Food Web Structure Shapes Foraging Costs in Lake Trout

Food web structure affects habitat use by Lake Trout and, as a consequence, foraging costs may also be influenced.  Depending on the season, Lake Trout, a top predator that requires cold (<15 C), well-oxygenated water, use different parts and depths of a lake.  During the summer, habitat constraints force fish to make choices regarding depth occupancy in order to maintain access to their prey.  As a result fish may engage in excursions to shallower or deeper parts of the lake to forage that are outside of their thermal preferenda.  The metabolic cost of these excursions can be significant.  In this study we evaluated the foraging costs of Lake Trout using depth - acceleration telemetry transmitters that provide a direct measure of activity.  This measure is a useful proxy for energy expenditure, as confirmed by laboratory trials.  We compared summer activity in lake trout in four lakes that differed in possible foraging costs due to differences in prey availability within thermal preferenda.  The main prey type in each lake was: 1) Cisco; 2) littoral minnows and Mysis; 3) Yellow Perch and littoral minnows; and 4) littoral minnows.  We found different movement/acceleration patterns in each lake.  In lakes with prey that live in warm water, Lake Trout made frequent high-acceleration excursions to shallow water.  However, in the lake with only littoral minnows available, lower than expected values of acceleration were observed, but the depth occupancy data showed that Lake Trout position themselves close to the thermocline, where prey are accessible and temperatures are still favourable.  In general, differences in movement behaviour were strongly associated with differences in the prey and food web structure across the four lakes in this study.