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Research publication: Relevance for economic and biological management objectives of MSY-based ref. points for fisheries (North Sea plaice and sole)

Plaice (Pleuronectes platessa) and sole (Solea solea) are the primary targets of a mixed flatfish beam-trawl fishery in the North Sea. The fishery is currently managed through single-species Total Allowable Catches and trawl mesh size regulations, with fleets fishing in the northern North Sea required to use a larger mesh size than in the south. We modelled interactions between these fish stocks and fishing fleets, taking into account the impact of changes in fish distribution, stock productivity, and resilience on fishery sustainability and profitability.

Over time, the plaice population appears to have shifted northwards and the productivity of both plaice and sole has varied. Biological changes had a considerable impact on the limit management reference points currently used by ICES. Biomass reference points shifted with changes in productivity, whilst fishing mortality reference points remained constant (although the resulting impact on the stock changed). Single-species reference points currently used are often conflicting and encourage high-grading and discarding of smaller fish. As fishing activity is largely driven by economic considerations (in the light of management controls), alternative economic management targets were considered. In order to achieve maximum economic yield from the fishery, a considerable decrease in fishing effort of the southern fleet is required, owing to the smaller mesh size and increased vulnerability of young plaice before they move north with age. Sustaining maximum employment within the fishery could, in contrast, be achieved when fishing effort was close to the break-even level.

The analysis suggested that at 2002 effort levels, at equilibrium fleets would break even in only one scenario of productivity and resilience. Common alternative biological target reference levels (e.g. maximum sustainable yield and its proxy F0.1) were also examined. They resulted in considerably different effort levels in the north and south, and different profit levels. While we examined a range of possible management objectives in a multifleet, multispecies fishery, selection of the most appropriate depends on priorities assigned by managers and users. Distribution of resulting effort between fleets in the fishery must then be considered, which as shown in this study may not conform to the European aim of relative stability. The choice of alternative reference points should be evaluated through simulation, considering the system in place and the uncertainties inherent within it.

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