Influence of Predatory Fish Density on Trapability of Crayfish in Ontario Lakes

Abstract

In 19 central Ontario lakes the catches of crayfish in baited minnow traps usually were well predicted by numbers of crayfish counted in timed underwater searches. Exceptions were lakes with relatively high predator densities, where trap catches were far less than expected from search counts. In a stepwise linear regression, an index of the density of rock bass (Ambloplites rupestris) in the lakes was significantly negatively correlated with trapability. Smallmouth and largemouth bass (Micropterus dolomieui and M. salmoides), despite their reputation for consuming crayfish, were not significantly correlated with variations in trapability across lakes, but they reduced trapability and altered crayfish behavior in one lake. Yellow perch (Perca flavescens) density was positively correlated with trapability. The predatory basses appear to reduce trapability of crayfish more than they affect crayfish density; high crayfish populations were found in some lakes with dense bass populations. Predator-mediated changes in trapability seem consistent across the crayfish species Orconectes propinquus, O. virilis, and Cambarus robustus. In aquaria without predators, O. propinquus from a lake with high bass density were more nocturnal, more shelter-bound, and less active than those from a lake without bass. Behavior of females differed more between these two lakes than that of males, which are less vulnerable to predation than females. These behavior differences persisted for at least 3 wk in the laboratory, indicating that interspecific comparisons of behavior could be misleading unless the populations studied had similar histories of predation risk. The observed interlake variation in trapability suggests that catches in passive traps may be poor indices of crayfish density.