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Fishing Effects on Fish Species Diversity in Different Environmental Scenarios: a Comparison of Case-Studies in Scottish, French and Italian Waters

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Fishing Effects on Fish Species Diversity in Different Environmental Scenarios: a Comparison of Case-Studies in Scottish, French and Italian Waters Not to be cited without prior reference to the Authors ICES CM 2007/D:19 Fishing effects on fish species diversity in different environmental scenarios: a comparison of case-studies in Scottish, French and Italian waters Catherine S. Longo, Marie-Joëlle Rochet, Francesco Colloca, Graham J. Pierce, Fabrizio Serena, and Simon P. R. Greenstreet 1. The protection of species diversity has been recognised as a priority for sustainable fisheries management in international agreements (e.g. the EU CFP, 2003). Nevertheless, an appropriate indicator of the impact of fishing on fish species diversity is still lacking. The way that fish assemblages respond to fishing exploitation in terms of diversity remains poorly understood – available studies show contradictory or inconclusive results. 2. Since many processes in marine communities are known to be size-based, including fishing effects and predation, we argue that an appropriate metric to detect fish species diversity changes should also be size-based. 3. Therefore we subdivided trawl survey data on demersal fish communities into large (predator) and small (prey) size categories. We compared how large versus small fish diversity changed moving from less to more exploited areas. 4. As a working hypothesis we tested whether these changes could be explained as the effect of fishing-driven changes in trophic dynamics. 5. However, the nature of the size-based mechanisms mentioned above depends on the system considered. To understand how local factors affect the relationship between fishing disturbance and fish species diversity, we present a comparative study of 3 different European systems: the North-western North Sea, the Bay of Biscay and the Tyrrhenian Sea. 6. Our analysis shows that fisheries induce different types of responses in large versus small fish diversity (species evenness). 7. The type of response varied between systems, but the direction of change could be only in part explained by means of trophic dynamics. Therefore, a model taking into account more variables to characterise the system is required. For example, especially when the spatial scale of the study is small, local characteristics such as habitat heterogeneity may be more relevant size-structuring drivers than fishing disturbance. Keywords: biodiversity; demersal fish; fisheries; trophic interactions; Tyrrhenian Sea; Bay of Biscay; North Sea. Contact author: Catherine S. Longo, Laboratoire d’Ecologie et Modèles pour l’halieutique (EMH), Rue de l’Ile d’Yeu, IFREMER, BP 21105, 44311, Nantes CEDEX 03, France [e-mail: [email protected]; [email protected] ]. Introduction The protection of species diversity has been recognised as a priority for sustainable fisheries management in international agreements (e.g. the EU CFP, 2003). Nevertheless, an appropriate indicator of the impact of fishing on fish species diversity is still lacking. The main issues raised against the use of this type of indicators in the published reviews (ICES 2001; Rice & Rochet 2005; Link 2005) can be summarised as: - lack of an unambiguous relationship between cause (fishing) and effect - lack of predictability of the direction of change in diversity induced by fishing (increase? decline?) (e.g. Bianchi et al, 2000) These criticisms are symptomatic of gaps in the understanding of the mechanisms underlying the fishing disturbance-fish species diversity relationship. Many aspects of the functioning of marine communities are size-structured such as trophic dynamics and life-history traits. Often the ecological effects of fishing are size-structured as well (e.g. Kerr & Dickie, 2001). We suggest that where fisheries produce size-based effects on fish communities, and thus affect the communities’ trophic dynamics, also size-structured changes in fish diversity should be expected. Here we tested the hypothesis that different sizes respond differently to fishing pressure and thus undergo different changes in species diversity. To this end we compared the species diversity in demersal fish communities sampled during trawl surveys from heavily fished and less fished areas, separating the fish into large and small size categories. We constructed some working hypotheses to predict the size-based patterns, founded in theoretical knowledge on the relationship of fisheries with trophic dynamics (Pauly et al, 1998) and on that of trophic dynamics with species diversity (Huston, 1994). Many studies in the published literature have suggested that, in those areas where intense fishing has sensibly reduced large fish abundance, an indirect effect has been the increase of the smaller fish they prey upon (e.g. Daan et al. 2005). If fishing induces a predatory release effect in small fish, according to niche theory the most competitive species will expand at the expense of others, becoming more dominant (Huston, 1994). If this occurs, species evenness (i.e. the relative proportion of individuals among species) of small fish should be found to decline as an indirect effect of fishing. In large fish, that represent often the main target of fisheries, we assume here that the direct impact of fishing will mask any other potential indirect effects, such as changes in trophic dynamics (Rice & Kronlund, 1997). Therefore, in this case, we do not expect to observe any change in species evenness. To test these assumptions the demersal fish community was devided into size groups as follows: a ‘large’ fish group composed by the predator size range of the fish community and a ‘small’ fish group composed by the size range they are expected to prey upon, based on prey- predator size ratios available in the literature (Frœse & Pauly, 1998; Jennings et al., 2001). The increase/decline in biomass and species evenness of ‘small’ and of ‘large’ fish found in the more exploited areas was compared against expected results described above. This trophodynamic explanation can apply to those systems where fisheries are strongly size- selective (so that small sized fish escape the fishermen and increase when their predators decline) and where the predation of large fish upon small fish is ecologically significant (so that the decline of large fish in a heavily exploited state determines a significant alteration in the predator-prey dynamics). On the other hand, in areas where fishing disturbance affects directly most fish sizes and where the percentage of piscivorous fish is low, no cascade effects should be observed. Based on this, size-structured effects of fisheries on diversity should vary depending on the degree of piscivory within the fish community and on the degree of size- selectivity of fisheries in the ecosystem considered. Therefore, to appreciate the role played by the system’s characteristics in determining the observed patterns, we repeated the hypothesis testing in areas located in 3 different European eco-regions: the North-western North Sea, the Bay of Biscay and the Tyrrhenian Sea. Methods Hypotheses testing design: We assume that, in heavily fished areas, there is a fisheries-mediated predatory release effect. In those areas where fisheries are size-selective, the consequences of this effect should be the declining biomass of large fish (the ‘predators’) and an increased biomass of small fish (their ‘prey’) (e.g. Daan et al 2005). We test here that this predatory release entails size-based changes in species evenness. In particular, we assume fish species evenness, in a heavily fished state, to change as follows: • in an eco-region where fisheries are size-selective and piscivory is ecologically relevant: a. small fish evenness declines b. large fish evenness shows no trend • in an eco-region where fisheries are not size-selective and piscivory is not ecologically relevant: a. small fish evenness shows no trend b. large fish evenness shows no trend To perform this test, in each eco-region selected, we 1. compared the demersal fish community in a heavily fished and a less fished state. To do so, within the area sampled by trawl surveys, we selected sub-areas that were as similar as possible in terms of habitat but differed in fishing effort intensity. 2. defined the size-categories to be analysed: To test for a predatory release effect we estimated the size-range of the fish predators that are removed by fisheries. Then, under the assumption of an opportunistic size-based diet (Jennings et al., 2002), we estimated the size range of fish that these predators would generally prey upon. If the ‘prey’ size-range is too small to be caught by fishermen, these will be the individuals we expect to undergo predatory release effects (Fig.1). To establish expected results for each eco-region, we 3. defined the eco-regions on a scale of “size-selectivity” of the fishing strategy, based on the legal minimum landing sizes relative to the size frequency distribution of the community. 4. determined the ”ecological relevance of piscivory” within the demersal community comparing the relative proportion of piscivores to the total biomass of the fish assemblage, but also referring to diet information in the published literature. The categories to which each case-study was assigned are shown in Tab.1. For further details on methods used see Anon. (2007a), Anon. (2007b). The hypothesis testing within each case-study is based on the assumption that the most ecologically relevant parameter that differs between areas compared is fishing intensity. However, locating two equivalent areas in terms of habitat, but with very different levels of fishing exploitation is problematic because
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