Significance of the Functional Response of Predators to Changes in Prey Abundance in Multispecies Virtual Population Analysis

Abstract

Recent applications of multispecies assessment methods (MSVPA) to North Sea Fish stocks indicate that MSVPA and single-species assessments give highly corrected recruitment estimates. This may be an artifact due to the choice of the relationship between predation mortality and prey cohort size. If the predation mortality is an increasing function of prey cohort size over a certain range of cohort sizes, even a constant predator biomass may cause fluctuations in the number of fish recruiting to the fishable part of the population. Thus the correlation between single-species and multispecies recruitment estimates may be lower than earlier analyses have suggested, and changes in predation mortality may have caused fluctuations in year class strengths which previously have been attributed to unspecified environmental conditions. Because a relatively minor alteration of the predation function can cause such a drastic change in the perceived dynamics of the fish stocks, I suggest that more studies of the predation processes in fish communities are needed before one can hope to gain deeper insights into the recruitment of fish stocks by applying multispecies assessment methods.