Summary
A littoral zone assemblage of six planktivorous fishes (Pimephales notatus, Notropis heterodon, Fundulus diaphanus, Lepomis macrochirus, Ambloplites rupestris, Perca flavescens), and five zooplankter species, was analysed relative to three hypotheses concerning prey consumption: (1) Size-dependent predation will operate, as elsewhere. (2) Small-bodied planktivores, unable to handle larger prey, will take the most abundant zooplankter and not show species specialization. (3) The strongly cyclical nature of zooplankton populations will not permit fish species to specialize exclusively on zooplankton; there will be a negative correlation between zooplankton numbers and use of alternative prey, and at this time the planktivores will minimize competition by choosing different alternative prey. The first hypothesis was supported, the second and third partially so. The small specialist planktivores, P. notatus and N. heterodon, did not take the commonest small zooplankter, Bosmina longirostris: rather, they specialized largely on Chydorus sphaericus, feeding on it even when rare. Lepomis macrochirus, a generalist, took largely B. longirostris, No species was exclusively planktivorous. The species only partly differed in alternative prey types eaten. Chironomid larvae were a regular item of diet of most species.
Methodology
Lake Opinicon is a 787-ha mesotrophic lake with mean and maximum water depths of 4.9 and 9.2 m, respectively. The work was carried out in 1980 along a 1000-m shoreline section of Cow Island Bay adjacent to the Queen's University Biological Station. Up to a depth of 2 m, the littoral zone becomes lightly to moderately weeded in late summer with the development of Potamogeton sp., Vallisneria americana, Najas flexilis, N. guadalupensis, Ceratophyllum demersum, Chara sp., and some Myriophyllum exalbescens and M. spicatum (Keast 1984).
Fish were netted between the 17th and 26th of each month, May—August, and between the 11th and 18th in September and October.