Thermal Habitat Shifts by Fishes at the Thermocline in Lake Michigan

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Abstract
We documented thermal habitats of Lake Michigan fishes by bottom trawling at the intersection of the thermocline and the lake bottom near Grand Haven, Michigan. Fish distribution by temperature strata was surveyed during day and night on September 7–13, 1977, and repeated on August 27–September 1, 1979. In the interim, bloaters (Coregonus hoyi) had increased substantially. By comparing these two periods, we suggest that bloaters dramatically altered the thermal distribution of adult alewife (Alosa pseudoharengus). While comprising less than 1% of the trawl catch in 1977, bloaters comprised 42% in 1979. Bloaters were the most-abundant species from 5 to 16 °C in 1979. Adult alewife shifted their thermal distribution from 11 to 16 °C in 1977 to 4 to 8 °C in 1979. This shift is interpreted in terms of competition for food and thermal habitat, or predation by salmonids. The idea that the altered thermal distribution of alewife results from competitive interactions with bloaters is favored by existing data.Key words: competition, fishes, habitat partitioning, Lake Michigan, niche shift, predation, salmonids, temperature, thermocline