PREDATOR RESPONSES TO PREY POPULATION-DYNAMICS - AN EMPIRICAL-ANALYSIS BASED ON LAKE TROUT GROWTH-RATES

Lake trout (Salvelinus namaycush) growth rates were lower in Lake Supe rior (with the lowest prey density) than in Lakes Michigan and Ontario . In contrast, consumption rates (estimated using bioenergetics models ) of lake trout were similar in Lakes Superior, Michigan, and Ontario because the primary prey in Lake Superior, rainbow smelt (Osmerus mord ax), have a lower energy content than the primary prey in Lakes Michig an and Ontario, alewife (Alosa pseudoharengus). Estimated consumption rates did not respond to three- to five-fold changes in prey density w ithin lakes and were similar across a 100-fold difference in prey dens ity among lakes. These results demonstrate that lake trout are able to sustain high predation rates at low prey densities and a type II func tional response fitted to these data rises very steeply at low prey de nsities. Two factors are implicated: the large search volume of these predators and disparity between the average density estimates of prey stock assessment and the spatial and temporal scaling appropriate for understanding predator-prey interactions. If prey are highly aggregate d or predators can search large areas, then a decrease in whole-lake p rey abundance will not necessarily result in a lower predation rate un til prey populations are severely reduced.