Two approaches of estimating energy expended by rainbow trout (Snlmo guirdneri) on density-dependent activities were tested. Metabolic rates were obtained from growth experiments in laboratory systems. Increases in metabolic rates at higher population densitics were attributed to three factors: (1) starvation, (2) increased levels of exercise by subdominant fish were forced to maintain stations in fast water due to saturation of territories of more dominant trout, (3) higher planes of excitation. It was observed that variance of energetic factors of routine metabolism, growth rate, consumption rate, and growth efficiency tended to increase with density. These responses seem to be a good indication of intraspecific competition. Little correlation was observed between density and average daily ration, or density and aggressive behaviour.Dominant trout, as a group grew faster, grew more efficiently, and contained greater lipid content than the rest of the population at each density. However, strictly defined, energetic advantages were not conferred on a ' peck-right ' basis. At higher densities, dominance conferred less benefits than was observed at lower population levels. Space limitation per se was not an influential factor affecting growth rates. Methods used to estimate densitydependent energy costs are evaluated. Both methods give nearly identical estimates.

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A difference was found in the summer distribution of underyearling brook trout, Salvelinus fontinalis (Mitchill), and planted rainbow trout, Salmo gairdneri Richardson, in Castle Lake, California. Brook trout underyearlings oriented to the bottom and were found primarily in shallow water on the eastern shore of the lake near springs. The rainbow trout underyearlings were more pelagic and were found in the littoral areas along the entire shoreline. Gravimetrically, the food eaten during the summer by brook trout underyearlings was 13% terrestrial, 11 % limnetic, and 76 % benthic. Rainbow trout ate 15% terrestrial, 15% limnetic, and 70% benthic food. In summer, rainbow trout adults are located in the epilimnion in Castle Lake, whereas adult brook trout are found near the bottom of the lake beyond the littoral zone. Due to this spatial isolation, their diets differ considerably. An earlier study showed that during the summer, adult brook trout ate 20% terrestrial, 31% limnetic, and 49% benthic food (by volume). Adult rainbow trout ate 49% terrestrial, 33% limnetic, and 18% benthic food.

Juvenile rainbow trout demonstrated recognizable differences in their ability to assimilate useable energy from food organisms at three different water temperatures. This paper describes the technique and computations used to document these differences and discusses possible reasons for such differences.