Mapping spatial variation in demersal fish species diversity and composition in the North Sea: accounting for species and size-related catchability in survey trawls

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.