Single and multispecies reference points for Baltic fish stocks

Gislason, H. 1999. Single and multispecies reference points for Baltic sh stocks. –ICES Journal of Marine Science, 56: 571–583.Single and multispecies models are used to examine the eect of species interaction onbiological reference points for cod, herring, and sprat in the Baltic. The resultsdemonstrate that reference points are dierent in single and multispecies contexts.Reference points for shing mortality based on single-species yield and SSB calcula-tions are dicult 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 benet from the higher productivity of herring and sprat,its major prey. Such a result stresses the need for incorporating socio-economicconsiderations in the denition of target reference points. Management advice basedon biomass reference points will also dier. In the single species situation thecombinations of cod and pelagic shing eort 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 sh cannot be dened withoutconsidering changes in the biomass of their natural predators. Likewise, referencelimits for predators cannot be dened without considering changes in the biomass oftheir prey.