Scottish Marine and Freshwater Science Report Volume 1 No 3 | Assessment of the “State” of the Demersal Fish Communities in UK waters

To date, OSPAR has largely been responsible for the development of an ecosystemapproach to the management of marine natural resources within the convention area. TenEcological Quality Issues have been identified, with the intention of setting Ecological QualityObjectives (EcoQOs) for each issue. Since 2000, this scheme has been developed andpiloted 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 concernsregarding anthropogenically induced change in fish communities therefore, a communitylevelapproach has been widely adopted. This has generally involved the application ofunivariate metrics to groundfish survey data to quantify change in various aspects of thecommunity’s composition, structure and function. The element of Ecological Quality for theNorth Sea fish community focuses on “changes in the proportion of large fish and hence theaverage weight and average maximum length of the fish community”, thus clearly identifyingthe need for a community size composition metric as the “indicator” on which to base anEcoQO for the “fish community” Ecological Quality Issue. The chosen metric, the Large FishIndicator (LFI) was eventually defined as “the proportion by weight of fish greater than 40cmin 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 fishcommunity. Indeed DEFRA have adopted the LFI as a means of monitoring change in thetrophic structure of demersal fish communities, since in marine systems, ontogenetic changeand inter-specific variation in diet is so strongly linked to predator body-size. However, otheraspects of the composition, structure and functioning of fish communities, such asabundance, biomass, productivity, species richness, species diversity and mean life-historytrait composition, can also be summarised using univariate metrics. Any one, or all, of thesealternative metrics might also be considered to be indicative of the “health” of fishcommunities. The LFI was chosen ahead of these alternative metrics because it wasbelieved to be particularly sensitive to variation in fishing pressure, and therefore to indicatedirectly the effect of fishing on the state of the fish community. But this raises the questionas to whether one indicator is sufficient to inform on the general health of the demersal fishcommunity, or is a suite of indicators necessary in order to provide information on variousdifferent aspects of a community’s composition, structure and function? In this assessmentof the state of the demersal community in UK waters, fifteen univariate metrics are applied togroundfish 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 fisheriesmanagement process for several decades. These surveys routinely provide point estimatesof the abundance at length of each species sampled; therefore providing the ideal data setsfor the application univariate community metrics. Many of these surveys have run for two orthree decades now, providing appropriate time series with which to evaluate changes in thecomposition, structure and function of fish communities. Furthermore, most coastalEuropean nations have been involved in survey activity, providing data from most westerncontinental shelf waters. In this assessment, otter trawl survey data is analysed to assesschanges in the demersal fish communities present in the sea area that constitutes the UK’sExclusive 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-regionwas required. However, one of the sub-regions, the Scottish Continental Shelf, straddled theboundaries 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 ICESdesignated marine regions, covered the entire Scottish Continental Shelf sub-region. Asingle assessment for the Scottish Continental Shelf was therefore not possible. The subregionhad to be further sub-divided in line with the data available for analysis, and threeAssessment of the “State” of the Demersal Fish Communities in UK Waters2separate assessments made for the southwestern and northeastern sectors of this largearea (Figure 1).To assess the state of the demersal fish community and make a judgement on the changesobserved, it was necessary to decide what constituted “beneficial” and “detrimental” change.Emphasis is generally placed on conserving and restoring biodiversity; implying that declinesin species diversity (both richness and evenness) are detrimental. The converse wastherefore also assumed to hold. The life-history trait composition responses of populationsand communities to anthropogenically raised levels of mortality have recently receivedconsiderable attention. Declines in population age and length at maturity, decreasedcommunity average ultimate body length, and increased in community average growth rateare all considered to be detrimental consequences. Improved management would thereforebe expected to induce the opposite, beneficial trends.Established population dynamics theory predicts that size-related fishing mortality reducesthe mean size and proportion of large fish in exploited populations (including non-targetspecies taken as by-catch). This concept underpins development of the LFI as the basis forthe OSPAR North Sea Fish Community EcoQO. Reductions in LFI are therefore considereddetrimental. Changes in LFI were generally inversely correlated with changes in theabundance, biomass and (growth) productivity of the fish community, implying that decliningabundance, biomass and productivity are indicative of a fish community returning to a morenatural undisturbed state. The von Bertalanffy growth equation makes it clear that largerfish, closer to their ultimate body length (L8), have lower daily specific growth rates. Largefish exert a strong predation loading on small fish abundance. Since trophic transferefficiency is around 10%, every kg of production by larger fish requires 10kg of production intheir smaller prey fish populations. Specific growth rates among smaller fish areapproximately twice that of larger fish, so 5kg of prey fish are required to support every kg oflarger fish. Reductions in the abundance of larger fish, with the consequent reduction inpredation loading on smaller prey fish, would therefore tend to result in a rapid increase inthe 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 to2008; the period of particular interest to the current report. Secondly, how does the state ofthe community over the period 2004 to 2008 (i.e. now) compare with that prevalent duringthe eight-year period when data were first available for analysis. Detrimental trends wereassigned a “red” colour code and beneficial trends “green” (e.g. Table 2). Where nodiscernable trend was apparent, an “orange” colour code was applied. Five separateaspects of composition, structure and function of the demersal fish community wereconsidered, and a judgement was made based on the trends observed in the 15 univariatecommunity metrics applied to the groundfish survey data. Linear regression was used tomake an assessment of metric trends over the last decade. In comparing the currentsituation with the earlier “reference period”, mean metric values over the period 2004 to 2008were determined and the assessment was scored red or green depending on whether therecent mean value differed by more than one standard deviation either side of the meanvalue determined for the earliest possible “reference period”.

To determine an overall regional assessment, weighted average scores for each aspect ofthe demersal fish community composition, structure and function were calculated. In theindividual sub-region assessments, red cells were given a value of 1, orange a value of2, and green a value of 3, and cells were then weighted by the number of ICES statisticalrectangles in each sub-region. For the final regional assessment, weighted average scoresgreater than 2.35 were considered good and assigned a “green” code; scores of less than1.65 were considered poor and assigned a “red” code; while scores of 1.65 to 2.35 weredeemed to indicate little change and assigned an “orange” code.