A quantitative evaluation of the impact of beam trawling on benthic fauna in the southern North Sea

Data on density of benthos species and on direct mortality caused by the passing of a beam trawl, together with fishing effort data for the Dutch beam-trawl fleet, were used to evaluate the annual population mortality caused by beam trawling in the Dutch sector of the North Sea. The effects of using environmental strata, instead of ICES rectangles for density distributions, and of using higher-resolution fishing-effort data on the population mortality estimates of 21 infauna and epifauna species were investigated. Variation in species abundance was markedly smaller based on sediment-depth strata than based on ICES rectangles, and the resulting population mortality estimates differed significantly among species (ratio ranged from 0.3 to 1.6) depending on the overlap of the spatial distribution of a species and of beam-trawl effort. Changing the resolution of fishing effort from ICES rectangles or sediment-depth strata to 1' minute latitude x 2' minute longitude square (+/-1 x 1 nm) resulted in a systematic reduction of population mortality by a factor 0.7 due to the patchy effort distribution. We argue that annual fishing mortality should preferably be based on relevant environmental strata, and accuracy of the estimates increases markedly when the resolution of spatial fishing effort data sufficiently reflects the patchiness of the fleet's activities. (C) 2000 International Council for the Exploration of the Sea.