OPTIMAL RESPONSES TO ENVIRONMENTAL AND PHYSIOLOGICAL CONSTRAINTS - EVALUATION OF A MODEL FOR A PLANKTIVORE

A numeric model for the life history of the mesopelagic planktivore Ma urolicus muelleri is tested for sensitivity to different parameter val ues. The model is based on stochastic dynamic programming with decisio n variables for habitat choice (depth) and energy allocation (between somatic or reproductive growth). The model includes sub-models to repr esent the foraging, mortality and growth processes. The parameters tha t induce large changes in most variables are those related to foraging and visual predation risk. By inducing changes in the strategic habit at choices, they have a large impact on other processes like growth, s urvival and reproduction. The other parameters tested generally have l ocal impact (e.g, on weight or survival), with less impact on other va riables of the model. Seasonality in prey abundance and larval surviva l both have to be present for the model to predict the seasonal strate gies observed for M. muelleri in Norwegian fjords. Size dependent fact ors in the prey encounter, predation and growth processes may explain the observed differences amongst different sized groups of the fish. T he strategies predicted for different seasons and size groups of fish, however, are very dependent on the ratio between food abundance and v isual predation risk. Size dependent processes in general seem importa nt to understand the selective forces that shape the observed life his tory patterns of the fish.