Simon P.R. Greenstreet, Helen M. Fraser, John Cotter and John Pinnegar
Introduction and Methods
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. 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 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 ideal 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 sub-regions (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 sub-region had to be further sub-divided in line with the data available for analysis, and three 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 more natural undisturbed state. The von Bertalanffy growth equation makes it clear that larger fish, closer to their ultimate body length (L∞), 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 of 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 (ie. 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”. 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”.
To determine an overall regional assessment, weighted average scores for each aspect of the demersal fish community composition, structure and function were calculated. In the individual sub-region assessments, red cells were given a value of 1, orange a value of 2, and green a value of 3, and cells were then weighted by the number of ICES statistical rectangles in each sub-region. For the final regional assessment, weighted average scores greater than 2.35 were considered good and assigned a “green” code; scores of less than 1.65 were considered poor and assigned a “red” cod; while scores of 1.65 to 2.35 were deemed to indicate little change and assigned an “orange” code.