Seasonal Food Habits of Bull Trout from a Small Alpine Lake in the Canadian Rocky Mountains

Abstract

We investigated the seasonal diet of a native, undisturbed population of bull trout Salvelinus confluentus in an alpine lake to examine predation patterns between fish size-classes and in relation to available invertebrate prey. The diets of small (≤250 mm in fork length, FL) and 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 chironomid pupae. Daphnia pulex var. and the amphipod Gammarus lacustris dominated 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. Large bull trout preyed on larger Daphnia than did small bull trout. Fish of both size-classes consumed large Gammarus. Bull trout were spatially segregated; small fish occupied shallow water (<1 m deep), while large fish occupied the profundal offshore zone. Spatial segregation prevented small bull trout from cropping small immature Daphnia in offshore areas. Average total food volume in stomachs of small fish increased between July and September whereas it decreased in large fish. The latter were frequently emaciated, indicating that large individuals may be food limited for much of the open-water 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 Gammarus, in the presence of bull trout suggests that stocks of this fish species could be increased by stocking small mountain lakes without severely affecting the native invertebrate fauna.