Modelling fisheries

Background

Conventional models of fisheries, based on stock assessments, are constructed in order to predict what will happen to a fishery under various conditions. Scenarios governed by a model can guide the decisions of the professional fishery manager. Based on the model, the fishery scientist can predict, with a variable degree of precision and confidence, for example, how particular levels of exploitation will effect the stock and influence catches spawning stock size, or revenues.

The model can also be used to assess the effectiveness of proposed actions to regulate the fishery, such as setting catch quotas, increasing the mesh sizes of nets, banning certain types of fishing, or closing protected areas. At its most refined level, a model can be used to set the requirements for shore-based handling and processing facilities on a year-to-year basis.

Model designs

Models designed to explain the reaction of fish populations to fishing generally start from the premises that the recruitment of young fish to an unexploited stock and the average growth of the individuals in the stock approximately balances the loss through natural causes such as predation. When fishing starts, recruitment and average growth will change, compensating to some extent loses to the population due to fishing. Nevertheless, the additional (fishing) mortality will drive the stock to new and lower levels of equilibrium with lower biomass and younger fish on average. This concept of "equilibrium" is simplistic and does not account for natural variations from year too year. It established however that the "equilibrium"(or sustainable) catch is maximum somewhere around the level at which biomass is about half of the biomass in the unexploited stock.

Conventional models also indicate that since fish grow very fast until they reach the age of sexual maturity, it pays to avoid catching them before this age. From these points, it follows that the pattern of fishing featured in the model must strive to maintain the mature breeding stock at a level that provides adequate recruits to it. Overfishing can occur through the excessive removal of either the mature breeding stock or of fish about to be recruited to it.

Conventional models aimed to set the level of fishing effort to meet predetermined biological, economic or social objectives, such as Maximum Sustainable Yield (MSY) or maximum net economic return. They were usually based on single species fisheries. They are still be applied in many areas but the multispecies fisheries commonly found in tropical waters require models that take a more holistic approach making far greater allowances for interactions between species and their place in aquatic ecosystems.

Finally, the need to allow for the increasing impact on fisheries of human activities such as waste disposal and coastal area development, as well as the need to reflect natural variability and uncertainty add greatly to the complexity required for more realistic of fishery management models. At the same time, the tremendous advances in data-processing technology offer the possibility of dynamic modelling moving away from "equilibrium" models.

Multispecies and multifleet models for instance, are used when the data are available and integrate the relationships between species and fisheries. New developments in ecosystem modelling of fisheries (e.g. Ecopath, Ecosim, Ecospace) represent a new generation of models, the practical impact of which on fisheries science are still to be fully appreciated and, by lack of practical alternative also assume ecosystem "equilibrium".