Implications for current management advice for North Sea plaice: Part I. Migration between the North Sea and English Channel

Twentieth century mark-recapture studies of conventionally tagged plaice suggested that the maximum exchange rate of fish between the North Sea and the eastern English Channel did not exceed 10%. However, recent studies with electronic data storage tags have suggested that this figure may be significantly greater. In spite of this, current stock assessment and management advice is predicated upon the assumption that the eastern English Channel and North Sea contain two distinct stocks, with no mixing between them. Previous studies on other stocks have shown that different mixing rates can yield grossly different predictions of abundance trends. Using a simulation framework based on the current management regime and simple assumptions regarding the mixing scenario, the consequences of ignoring migratory behaviour between independently managed stocks was investigated. Results indicate that the effects of stock mixing can generate considerable bias in the perceived state of a stock relative to its true state and that changes in the management applied to one stock may cause changes in the level of bias for neighbouring stocks. However, it is concluded that improved management will not necessarily be achieved by developing increasingly complex spatial and temporal assessment methods but rather by developing simple but robust management strategies that have been evaluated before implementation against plausible hypotheses about the dynamics of the stocks and fisheries to be managed. Evaluation should be undertaken using a simulation framework that allows a range of plausible hypotheses about stock and fishery dynamics to be included.