A BIOENERGETICS ANALYSIS OF DIEL VERTICAL MIGRATION BY KOKANEE SALMON, ONCORHYNCHUS-NERKA

Diel vertical migration of fishes is probably a result of the combined effects of several selective forces, including predator avoidance, fo raging efficiency, and bioenergetic efficiency. We considered both for aging efficiency and energetic efficiency as a combined effect which w e called growth maximization. The importance of growth maximization as a selective force was evaluated with a bioenergetics-based model to e stimate growth rates of various migration scenarios of kokanee salmon, Oncorhychus nerka. Environmental parameters (temperature and zooplank ton distributions) in the model were obtained from a North Carolina re servoir with an established population of kokanee. The simulations dem onstrated that vertical migrations can be energetically advantageous w hen kokanee and their prey are thermally segregated and that ontogenet ic and seasonal differences in the optimal migration strategy should b e expected. The general rule for vertical migration as determined from the simulations is to feed where net energy intake is maximized and t hen reside when not feeding where energetic costs are minimized and fo od is digested to the point that consumption during the next feeding p eriod is not limited by the amount of undigested food remaining in the stomach. Data obtained from vertical gill nets and hydroacoustics wer e compared with model predictions.