The effect of management choices on the sustainability and economic performance of a mixed fishery: a simulation study

Alternative management scenarios were evaluated in a simulation framework that mimicked the recent exploitation of sole and plaice in the North Sea. A large proportion of plaice is taken as bycatch of the beam trawl fleet targeting sole, yet current management of the two stocks assumes no interaction in their exploitation. The evaluation criteria included biological and economic sustainability, and stability in the management measures. The fishery was assumed to respond to management restrictions by dropping the least profitable trips. We investigated two contrasting management strategies, single-species total allowable catches, and effort regulation. Under the assumptions made, the latter strategy performed better. The results suggest that, given assessment error and bias, a strategy that accounts for the mixed nature of a fishery and that occasionally results in perceived underexploitation may work best. Stability in fishing mortality reinforces itself, through lower assessment bias, and management corrections become less frequent. The common assumption in many stock assessments in EC waters that fishing mortality in the most recent year should resemble the value obtained in previous years (shrinkage) had a negative effect on the stability of control measures.