The impacts of increased codend mesh size on the northern Baltic herring fishery: ecosystem and market uncertainties

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The short and long term eVects of an increase in trawl codend mesh size (from 20-mm to 36-mm) on catch weight and value per recruit and on the stock biomass of herring in the northern Baltic were assessed. Calculations were carried out using an agestructured model. Yearly estimates of growth rates for commercial catch sample data for 1974‐1992 were used in the analysis. Natural mortality estimates were from the multispecies population analysis. Under the conditions prevailing in 1974‐1992, the increase in codend mesh size would have led to a reduction in catch and in the value of the catch per recruit. The magnitude of the estimated reduction in catch varied greatly with the growth and natural mortality of the population. If an increase in mesh size is to be profitable for this fishery in the long run, the price of large herring processed for human consumption would have to be six times greater than the price of the smaller fodder herring (the price diVerential has been around four-fold), or the survival of codend escapees would need to be increased to 80% from the current estimated level of about 15% (e.g. with the aid of new fishing technology). ? 1996 International Council for the Exploration of the Sea