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 (Յ250 mm in fork length, FL) and large (Ͼ250 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 (Ͻ1 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.

Trophic relationships between a native and a nonnative predator in a system of natural lakes

“…For example, d 13 C of primary consumers was )23.8& in littoral area, )28.4& in pelagic areas and )30.5& in profundal areas among 14 lakes in Ontario, Quebec , and a mean difference of 1.7& between d 13 C values for lake trout in bass invaded lakes and noninvaded lakes was used to infer greater dependence on littoral prey in noninvaded lakes (lake trout d 13 C = )27.5&) and a greater dependence on pelagic prey in invaded lakes (lake trout d 13 C = )29.2&). <1 m) and larger bull trout observed in the profundal offshore waters (Wilhelm et al 1999). Therefore, the observed trends are suggestive of bull trout foraging in littoral habitat more than lake trout and of bull trout shifting from foraging in littoral habitat to foraging in pelagic or profundal habitat as they increase in length.…”

“…Within GNP, lake trout were first documented in Lake McDonald in 1959, Bowman Lake and Kintla Lake in 1962, Logging Lake in 1984, Harrison Lake in 2000, Lower Quartz Lake in 2003 and Quartz Lake in 2005 (Fredenberg 2002;Meeuwig & Guy 2007). see Martin 1966;Scott & Crossman 1973;Donald & Alger 1993;Wilhelm et al 1999;Beauchamp & Van Tassell 2001); however, when present, fish compose the majority of bull trout and lake trout diets (Donald & Alger 1993). Many bull trout populations within GNP exhibit a lacustrineadfluvial life history in which lake environments are occupied during much of their life history other than for spawning and rearing of juveniles.…”

Bull Trout Life History, Genetics, Habitat Needs, and Limiting Fact in Central and Northeast Oregon. Annual Report 1999.

“…Values of D range from 0, representing no overlap, to 1, for complete overlap. The index provides a comparative measure rather than a statistical measure (Townsend and Hildrew 1976) where values greater than 0.60 are generally considered to indicate biologically significant overlap in the resource use of two species (Wallace 1981;Wilhelm et al 1999).…”

Higher temperatures enhance the effects of invasive sportfish on mountain zooplankton communities

“…Plankton were preserved in 95% ethanol for enumeration back in the laboratory. Species richness and total biomass values for each study site included crustacean zooplankton only; however, biomass values for G. lacustris and Chaoborus were also used for ordination analysis, as these organisms can serve as significant food sources for trout in alpine lakes in our study region (Wilhelm, Parker & Schindler, 1999). Species biomass estimates from previous investigations in our study area (Anderson, 1980;Donald et al, 2001;Schindler & Parker, 2002) were used to calculate the total biomass for each sample.…”