To date, OSPAR has largely been responsible for the development of an ecosystem
approach to the management of marine natural resources within the convention area. Ten
Ecological Quality Issues have been identified, with the intention of setting Ecological Quality
Objectives (EcoQOs) for each issue. Since 2000, this scheme has been developed and
piloted in the North Sea. Fish Communities is the fifth in the list of Ecological Quality Issues,
and is considered to be one of three community-level issues. In addressing concerns
regarding anthropogenically induced change in fish communities therefore, a communitylevel
approach has been widely adopted. This has generally involved the application of
univariate metrics to groundfish survey data to quantify change in various aspects of the
community’s composition, structure and function. The element of Ecological Quality for the
North Sea fish community focuses on “changes in the proportion of large fish and hence the
average weight and average maximum length of the fish community”, thus clearly identifying
the need for a community size composition metric as the “indicator” on which to base an
EcoQO for the “fish community” Ecological Quality Issue. The chosen metric, the Large Fish
Indicator (LFI) was eventually defined as “the proportion by weight of fish greater than 40cm
in length”, based on ICES first quarter (Q1) International Bottom Trawl Survey (IBTS) data.
The LFI was intended to be an indicator of the “general health” of the demersal fish
community. Indeed DEFRA have adopted the LFI as a means of monitoring change in the
trophic structure of demersal fish communities, since in marine systems, ontogenetic change
and inter-specific variation in diet is so strongly linked to predator body-size. However, other
aspects of the composition, structure and functioning of fish communities, such as
abundance, biomass, productivity, species richness, species diversity and mean life-history
trait composition, can also be summarised using univariate metrics. Any one, or all, of these
alternative metrics might also be considered to be indicative of the “health” of fish
communities. The LFI was chosen ahead of these alternative metrics because it was
believed to be particularly sensitive to variation in fishing pressure, and therefore to indicate
directly the effect of fishing on the state of the fish community. But this raises the question
as to whether one indicator is sufficient to inform on the general health of the demersal fish
community, or is a suite of indicators necessary in order to provide information on various
different aspects of a community’s composition, structure and function? In this assessment
of the state of the demersal community in UK waters, fifteen univariate metrics are applied to
groundfish survey data to quantify changes in five main aspects of community composition,
structure and function: abundance/biomass/productivity; size composition; species richness;
species diversity; and life-history trait composition (Table 1).
Bottom trawl surveys have been carried out as part of the traditional annual fisheries
management process for several decades. These surveys routinely provide point estimates
of the abundance at length of each species sampled; therefore providing the ideal data sets
for the application univariate community metrics. Many of these surveys have run for two or
three decades now, providing appropriate time series with which to evaluate changes in the
composition, structure and function of fish communities. Furthermore, most coastal
European nations have been involved in survey activity, providing data from most western
continental shelf waters. In this assessment, otter trawl survey data is analysed to assess
changes in the demersal fish communities present in the sea area that constitutes the UK’s
Exclusive Economic Zone (EEZ). The UK EEZ has been divided into nine principal subregions
(Figure 1), and assessment of the state of the fish community in each sub-region
was required. However, one of the sub-regions, the Scottish Continental Shelf, straddled the
boundaries between OSPAR Regions II and III, and between ICES area IV and VI.
Consequently, none of the groundfish surveys, which are normally aligned with the ICES
designated marine regions, covered the entire Scottish Continental Shelf sub-region. A
single assessment for the Scottish Continental Shelf was therefore not possible. The subregion
had to be further sub-divided in line with the data available for analysis, and three
Assessment of the “State” of the Demersal Fish Communities in UK Waters
2
separate assessments made for the southwestern and northeastern sectors of this large
area (Figure 1).
To assess the state of the demersal fish community and make a judgement on the changes
observed, it was necessary to decide what constituted “beneficial” and “detrimental” change.
Emphasis is generally placed on conserving and restoring biodiversity; implying that declines
in species diversity (both richness and evenness) are detrimental. The converse was
therefore also assumed to hold. The life-history trait composition responses of populations
and communities to anthropogenically raised levels of mortality have recently received
considerable attention. Declines in population age and length at maturity, decreased
community average ultimate body length, and increased in community average growth rate
are all considered to be detrimental consequences. Improved management would therefore
be expected to induce the opposite, beneficial trends.
Established population dynamics theory predicts that size-related fishing mortality reduces
the mean size and proportion of large fish in exploited populations (including non-target
species taken as by-catch). This concept underpins development of the LFI as the basis for
the OSPAR North Sea Fish Community EcoQO. Reductions in LFI are therefore considered
detrimental. Changes in LFI were generally inversely correlated with changes in the
abundance, biomass and (growth) productivity of the fish community, implying that declining
abundance, biomass and productivity are indicative of a fish community returning to a more
natural undisturbed state. The von Bertalanffy growth equation makes it clear that larger
fish, closer to their ultimate body length (L8), have lower daily specific growth rates. Large
fish exert a strong predation loading on small fish abundance. Since trophic transfer
efficiency is around 10%, every kg of production by larger fish requires 10kg of production in
their smaller prey fish populations. Specific growth rates among smaller fish are
approximately twice that of larger fish, so 5kg of prey fish are required to support every kg of
larger fish. Reductions in the abundance of larger fish, with the consequent reduction in
predation loading on smaller prey fish, would therefore tend to result in a rapid increase in
the abundance and biomass of small fish; a typical trophic cascade effect.
In assessing the state of the demersal fish community, two questions were addressed.
Firstly, how has the state of the community changed over the last decade, from 1999 to
2008; the period of particular interest to the current report. Secondly, how does the state of
the community over the period 2004 to 2008 (i.e. now) compare with that prevalent during
the eight-year period when data were first available for analysis. Detrimental trends were
assigned a “red” colour code and beneficial trends “green” (e.g. Table 2). Where no
discernable trend was apparent, an “orange” colour code was applied. Five separate
aspects of composition, structure and function of the demersal fish community were
considered, and a judgement was made based on the trends observed in the 15 univariate
community metrics applied to the groundfish survey data. Linear regression was used to
make an assessment of metric trends over the last decade. In comparing the current
situation with the earlier “reference period”, mean metric values over the period 2004 to 2008
were determined and the assessment was scored red or green depending on whether the
recent mean value differed by more than one standard deviation either side of the mean
value determined for the earliest possible “reference period”.