To date, OSPAR has largely been responsible for the development of an ecosystem
approach to the management of marine natural resources. 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 community-level 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. 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 four OSPAR
Regions, 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 ideal data sets for
the application of 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 of the
continental shelf waters in the OSPAR area. In this assessment, otter trawl survey data is
analysed to assess changes in the fish communities present in four OSPAR Regions (Figure
1).
To assess the state of the demersal fish community in OSPAR Regions, II, III, IV and V, 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 lifehistory
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 an increase in community average growth rate are all considered to be detrimental
Assessment of the “State” of the Demersal Fish Communities in OSPAR Regions
3
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 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.