Evidence of Diel Vertical Migration by Lake Trout

Lake trout (Salvelinus namaycush) is a cold water fish species with high variation in habitat selection. Various factors influence the depth occupancy, such as temperature, oxygen concentration and prey distribution. The diel pattern of depth occupancy by lake trout was related to prey distribution for Lake Superior, but a more general pattern has not been described for other types of lakes. We hypothesized that during the summer, lake trout will show patterns of diel vertical movements that will be dependent upon the food web of the lake. This study used acoustic telemetry to examine the diel vertical migration by lake trout in four lakes that represent different prey availability: 1) cisco; 2) littoral minnows and Mysis; 3) yellow perch and littoral minnows; and 4) littoral minnows. We analyzed a period of 20 days during the summer in which temperature differences between the epilimnion and hypolimnion were greatest. We observed patterns of diel vertical migration by lake trout in all study lakes.  The results showed individual variation within and across lakes, but two generalities emerged.  First, food webs dominated by warm-water forage showed daily movements to shallower water during daylight hours, while lake trout depth occupancy increased in lakes with cold-water forage.  Second, more complex food webs supported different patterns of diel vertical movements.  Combined these findings suggest that food web structure is a strong determinant of lake trout habitat use.