Assessment of the "State" of the Demersal Fish Communities in OSPAR regions II, III, IV and V
Scottish Marine and Freshwater Science Volume 1 No 2
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.