MODELING FLUCTUATIONS AND OPTIMAL HARVESTING IN PERCH POPULATIONS

A mathematical model of perch (Perca fluviatilis L.) was elaborated, b ased on the data for ten Swiss lakes. The aim is management of a comme rcially exploited population. This required elucidation of dynamics of perch stocks and catches. Statistical data analysis demonstrated comp licated population dynamics, including cycles of different periods and chaos. The model describes age and sex structure, nonlinear reproduct ion (including the effects of cannibalism and competition), individual growth, and harvesting, which depends on fishing effort and mesh size of nets. Bifurcation analysis of the model showed the presence of cha otic dynamics and, hence, high sensitivity to initial conditions. Sens itivity to model parameters was also investigated. Simulation analysis of the fishing process was done on the basis of data from Lake Consta nce. A multiobjective problem of harvest maximization combined with va riance minimization was investigated. The isopleths of the annual harv est and of its dispersion were calculated. Pareto-effective strategies , accounting for trade-off between average harvest and its stability w ere evaluated. A recommendation was given for optimal mesh size and fi shing effort. User-friendly software was developed for IBM PC compatib le computers, which can be used to investigate a broad class of age-st ructured populations.