Species interaction in assessment of fish stocks with special application to the North Sea

A multispecies Virtual Population Analysis model is presented in which natural mortality is split into a variable mortality due to predation and a constant mortality due to other causes. Predation is calculated within the model from the food selection of the predators, their yearly food intake and the average stock sizes. The model is tested on a set of data from the North Sea. The results indicate that predation is very important. On average the total biomass of fish dead due to predation amounts to 1.6 times that removed by the fishery. Furthermore the introduction of predation mortality changes the exploitation pattern calculated by traditional VPA.