EFFECT OF LAKE-WIDE PLANKTIVORY BY THE PELAGIC PREY FISH COMMUNITY INLAKES MICHIGAN AND ONTARIO

We compared predatory demand by pelagic planktivorous prey fish with i nvertebrate production in Lake Michigan during 1987 and in Lake Ontari o during 1990. Predation by the planktivores in Lake Ontario was nearl y fourfold higher than in Lake Michigan (approx. 87 g wet weight . m(- 2). year(-1)). Predation rates on Mysis were comparable in Lakes Michi gan and Ontario (approx. 21 g . m(-2). year(-1)), while predation on D iporeia was markedly higher in Lake Michigan than in Lake Ontario (21. 3 vs. 8.5 g wet weight . m(-2). year(-1)). In Lake Ontario, predatory demand on zooplankton exceeded our best estimate of production by a fa ctor of 1.7. Similarly, predation estimates on Mysis in Lake Ontario w ere 1.2-2.0 times the estimated rate of production, depending on the p roduction model used. Lake Michigan planktivores consumed approximatel y 55% of available zooplankton production in 1987, indicating that com petition for prey resources, if operating, was not as intense as that in Lake Ontario in 1990. It is unclear how to resolve the paradox that predation could markedly exceed available prey production in some cas es. There could be sources of error in the estimates of both the suppl y and demand sides of these trophic relationships.