosys-
tem (Whitfield and Elliott, 2002).
The WFD specifies that the transitional fish quality ele-
ment is to be assessed by taking account of the composition
and abundance of the fish fauna and that of disturbance-
sensitive taxa. In order to carry out an integrated approach
to assess the fish community of the Thames estuary, a num-
ber of attributes have been incorporated into a single multi-
metric index. This methodology has been used in many
other studies, (Miller et al., 1988; Deegan et al., 1997; Har-
rison et al., 2000; USEPA, 2000; Goethals et al., 2002;
Borja et al., 2004; Breine et al., 2004).
As part of the assessment of the fish faunal assemblage
within an estuary, a number of monitoring techniques
and sampling strategies have been developed (Hemingway
and Elliott, 2002). Environment Agency, Thames Region
has established a long-term monitoring programme based
on the recovery of the Thames estuary, with the initial sur-
vey work based on power station fish impingement
(Wheeler, 1979; Attrill, 1998; Kirk et al., 2002). However,
with the decommissioning of the Thames power stations
and the need to address the data gaps caused by this sin-
gle-strand survey approach, a multi-method monitoring
programme was established (Colclough et al., 2000,
2002). This approach combines a variety of methods such
as seine netting, beam trawling and otter trawling. Differ-
ent survey techniques have varying gear selectivities so it
is important to incorporate a suite of techniques (von
Brandt, 1964) to obtain a comprehensive picture of each
0025-326X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpolbul.2006.08.029
*
Corresponding author.
E-mail address: steve.coates@environment-agency.gov.uk (S. Coates).
www.elsevier.com/locate/marpolbul
Marine Pollution Bulletin 55 (2007) 225–240
2. fish community assessed e.g. benthic, pelagic and marginal.
The Thames multi-method monitoring strategy has now
been recognised as an example of ‘European Best Practice’
in establishing an estuarine fishery-monitoring programme
(European Commission, 2000).
Karr (1981) derived the basis of a metric scoring system
from work in assessing the ‘Biotic Integrity’ of North
American fish communities. The principles of the Index
of Biotic Integrity (IBI) established by Karr are widely
accepted and have been used as a classification tool for
fisheries assessment (Harrison and Whitfield, 2004; Borja
et al., 2004; Breine et al., 2004; Coates et al., 2004). These
principles have been used recently for WFD purposes
within the ‘Fish-based Assessment Method for the Ecolog-
ical Status of European Rivers’ (FAME) (Kestemont et al.,
2002). To consider the biotic integrity of a water body, a
comparison must be made between the data and a ‘refer-
ence’ community.
The approach described here aims to devise metrics suit-
able for analysing fisheries data that are appropriate for the
WFD transitional fish component requirements. Further-
more, by using the Thames estuary fish long-term data in
a multi-method, reach-based format the efficacy of the met-
rics can be examined in detail.
2. Materials and methods
2.1. Selection of metrics
USEPA (2000) define a metric as a ‘‘measurable factor
that represents some aspect of biological assemblage, struc-
ture, function, or other community component’’. The selec-
tion of the candidate metrics were devised partially on a
classification scheme developed for use in South African
estuaries (Harrison et al., 2000; Harrison and Whitfield,
2004) and from the output of an Environment Agency
R&D report to develop classification tools for the WFD
(Coates et al., 2004). These metrics also reflect the ‘norma-
tive definitions’ for the assessment of biological quality as
defined by the WFD. The candidate metrics are based upon
either presence/absence data, ‘relative’ abundance data or
number of taxa present (Table 1). Combined, the metrics
provide an overall Transitional Fish Classification Index
(TFCI). The measured element of the TFCI is a relative
score (RS). Created for each metric, the RS indicates the
sample’s proximity to an ideal or ‘reference’ community.
An RS is created for each sampling occasion. These are
then averaged to create a combined RS for each sampling
regime. These are again averaged to create an overall RS
for each reach.
2.1.1. Metric 1 – ‘Species Composition’
A key indicator of the biotic integrity of a faunal com-
munity is the composition of species within a sample (Har-
rison and Whitfield, 2004). To create the RS, firstly the
inherent variability or ‘noise’ within each catch was
reduced by removing all but the 20% most frequent species.
This top quintile was analysed using a Bray-Curtis similar-
ity index to determine the percentage similarity of each
sample compared to the reference. An RS of one to five
is weighted evenly between 0 and 100 (Table 2).
2.1.2. Metric 2 – presence of ‘Indicator Species’
Metric 2 provides a measure of ‘disturbance-sensitive
species’ termed ‘indicator taxa’ by the WFD and was calcu-
lated for each sample. Species were selected on the basis of
their conservation status and protection under EU legisla-
tion (European Council Directive, 1992), their sensitivity to
dissolved oxygen (Turnpenny et al., 2004), or other traits
that make them sensitive to disturbance. Lampreys (Lam-
petra fluviatilis & Petromyzon marinus) were selected for
their sensitivity to water quality and spawning habitat
quality, as were Allis shad (Alosa alosa) and Twaite shad
(Alosa fallax). Salmonids (Salmo salar and Salmo trutta)
were selected for their conservation status and sensitivity
to dissolved oxygen (DO) and temperature (T). Smelt
(Osmerus eperlanus) were also chosen for their sensitivity
to DO and the European eel (Anguilla anguilla) was chosen
for its present sensitivity to anthropogenic exploitation. All
of the above species are sensitive to hydromorphological
changes, which have occurred to most UK TWs, the
Thames being an example of one of the most severely
disturbed.
Table 1
Candidate fish metrics
Metric type No. Metric
Species diversity and
composition
1 ‘Species composition’
2 Presence of ‘Indicator Species’
Species abundance 3 Species relative ‘abundance’
4 Number of taxa that make up 90% of
the ‘abundance’
Nursery function 5 Number of estuarine resident taxa
6 Number of estuarine-dependent marine
taxa
Trophic integrity 7 Functional guild composition
8 Number of benthic invertebrate feeding
taxa
9 Number of piscivorous taxa
10 Feeding Guild Composition
Metric 1 is based on presence/absence. Metric numbers 3 and 4 are based
on species relative abundance. The remaining metrics are based on the
number of taxa present.
Table 2
Scoring system for metrics 1 and 3, based on percentage similarity of each
sample to reference, calculated using a Bray-Curtis similarity index
Percentage similarity Relative score
0–19.9 1
20–39.9 2
40–59.9 3
60–79.9 4
80–100 5
226 S. Coates et al. / Marine Pollution Bulletin 55 (2007) 225–240
3. If any of the above taxonomic groups were present, a
score of 1 was assigned to the sample. Scores were adjusted
according to the sample method and are based upon the
probability of capture for each species per sample tech-
nique. Seine net samples, for example, included shads, lam-
preys, salmonids, eels and smelt whereas only eels, smelt
and lampreys were included for beam and otter trawling.
2.1.3. Metric 3 – species relative ‘abundance’
Similarly to Metric 1, the abundance of species provides
an excellent indicator of biotic integrity (Harrison and
Whitfield, 2004). Unlike Metric 1 where a presence or
absence of taxa was used to create an RS, the relative abun-
dance of individuals per sample was used to create the
score. Similarly to metric 1, only the top quintile of most
abundant species per sample were compared to the refer-
ence using a Bray-Curtis similarity index. The RS was
derived from the per cent similarity values. It is calculated
as a percentage of each taxa present in relation to total
number or relative abundance caught.
2.1.4. Metric 4 – number of taxa that make up 90% of
abundance
A healthy, unimpacted estuary will contain many species
without a dominating presence from one or only a few spe-
cies. By considering how many species make up 90% of the
catch, one can determine whether dominating taxa are
present.
The number of taxa were counted for each sample and
ranked from the most to the least frequent. The mean num-
ber of taxa within the upper quintile (top 20%) was deter-
mined and used as the boundary value between RS4 and
RS5. Percentages of this value were used to calculate the
boundaries for each metric. Each boundary range had an
associated score (Table 3).
2.1.5. Metric 5 – number of estuarine resident taxa
Metrics 5–7 are based on the number of functional
guilds represented by the fish in each sample. Each fish spe-
cies was allocated to a ‘Guild’ that best describes its life his-
tory characteristics within an estuary (Elliott and Dewailly,
1995; Hemingway and Elliott, 2002):
• Estuarine residents (ER) – Fishes that spend their entire
life in estuaries.
• Marine seasonal species (MS) – Fishes that use estuaries
for part of the year.
• Freshwater species (FW) – Fishes that are present
mainly or exclusively at low salinity values.
• Marine juvenile species (MJ) – Fishes that use estuaries
as nursery grounds or during juvenile phases of their life
cycle.
• Diadromous species (CA) – Species that migrate
between fresh and salt water during different life stages.
• Marine adventitious species (MA) – Species that are
considered fully marine but inhabit estuaries
temporarily.
For Metric 5 the estuarine resident species were filtered
in each sample. The method described above in Metric 4
was followed to create a score from one to five.
2.1.6. Metric 6 – number of estuarine-dependent marine taxa
For this metric, fish that are dependent on estuaries dur-
ing the early life phases or during particular parts of the
year (marine juveniles or marine seasonal) were treated in
the similarly to Metric 4.
2.1.7. Metric 7 – functional guild composition
An unimpacted, healthy estuary should contain species
of fish that represent all functional guilds (Elliott and
Dewailly, 1995; Hemingway and Elliott, 2002), although
one must account for the fact that an estuary is highly het-
erogeneous. It is therefore unlikely to catch freshwater spe-
cies in the lower estuary, for instance. So Metric 7 was
adjusted according to the section of the transitional water
(TW) analysed. All functional guilds apart from the fresh-
water taxa were scored in the mid and lower data sets and
in the upper TW, all guilds were scored except marine
adventitious species. Scores were assigned according to
the number of taxa present (Table 4).
2.1.8. Metric 8 – number of benthic invertebrate feeding taxa
Feeding guilds have been used for some time as a way to
evaluate fish communities and structure (e.g. Goldman and
Talbot, 1976). Increased levels of anthropogenic stress have
also been shown to remove one of more of the feeding
guilds (Harrison and Whitfield, 2006). The feeding guilds
used for metrics 8–10 were developed by Whitfield (1998):
benthic invertebrate feeders, zooplankton feeders, piscivo-
rous feeders and detritus feeders. For this metric, fish
caught that are considered to feed exclusively or mainly
on bottom-dwelling invertebrates were considered for anal-
ysis. RS creation and score allocation was derived in a sim-
ilar fashion to metric 4.
Table 3
Example of scoring system used for metrics 4–6, 8 and 9. Example is based
on the upper quintile having a mean of 6 taxa
Percentage of mean Boundaries RS
0–19.9 <1.19 1
20–39.9 1.2–2.39 2
40–59.9 2.4–3.59 3
60–79.9 3.6–4.79 4
80–100 4.8+ 5
Table 4
Scoring system for metric 7 showing the score assigned according to the
number of taxa present
Number of taxa Score
0–1 1
2 2
3 3
S. Coates et al. / Marine Pollution Bulletin 55 (2007) 225–240 227
4. 2.1.9. Metric 9 – number of piscivorous taxa
For this metric, only fish caught that are recognised as
only or exclusively feeding on other fish were analysed
(Hemingway and Elliott, 2002). The method used to create
the RS and score for metric 4 was used.
2.1.10. Metric 10 – feeding guild composition
The premise of metric 7, that a healthy estuary should
contain certain functional guilds of fish, is presented here
for feeding guilds. For an estuary to be considered of good
quality in terms of fish species, a member of each guild
should be represented in each sample (Table 5).
2.2. Method-specific reference
An ideal reference community is derived from the same
site at the same time of year using the same methods, dur-
ing a period when the environment is pristine and no
anthropogenic changes have occurred. For many water
bodies, samples have only been taken after significant
changes in hydrology and morphology are present so no
‘reference’ data is available for the actual site analysed.
Such a situation exists for the Thames estuary. London
was the first city in the world to witness changes from a less
agrarian towards a more urban society during the indus-
trial revolution. At this point 150 years ago, changes in
water quality, community structure and hydrology began,
causing a change away from a ‘reference’ community
(Wheeler, 1979).
Spatial and temporal variations in fish communities can
lead to bias in both sampling and analysis and can result in
a false representation of the ecosystem measured (Malavasi
et al., 2004). Therefore the data was compared to a refer-
ence community that was limited spatially, temporally
and by type.
The Thames TW is defined by UK typology (UK TAG,
2004; Vincent et al., 2002) of ecotype E4T3 i.e. the water-
body is within the North Sea (WFD Ecoregion 4) and is
a fully mixed, polyhaline, macrotidal, sheltered estuary
with extensive intertidal areas (Type 3). All available data
from all TWs of E4T3 was used as a reference against
which to compare the Thames data.
The abundance data for each of these TWs (Thames,
Medway, Swale, Humber, Great Ouse and the Wash) was
divided into three reaches or ‘bodies of surface water’
(WFD Ref Article 2.9). These reaches represent an upper,
middle and lower section consistent with local sampling
divisions, salinity changes and the fish community changes
from freshwater to marine species. The data was also
grouped by spring and autumn, to take into account sea-
sonal variation. The data for each separate sample method
(seine net, beam and otter trawl) was aggregated by sam-
pling gear for all E4T3 TWs to form a ‘method-specific’
reference. Therefore nine separate reference communities
were created to compare against each sampling occasion.
The reference community used to compare the Thames
(upper reach) spring 1993 seine netting raw results is sum-
marised in Table 7. The reference data was ranked, then
divided into quintiles and the mean species richness for
the upper quintile calculated (Fig. 1). This mean was then
used as a ‘reference’ for calculating metrics 1 and 3. Tables
8–17 list the reference figure used to create the RS. These
relative scores were then averaged per year.
2.3. Metric scoring
Karr et al. (1986) assigned grades of 1, 3 or 5 to quantify
the metric scores to develop a scoring system of biotic
integrity. This approach has been adapted to a 1, 2, 3, 4
or 5 system as the five bands corresponded more appropri-
ately to the ecological status bands of the WFD (WFD CIS
Working Group 2.4 (COAST), 2003; Vincent et al., 2002).
Once the metrics had been calculated for each method-
specific reference for each reach, the sample-level data were
compared to its ecotype ‘reach-reference’. For example, the
upper Thames seine net samples were compared against a
reference value of all E4T3 upper seine net samples. Each
metric was scored according to its respective reference, with
the exception of metrics 2, 7 and 10, which were based
directly on the number of taxa present.
Once the metrics were calculated, the scores were
totalled for each sample and a ‘Relative Score’ (RS) was
generated using the following formula:
RS ¼
Total score of the 10 metrics
Maximum score possible
2.4. Sample methods
Historically, estuarine fish monitoring in the UK has
concentrated on localised surveys within impacted, indus-
trialised estuaries (CEFAS, 2004; Rogers and Millner,
1996). Since 2002, the multi-method approach has been
considered a more effective way of assessing the ‘ecological
status’ of a TW across the UK (Coates et al., 2004).
The use of fish communities in estuaries as indicators of
biotic integrity has received much attention from many
authors (Harrison and Whitfield, 2004; Karr, 1981; Karr
et al., 1986; USEPA, 2000). Using fish as environmental
indicators has many advantages including relative ease of
identification and presence in most estuaries.
The Thames estuary is one of the few TWs in the UK
that has robust data sets using the multi-method approach
Table 5
Scoring system for metric 10 showing the score assigned according to the
number of feeding guilds present
No. of guilds present Score
0 1
1 2
2 3
3 4
4 5
228 S. Coates et al. / Marine Pollution Bulletin 55 (2007) 225–240
6. designed to assess both the pelagic and benthic communi-
ties using seine nets, otter trawls and beam trawls at a vari-
ety of locations throughout the estuary.
The Thames estuary also provides the largest long-term
UK fish data set (Colcough, pers. com.), incorporating a
range of sampling techniques and monitoring sites from
freshwater to marine. Fig. 2 displays the sites sampled. Site
locations were chosen mainly on their substrate regime and
ease of access. A stable shingle shore allows researchers to
gain purser underfoot when pulling in the seine net and
prevents a build up of material in the net, which can cause
the net to roll and loose fish. A firm and even riverbed also
proves a better site for trawling, since less inorganic matter
is disturbed. The sites allowed an extended sampling win-
dow, ranging from almost freshwater upstream to almost
marine downstream.
This approach combines a variety of methods such as
seine netting, beam trawling and otter trawling. Different
survey techniques have varying gear selectivities so it is
important to incorporate a suite of techniques to obtain
a comprehensive picture of each fish community assessed
e.g. benthic, pelagic and marginal.
Table 7
E4T3 (upper reach) spring seine netting results
Functional guild Rare/threatened Feeding guild n % Relative richness Total abundance
Abramis brama FW BI 49 38.6 414
Alburnus spp. FW Y Z 26 20.5 622
Anguilla anguilla CA Y P 56 44.1 145
Atherina presbyter MJ Z 29 22.8 217
Barbatula barbatula FW BI 1 0.8 1
Barbus barbus FW BI 1 0.8 7
Chelon labrosus MS D 4 3.1 32
Clupea harengus MJ Z 1 0.8 27
Cottus gobio FW Y BI 4 3.1 4
Cyprinus carpio FW BI 3 2.4 5
Dicentrarchus labrax MJ P 46 36.2 904
Esox lucius FW P 1 0.8 1
Gadus morhua MJ P 0 0.0 0
Gasterosteus aculeatus CA Z 40 31.5 111
Gobio gobio FW D 13 10.2 67
Leuciscus cephalus FW P 5 3.9 47
Leuciscus leuciscus FW Z 100 78.7 3490
Liza ramada CA D 10 7.9 34
Merlangius merlangus MJ P 0 0.0 0
Osmerus eperlanus CA Y Z 30 23.6 872
Perca fluviatilis FW P 49 38.6 215
Phoxinus phoxinus FW Z 6 4.7 7
Platichthys flesus ER BI 102 80.3 2798
Pleuronectes platessa MJ BI 0 0.0 0
Pomatoschistus microps ER BI 41 32.3 972
Pomatoschistus minutus ER BI 24 18.9 415
Pungitius pungitius CA Z 3 2.4 3
Rutilus rutilus FW Z 103 81.1 4579
Salmo salar CA Y P 3 2.4 5
Salmo trutta CA P 6 4.7 6
Scardinius erythrophthalmus FW Z 1 0.8 1
Solea solea MJ BI 0 0.0 0
Sprattus sprattus MS Z 3 2.4 31
Syngnathus rostellatus ER Z 0 0.0 0
Tinca tinca FW BI 1 0.8 1
Mean 21.7 17.1 458.1
Total 761 599.2 16033
(TWs = 3; years = 11; sampling occasions = 127; total species = 35).
Fig. 1. Example of species richness against rank, generated for each
‘reference’ list. Lines denote divisions into five equal quintiles. The mean
richness of the upper quintile was used to generate a reference for
calculating metrics 1 and 3.
230 S. Coates et al. / Marine Pollution Bulletin 55 (2007) 225–240
12. In the upper to mid estuary a 45 · 3.5 m seine net with a
5 mm knotless mesh centre and 20 mm wings was deployed
from the shore with a 17 ft open dory. The net was
deployed during the low water slack period twice. This
method was designed to capture small to large adult active
fish within the margins of the river. A 1.52 m (50
) wide
beam trawl with a 20 mm knotless outer mesh and 5 mm
knotless cod end, designed to capture demersal species,
was trawled for 250 m parallel to the seining site. In the
mid and lower estuary seine netting & beam trawling was
complimented by paired 8 m wide otter trawls with a
40 mm outer mesh with a 5 mm knotless ‘cod end’ mesh.
It was deployed usually during low water slack or on a
flooding tide and was used to assess the pelagic & benthic
fish community present within the main channel of the
Thames estuary.
3. Results
Table 6 shows the raw results for Thames (upper reach)
spring 1993. Table 7 includes the data for all years for
spring seine netting, that was used to create ‘reference’ con-
ditions for Table 6 values. Reference conditions were cre-
ated in the same way for spring and autumn for seine,
beam and otter trawling for the upper, middle and lower
reaches. Tables 8–17 display the metric scores for Thames
(upper reach) spring 1993 results. The total count, the ref-
erence total, % similarity to reference (where appropriate)
and the RS is displayed for each metric.
The annual RSs for the upper, mid and lower Thames
compared to the ecotype reference values between 1992
and 2004 are shown in Figs. 3–5. It is evident that the rel-
ative scores are higher in the lower TW than those in the
mid TW and those in the mid TW are higher than those
in the upper TW.
Analysing variations between methods, the seine net
has a higher RS compared to the beam trawl in every year
in the upper Thames. The difference between the two
methods is lowest in 1992 and highest in 2000. Similarly,
within the mid Thames, the seine net and otter trawl have
a higher RS compared to the beam trawl in all years.
Otter trawling was introduced in 1997 as part of the
CEFAS & EA bass (Dicentrarchus labrax) population sur-
vey work. Between 1997 and 2000, the seine net had a
slightly higher RS than the otter trawl samples whilst in
2002 and 2003, the otter trawl had a higher RS which var-
ied between 0.1 and 0.4. Within the lower Thames, there
is a similar pattern evident to that in the mid Thames,
where the seine net samples have a higher RS compared
to the beam and otter trawl in most years during the
1990s. Again between 2000 and 2003, the otter trawl sam-
ples have the highest RS.
Fig. 2. Thames estuary indicating division into separate upper, mid and lower reaches for statistical analysis. Points indicate all sampling locations.
236 S. Coates et al. / Marine Pollution Bulletin 55 (2007) 225–240
13. Comparing within-method variation along the TW, the
beam trawl samples had a higher RS in most years in the
lower Thames than further upstream. In the lower TW,
the beam trawl samples also had the greatest range of RS
(0.1–0.6). The overall RSs of the seine net samples
increased slightly between the upper to lower reaches.
There was little difference between the RSs of the otter
trawl samples in the mid and lower Thames.
Initial assessment of dissolved oxygen and the relative
scores within the upper reaches of the Thames suggest that
there may be some correlation between the RS and DO
concentration, for example when DO levels are reduced,
RSs are also low. This exploratory work will be investi-
gated further as part of developing physicochemical stan-
dards for WFD in support of the ‘biological quality
elements’ for transitional and coastal waters.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
92 93 94 95 96 97 98 99 00 01 02 03 04
Year
RS
Beam
Seine
B & S Mean
Fig. 3. Mean annual relative scores for beam trawl and seine net samples in the upper Thames. Samples were compared against a method-specific
reference based on TWs in E4T3. Bars indicate minimum and maximum relative scores per annum.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
92 93 94 95 96 97 98 99 00 01 02 03 04
Year
MeanRS
Beam
Seine
Otter
Mean (B, S & O)
Fig. 4. Mean annual relative scores for beam trawl, seine net and otter trawl samples in the mid Thames. Samples were compared against a method-
specific reference based on TWs in E4T3. Bars indicate minimum and maximum relative scores per annum.
S. Coates et al. / Marine Pollution Bulletin 55 (2007) 225–240 237
14. 4. Discussion
Prior analysis indicated that the assessment methodol-
ogy used provided a statistically robust way of analysing
the long-term fish data of the Thames (Coates, unpublished
data). An alternative was to compare each data set against
a ‘multi-method’ reference incorporating all survey tech-
niques (Coates et al., 2004). This was initially trialed but
because it was not possible to have standard ‘sampling-
effort’ for each monitoring technique within reference, the
resulting assessment of the single-strand data against the
reference was not statistically comparable.
The progressive downstream increase in RS and possible
increase in ecological status could indicate a general
increase in the health of the ecosystem and therefore in
the diversity and abundance of the fish communities. This
change is likely to reflect a shift from the urbanised, mor-
phologically modified channel of London to that of the
lower Thames estuary, which has a more diverse fish com-
munity (Astley, 2004; Kirk et al., 2002). Upper reaches of
the Thames have a greater freshwater influence and sup-
port less species diversity than lower reaches (Colclough
et al., 1992, 1993, 2000).
The mid reach of the Thames is highly channelised and
provides few habitats for juvenile and adult fish (Astley,
2004; Attrill, 1998). There are also high fluctuations in
salinity and so fewer species are able to tolerate the
dynamic macro-tidal environment (Colclough et al., 1992,
1993, 2000). Channel morphology and habitat niche
requirements are known to influence fish communities
(Hemingway and Elliott, 2002). Lower reaches are more
stable in terms of salinity and anthropogenic and there is
a greater diversity of habitat (Astley, 2004) thereby provid-
ing suitable refuges/food sources to support more diverse
communities (Hemingway and Elliott, 2002).
The variation between the results of the methods in all
reaches of the TW is unlikely to be due to variation in sam-
pling effort as each method is based upon consistent sam-
pling effort and gear selectivity (Coates et al., 2004). The
beam trawl is likely to produce samples with lower relative
scores than the seine net and otter trawl because it targets
benthic fish communities. It is a much more discriminative
technique than the other methods and therefore captures
lower species diversity than seine and otter trawling (Colc-
lough et al., 2000). Seine netting is carried out from the
shore and is used for marginal habitat sampling of pelagic
species and those fish communities utilising this habitat.
The otter trawl is towed through the water column and
samples a wide range of benthic and pelagic species, which
would account for the higher RSs during 2000–2004.
5. Conclusions
Although the patterns evident from the analysis fit with
what would be biologically expected to happen within the
Thames TW, the multi-metric tool and assessment outlined
in this paper requires further development as part of WFD
implementation in December 2006.
WFD guidance from Ecostat states that current refer-
ence conditions are not valid as they have been derived
from TWs that are not at ‘Reference’ or High Status (Euro-
pean Council Directive, Section 1.3 (iv), 2000). As the ref-
erence sites selected for TW3 within Ecoregion 4 are not
‘existing undisturbed sites with only very minor distur-
bances’ (WFD CIS Working Group 2.4 (COAST), 2003),
the data comparison needs to be revised because it is based
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
92 93 94 95 96 97 98 99 00 01 02 03 04
Year
MeanRS
Beam
Seine
Otter
Mean (B, S & O)
Fig. 5. Mean annual relative scores for beam trawl, seine net and otter trawl samples in the lower Thames. Samples were compared against a method-
specific reference based on TWs in E4T3. Bars indicate minimum and maximum relative scores per annum.
238 S. Coates et al. / Marine Pollution Bulletin 55 (2007) 225–240
15. on abundance data from TWs already impacted by certain
pressures.
Reference ideally needs to be based on the fish commu-
nities found in historically unimpacted sites (Coates,
unpublished data). If the abundance data per sample from
the current analysis were compared to such a pristine refer-
ence, the resulting RSs would be particularly low, reflecting
poor conditions compared to an unimpacted situation.
Although the method used is statistically robust at the sam-
ple level, the inherent temporal and spatial variability
would be too high to provide any meaningful results. Fish
populations do not aggregate spatially within TWs and are
highly variable throughout the year. These biological char-
acteristics, coupled with the fact that sampling has not been
historically consistent in effort or locality, mean that the
data cannot be analysed at the sample level. Further anal-
ysis will therefore involve pooling the data on an annual
survey basis from all reaches and methods and then com-
paring to reference. Two to three years of combined annual
sample data may also be used to assess ecological status in
this way.
Although the level of analysis can be revised, there are
no ‘reference’ TWs in E4T3 with long-term data sets
against which the Thames data can be compared. Expert
judgement will therefore need to be used in conjunction
with existing data to generate a suitable reference for each
of the candidate metrics (European Council Directive,
2000).
The assessment of the fish community also has to be
considered in relation to the assessment of the hydromor-
phology of the Thames estuary. The Thames is likely to
be classified as a Heavily Modified Water Body (HMWB)
under the WFD and has to meet the requirements of ‘good
ecological potential’ (European Council Directive, 2000).
The RSs will therefore have to be considered in relation
to ‘ecological potential’, rather than in relation to ‘ecolog-
ical status’. The criteria for ecological potential require a
water body to not deteriorate and will most probably will
be compared to the same reference conditions as those that
are not heavily modified, but the boundary criteria may be
different. Confirmation of the ecological potential criteria
have yet to be confirmed by the UK.
Although further development, testing and validation of
the TFCI is necessary, the current analysis has however
provided a valuable insight into comparing differing mon-
itoring techniques. The results indicate the varied selectivi-
ties of the methods and the benefits of the use of a range of
these techniques, in order to provide a complete picture of
functionality within transitional waters. Although still at a
preliminary stage, these results highlight the importance of
a multi-method sampling regime.
The current analysis of the Thames also highlights the
benefit of dividing large TWs into specific reaches and
analysing each independently. Each section is exposed to
different anthropogenic pressures and different hydromor-
phological regimes such as variations in salinity and
habitats. Fish composition and abundance is highly likely
to vary within each reach and as such, should be analysed
separately.
The TFCI developed here incorporates both structural
and functional attributes of estuarine fish communities. It
has been tested here to provide both a robust method for
assessing the ecological status of transitional waters. Fur-
thermore the TFCI could be refined and related to a range
of environmental pressures such water quality, shoreline
reinforcement and flow manipulation once reference condi-
tions have been established. Overall, the TFCI is an effec-
tive communication tool for converting ecological
information into an easily understood format for environ-
mental managers, policy makers and local communities
and stakeholders.
Acknowledgements
We would like to thank Peter Lloyd, Maxine Clement
and Lars Akesson of Environment Agency, Thames Re-
gion for their advice and support with the AQMS data.
We would also like to thank all our transitional fish col-
leagues and associates throughout the UK, Ireland and
Europe for their support and guidance in developing these
classification tools as part of WFD implementation.
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