The nature of prey selection by planktivorous fish is presented as a model. The model conceptualization is based on the predation cycle, and the effects of prey distribution, encounter rate, handling time, capture success, and optimal foraging on food selection are explicit. The model as presented assumes a multiple species prey assemblage admitting within—species size variation. Conditions are given so that the optimal strategy of prey utilization can be expressed as a breadth of diet, and the specific components of the predator—prey interaction of fish and zoo plankton are discussed in detail. A reduced form of model is applied to two experimental situations. Specific factors which influence prey selection are also discussed.

A simple differential equation model describing the level of food in the stomach, incorporating simultaneous ingestion and gastric evacuation, is used to gauge the sensitivity of methods that utilize diel change in food level of the stomach to determine diel feeding activity of fish. Analysis of model behavior under different feeding regimes shows that diel pattern of food level in the stomach is less sensitive to feeding regime than many authors claim.An expression is derived for the magnitude of bias associated with cessation of feeding and continuance of gastric evacuation while fish are retained by the fishing gear. This bias may be reduced by minimizing sampling time and accelerating preservation of captured fish in experiments to determine feeding chronology and daily ration.

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The reactive field volume model of prey encounter was shown to give a close approximation to the apparent size model of prey encounter rate for prey size distributions consisting of many prey types. The two models of prey encounter give almost identical predictions for the Lake Washington zooplankton community. The prey encounter models do not account for observed patterns of prey selection by Lake Washington juvenile sockeye salmon. This suggests an active preference for large nonevasive prey. This preference is dynamic. Small as well as evasive prey are pursued and eaten at times in the year when the large nonevasive prey are rare or absent from the water column.

Effects of different prey taxa and daily ration levels on fish growth and the relationship between fish growth rate and mean otolith increment width were investigated for juvenile chum salmon (Oncorhynchus keta) in saltwater aquaria. Growth was positively correlated with ration, and food conversion efficiency was much higher for fish fed the harpacticoid copepod, Tigriopus californicus, than either the calanoid copepod, Pseudocalanus minutas, or the gammarid amphipod, Paramoera mohri. Otolith increments were produced daily for at least the first 160 d after hatching and there was a direct relationship between mean daily otolith increment width and fish growth rate. These results illustrate the possibility that otolith microstructure recapitulates juvenile chum growth histories during estuarine residence.