Representing density dependent consequences of life history strategies in aquatic ecosystems: EcoSim II

EcoSim II uses results from the Ecopath procedure for trophic mass-balance analysis to define biomass dynamics models for predicting temporal change i n exploited ecosystems. Key populations can be represented in further detai l by using delay-difference models to account for both biomass and numbers dynamics. A major problem revealed by linking the population and biomass dy namics models is in representation of population responses to changes in fo od supply; simple proportional growth and reproductive responses lead to un realistic predictions of changes in mean body size with changes in fishing mortality. EcoSim II allows users to specify life history mechanisms to avo id such unrealistic predictions: animals may translate changes in feeding r ate into changes in reproductive rather than growth rates, or they may tran slate changes in food availability into changes in foraging time that in tu rn affects predation risk. These options, along with model relationships fu r limits on prey availability caused by predation avoidance tactics, tend t o cause strong compensatory responses in modeled populations. It is likely that such compensatory responses are responsible for our inability to find obvious correlations between interacting trophic components in fisheries ti me-series data. But Ecosim II does not just predict strong compensatory res ponses: it also suggests that large piscivores may be vulnerable to delayed recruitment collapses caused by increases in prey species that are in turn competitors/predators of juvenile piscivores.