Mapping spatial variation in demersal fish species diversity and composition in the North Sea: accounting for species- and size-related catchability in survey trawls
1 Fisheries Research Services, Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen AB11 9DB, UK
2 Wageningen–IMARES, PO Box 68 1970 AB, IJmuiden, The Netherlands
Correspondence to H. M. Fraser: tel: +44 1224 295439; fax: +44 1224 295511; e-mail: h.fraser{at}marlab.ac.uk
Fraser, H. M., Greenstreet, S. P. R., Fryer, R. J., and Piet, G. J. 2008. Mapping spatial variation in demersal fish species diversity and composition in the North Sea: accounting for species- and size-related catchability in survey trawls. – ICES Journal of Marine Science, 65: 531–538.The paper maps spatial patterns of groundfish species diversity. It considers how the catchability of different fish species in two different types of bottom trawls, the IBTS GOV and the 8-m beam trawl, influences the estimates of species diversity. Maps of groundfish species diversity derived from these two survey trawls are compared to determine the extent to which the maps of spatial variation in groundfish species diversity are influenced by gear type. Catchability-at-length coefficients were applied to the IBTS data to raise the observed catches to estimates of "actual" numbers of fish present in the path of the trawl, which are then used to produce maps of "actual" species diversity across the North Sea. Finally, these raised maps of "actual" groundfish species diversity are shown to be more explainable based on physical environmental parameters such as depth. We suggest that species diversity maps that take account of catchability provide more reliable information on which to base management decisions than "gear-biased" views. The implications for management are discussed, with particular emphasis on using closed areas to conserve marine biodiversity.
Keywords: groundfish survey data, marine protected areas, size-related catchability, species diversity, species-related catchability, species richness
Received 9 August 2007; accepted 1 February 2008.
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