Forage Fishes and Their Salmonid Predators in Lake Michigan

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Abstract
Alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax dominate the planktivorous fish fauna of Lake Michigan and are now the primary food of lake trout Salvelinus namaycush and introduced salmonids. Their fluctuations in abundance have been a concern due to their effects on native species and their present role as forage species. Each has been implicated as an important factor in the local reduction or extinction of important native species. Mechanisms for these interactions include competition for food and predation on eggs and larvae. Bioenergetic modeling simulations of alewife consumption by stocked salmonids suggest that as much as 20 to 33% of the annual alewife production may be consumed in some years. Increasing stocking rates of salmonids in Lake Michigan yield a predator‐prey system in which predator numbers become relatively independent of prey dynamics. This suggests possible declines in alewife production, changes in major forage available to predators, and perhaps destabilization of the current predator‐prey system. Results of simulations indicate that a given number of chinook salmon Oncorhynchus tshawytscha stocked in Lake Michigan will eat almost twice as much forage fish as the same number of coho salmon O. kisutch; equal numbers of lake trout will eat about 1.5 times as much as coho salmon.

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