Assessing Baltic herring stocks with a model that incorporates migration

Abstract

An age-structured stock assessment model incorporating fish migration was developed to test the migration effects on the assessment and management of the western and central Baltic herring stocks. The results of parasitological analyses provide the best evidence of the exchange between these stocks. In the model, part of the western stock migrates to central waters in the first half of the year, while a part of the central stock migrates to the western area in the second. Survivors return to each stock at the end of 6-month periods. The model was fitted for a range of assumed migration rates (from 0.1 to 0.4 for the western stock and from 0 to 0.1 for the central stock) in the 1991–2003 period. Next, catches and biomasses were projected for various fishing mortality options. The model, which incorporates migration, produces higher spawning stock biomass estimates for the western stock and lower ones or the central stock, in comparison with the model that omits migration. The catch and biomass projections performed indicate that the migration effect on catch and biomass ranges from low (less than 5% change) to moderate (up to 20% change) in comparison with projections that omit migration.