Modelling an exploited marine fish community with 15 parameters - results from a simple size-based model

To measure and predict the response of fish communities to exploitation, it is necessary to understand how the direct and indirect effects of fishing interact. Because fishing and predation are size-selective processes, the potential response can be explored with size-based models. We use a simulation approach to describe the relationship between size spectrum slope and overall fishing mortality and to try to understand how a linear spectrum might be maintained. The model uses 15 parameters to describe a 13 `species¿ fish community, where species are defined by their maximum body size and the general relationship between size and life history characteristics. The simulations allow us to assess the role of changes in the strength and type of density dependence in controlling the response to fishing and to investigate the tradeoffs between catches and the status of the different species. The outputs showed that the linear slope of the size spectrum was a function of community exploitation rate. Density dependent controls, specifically predation mortality and the extent of compensation in the stock-recruitment relationship, were key mechanisms in maintaining a linear spectrum. Compensation caused by the dependence of predation mortality on predator abundance can linearise the spectrum even when the compensation caused by the dependence of recruitment on spawning stock biomass is weak. However, as compensation in the stock-recruit relationship was increased, the effects of changes in fishing mortality dominated those of the dynamic changes in predation mortality. The approach allows us to explore the effects of different fishing mortality schedules on properties of the fish community, to assess how fishing affects species with different life histories in mixed fisheries and to assess the effects of selectively fishing different size classes. The simulations indicate that the size classes to be included when developing and interpreting sized-based metrics must be carefully considered in relation to the trophic structure and likely strength of predatory interactions in the community. Runs with differential fishing mortality by size suggest that the dynamics of predation cannot compensate fully for changing rates and patterns of exploitation, implying that the effects of selectively fishing different size classes should be assessed on a case by case basis