An overview of the multispecies VPA — theory and applications

Summary

Multispecies virtual population analysis is an attempt to take species interactions into account when assessing the status of fish stocks. It was introduced primarily with the aim of lowering the uncertainty in the natural mortality rate as used in single species VPA and to take account of variability between years and ages by calculating inside the model the part of the mortality rate caused by predation. The output of the MSVPA is therefore —in addition to stock sizes and fishing mortality rates as in single species VPA —the amounts consumed of the various species by the predators included in the analysis.

The MSVPA model of the predation interactions results in a set of coupled non-linear equations which must be solved for each time step in the analysis. Key parameters in the model are the so-called suitability coefficients, measuring the relative suitability of one species as prey for another species. These parameters must be estimated inside the model and this estimation requires data on the stomach contents of the predators in the model. The MSVPA makes two key assumptions: constant ration size (i.e. independent of time for each species-age combination) and hence fixed weights-at-age and a model of prey selection which leads to a type ii functional feeding response. These assumptions do not hold for all areas and therefore limit the applicability of the MSVPA in its present form.

The MSVPA is probably one of the more successful multispecies models in fisheries. Its main application to date has been to the North Sea, and although it has not been used directly as a management tool it has provided input values of parameters used in assessment models as well as valuable insights into the system. For example, it has demonstrated that an increase in mesh size can result in lower long-term yields, an effect opposite to what is predicted if species interactions are ignored. Such insights into the dynamics of the system are useful and MSVPA may therefore have an indirect role to play in management. Nevertheless, due to many uncertainties involved in multispecies modelling in general and MSVPA in particular, it seems doubtful that the use of MSVPA in fisheries management will be much greater in the immediate future than it is at present.