Effect of lake-wide planktivory by the pelagic prey fish community in Lakes Michigan and Ontario

We compared predatory demand by pelagic planktivorous prey fish with invertebrate production in Lake Michigan during 1987 and in Lake Ontario during 1990. Predation by the planktivores in Lake Ontario was nearly fourfold higher than in Lake Michigan (approx. 87 g wet weight∙m⁻²∙year⁻¹). Predation rates on Mysis were comparable in Lakes Michigan and Ontario (approx. 21 g∙m⁻²∙year⁻¹), while predation on Diporeia 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 factor of 1.7. Similarly, predation estimates on Mysis in Lake Ontario were 1.2–2.0 times the estimated rate of production, depending on the production model used. Lake Michigan planktivores consumed approximately 55% of available zooplankton production in 1987, indicating that competition 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 cases. There could be sources of error in the estimates of both the supply and demand sides of these trophic relationships.