Depth Distribution, Diet, and Overwinter Growth of Lake Trout ( Salvelinus Namaycush) in Southeastern Lake Michigan Sampled in December 1981 and March 1982

Abstract

Lake trout were collected in graded-mesh gill nets and forage fishes were collected in trawls in mid December 1981 and late March 1982. The length ranges of 317 lake trout caught in December and 138 in March were 280–767 and 286–857 mm, and the age ranges I–XI and II–XIV, respectively. Three year classes (1977–79) made up almost 80% of the catches of lake trout in both sampling periods. Lake trout were most abundant at depth of 18 to 37 m in December (water temperatures, 5.5–6.8°C) and at 28–64 m in March (water temperatures, 1.0–1.3°C). Fish of the 1977–79 year classes completed 9 to 24% of their annual growth in length, and 14 to 39% of their growth in weight, between mid December and late March. Lake trout ate mainly alewives ( Alosa pseudoharengus) especially young-of-the-year, in December, but primarily slimy sculpins ( Cottus cognatus) in March, when alewives were mainly at depths of greater than those occupied by most lake trout. Other important food items were rainbow smelt ( Osmerus mordax) and, in deeper water, deepwater sculpins ( Myoxocephalus thompsoni) Bloaters ( Coregonus hoyi) were eaten only sparingly, although they were abundantly available in both sampling periods. Perhaps this species, which coevolved with the lake trout in Lake Michigan and was important in the native trout's diet, is better able to avoid capture by the trout than are the exotic alewife and rainbow smelt. It may not again become a major forage species unless the other food sources become scarce.