Single and multispecies reference points for Baltic fish stocks

Single and multispecies models are used to examine the effect of species in teraction on biological reference points for cod, herring, and sprat in the Baltic. The results demonstrate that reference points are different in sin gle and multispecies contexts. Reference points for fishing mortality based on single-species yield and SSB calculations are difficult to use when nat ural mortality depends on the absolute abundance of the predators and their alternative prey. Reference points based on maximizing total yield from th e system may lead to impractical results when species interact. Multispecie s predictions suggest that the cod stock in the Baltic should be reduced to a very low level of biomass in order to benefit from the higher productivi ty of herring and sprat, its major prey. Such a result stresses the need fo r incorporating socio-economic considerations in the definition of target r eference points. Management advice based on biomass reference points will a lso differ. In the single species situation the combinations of cod and pel agic fishing effort for which the equilibrium spawning-stock biomass of the three species is above the biomass reference points forms a rectangular ar ea. When biological interaction is taken into account the limits of this ar ea becomes curved. Reference limits for forage fish cannot be defined witho ut considering changes in the biomass of their natural predators. Likewise, reference limits for predators cannot be defined without considering chang es in the biomass of their prey. (C) 1999 International Council for the Exp loration of the Sea.