Single and multispecies reference points for Baltic fish stocks

Abstract

Gislason, H. 1999. Single and multispecies reference points for Baltic fish stocks. –ICES Journal of Marine Science, 56: 571–583.Single and multispecies models are used to examine the effect of species interaction onbiological reference points for cod, herring, and sprat in the Baltic. The resultsdemonstrate that reference points are different in single and multispecies contexts.Reference points for fishing mortality based on single-species yield and SSB calcula-tions are difficult to use when natural mortality depends on the absolute abundance ofthe predators and their alternative prey. Reference points based on maximizing totalyield from the system may lead to impractical results when species interact. Multispe-cies predictions suggest that the cod stock in the Baltic should be reduced to a very lowlevel of biomass in order to benefit from the higher productivity of herring and sprat,its major prey. Such a result stresses the need for incorporating socio-economicconsiderations in the definition of target reference points. Management advice basedon biomass reference points will also differ. In the single species situation thecombinations of cod and pelagic fishing effort for which the equilibrium spawning-stock biomass of the three species is above the biomass reference points forms arectangular area. When biological interaction is taken into account the limits of thisarea becomes curved. Reference limits for forage fish cannot be defined withoutconsidering changes in the biomass of their natural predators. Likewise, referencelimits for predators cannot be defined without considering changes in the biomass oftheir prey.