Seasonal food habits of bull trout from a small alpine lake in the Canadian Rocky Mountains

We investigated the seasonal diet of a native, undisturbed population of bu ll trout Salvelinus confluentus in an alpine lake to examine predation patt erns between fish size-classes and in relation to available invertebrate pr ey. The diets of small (less than or equal to 250 mm in fork length, FL) an d large (>250 mm FL) bull trout were similar. Bull trout fed on seasonally abundant prey species. After ice-out in July, the diet was dominated by chi ronomid pupae. Daphnia pulex var. and the amphipod Gammarus lacustris domin ated the diet in August and September. Both Daphnia and Gammarus reproduced before bull trout switched to preying on them in early August. Bull trout fed size-selectively on large individuals of both Daphnia and Gammarus. Lar ge bull trout preyed on larger Daphnia than did small bull trout. Fish of b oth size-classes consumed large Gammarus. Bull trout were spatially segrega ted; small fish occupied shallow water(<l m deep), while large fish occupie d the profundal offshore zone. Spatial segregation prevented small bull tro ut from cropping small immature Daphnia in offshore areas. Average total fo od volume in stomachs of small fish increased between July and September wh ereas it decreased in large fish. The latter were frequently emaciated, ind icating that large individuals may be food limited for much of the open-wat er period. Our data and observations suggest that prey switching, timing of prey reproduction, and spatial segregation of the fish population by size are tightly coupled and contribute to the survival of the key prey species. The survival of a variety of invertebrate species, including large Gammaru s, in the presence of bull trout suggests that stocks of this fish species could be increased by stocking small mountain lakes without severely affect ing the native invertebrate fauna.