Management options for the Blackwater herring, a local spring-spawning stock in the Thames Estuary

摘要

Herring caught in the Thames Estuary sustain a small local commercial fishery (peak catch of 606 t during the 1972—1973 fishing season). Loss of local consumers' interest in the herring product has resulted in a gradual decline in catches and fishing effort for the stock. The stock is assessed using an age-structured model that relies on the information provided by a scientific trawl survey, and management advice is provided before the fishing season starts in October. Given its current low economic value, managers have requested evaluation of options for multi-annual Total Allowable Catches (TACs) in an attempt to reduce the frequency (and costs) of assessment and associated management advice. A tentative relationship between sea surface temperature and recruitment is used to predict the impact of increasing sea temperatures on future recruitment in the context of global warming. Hypotheses of auto-correlation and of an environmental effect on recruitment, together with trends in weight-at-age and the overestimation of spawning-stock biomass, form the basis for sensitivity tests of the management options considered. Implementation of a 3-year fixed TAC with 40% constraint in TAC variability and a slight reduction in target F would seem appropriate for the stock, given that it is within safe biological limits and compares well in terms of yield and risk with the current approach of annual TAC revision.