Shifts in depth distributions of alewives, rainbow smelt, and age-2 lake trout in southern Lake Ontario following establishment of dreissenids

In the mid-1990s, biologists conducting assessments of fish stocks in Lake Ontario reported finding alewives Alosa pseudoharengus, rainbow smelt Osmer us mordax, and juvenile lake trout Salvelinus namaycush at greater depths t han in the mid-1980s. To determine if depth distributions shifted coinciden t with the early 1990s colonization of Lake Ontario by exotic Dreissena mus sels, we calculated mean depth of capture for each of the three species dur ing trawl surveys conducted annually during 1978-1997 and examined the mean s for significant deviations from established patterns. We found that mean capture depth of alewives, rainbow smelt, and age-2 lake trout shifted deep er during the build up of the dreissenid population in Lake Ontario but tha t timing of the shift varied among seasons and species. Depth shifts occurr ed first for rainbow smelt and age-2 lake trout in June 1991. In 1992, alew ives shifted deeper in June followed by age-2 lake trout in July-August. Fi nally, in 1993 and 1994, the distribution of lake trout and alewives shifte d in April-May. Reasons why the three fishes moved to deeper water are not clear, but changes in distribution were not linked to temperature. Mean tem perature of capture after the depth shift was significantly lower than befo re the depth shift except for alewives in April-May. Movement of alewives, rainbow smelt, and age-2 lake trout to colder, deeper water has the potenti al to alter growth and reproduction schedules by exposing the fish to diffe rent temperature regimes and to alter the food chain, increasing predation on Mysis relicta in deep water and decreasing alewife predation on lake tro ut fry over nearshore spawning grounds in spring.