Modelling the effects of fishing on the North Sea fish community size composition

Ecosystem-based management of the North Sea demersal fish community uses the large fish indicator (LFI), defined as the proportion by weight of fish caught in the International Bottom Trawl Survey (IBTS) exceeding a length of 40cm. Current values of the LFI are ∼0.15, but the European Union (EU) Marine Strategy Framework Directive (MSFD) requires a value of 0.3 be reached by 2020. An LFI calculated from an eight-species subset correlated closely with the full community LFI, thereby permitting an exploration of the effects of various fishing scenarios on projected values of the LFI using an extension of a previously published multi-species length-structured model that included these key species. The model replicated historical changes in biomass and size composition of individual species, and generated an LFI that was significantly correlated with observations. A community-wide reduction in fishing mortality of ∼60% from 2008 values was necessary to meet the LFI target, driven mainly by changes in cod and saithe. A 70% reduction in cod fishing mortality alone, or a 75% reduction in otter trawl effort, was also sufficient to achieve the target. Reductions in fishing mortality necessary to achieve maximum sustainable harvesting rates are projected to result in the LFI over-shooting its target.