Foraging Ecology of the Freshwater Apex Predator Lake Trout (Salvelinus namaycush)

Movement is energetically expensive, yet necessary for energy acquisition by mobile predators, whose foraging over broad spatial scales links food webs in semi-discrete habitat patches. For the cold-water, apex predator lake trout (Salvelinus namaycush), movement into nearshore areas of lakes may be extremely costly because of thermally challenging conditions during summer stratification. However, in lakes without offshore forage fish, forays into these areas may be critical to amass enough energy for growth, fall spawning, and winter survival. In this thesis, I used a novel combination of data – nearshore prey transects, stomach content analysis, three-dimensional lake trout positioning with acoustic telemetry, and accelerometer data – to directly measure the impact of lake trout habitat and food web types on their energy acquisition and expenditure during the habitat-limited summer period in Northwestern Ontario. I studied lake trout behaviour in two small, neighbouring boreal lakes over two summers to ask how their habitat use differs (1) between lakes with different food webs and (2) within lakes between sites of different slopes. The study lakes have similar morphometric qualities but only one contains an offshore forage fish (yellow perch, Perca flavescens). Previous research indicates that steep-sided lakes allow for increased habitat coupling by lake trout during summer, but this is the first work to directly examine the effect of lake morphometry on lake trout foraging within lakes. I used kernel density estimates to show that lake trout tended nearshore with increasing acceleration in the lake without an offshore forage fish, whereas their core areas moved offshore with increasing acceleration in the lake with yellow perch. In nearshore areas, I observed a non-linear effect of slope on acceleration, indicating that lake trout use steep, thermally accessible locations to reach the nearshore fishes found in most of the stomachs analysed. Through stomach content analysis and metabolic calculations, I found that both populations tend towards planktivory and have similar metabolic costs despite behavioural differences, pointing to the energetic limitations of many small, boreal lakes. This thesis expands our understanding of lake trout habitat use during critical foraging events, which is essential for effective aquatic habitat management.

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