Eutrophication effects on prey size and food available

Abstract.--Before 1970, yellow perch Percafiavescens grew at similar rates in Lake Erie's western and central basins. At that time, high rates of nutrient loading were causing rapid eutrophication in both areas so that by the mid-1970s the central basin had become mildly eutrophic while the shallow western basin had reached hypereutrophy. From 1970 to 1983, growth rates of yellow perch in the western basin declined sharply while growth rates increased moderately in the central basin. Modeling of bioenergetics suggests that present differences in yellow perch growth rates are due to interbasin differences in food supply and not to temperature. Direct estimation of yellow perch feeding rates in each basin in 1983 supports this inference. Food limitation in the western basin appears related to a reduced size structure ofbenthic prey associated with advanced eutrophy. In the central basin, larger benthic prey are available and food consumption rates by yellow perch are high. Food limitation is more serious for larger than for smaller yellow perch in the western basin, and this has apparently caused stunting and population degradation. Further evidence from the literature suggests that changes in prey size structure which accompany eutrophication may be a primary cuase of the succession of fish species that occurs as lakes age. Mechanistic understanding of this succession will enhance present capacity to predict fish community responses both to ongoing eutrophication and to the more recent oligotrophication of lakes. Patterns of succession of fish species in response to lake eutrophication have been recognized from observations on individual systems (Biro 1977; Hartmann and Numann 1977) and from compar-isons among lakes in different trophic stages (Col