Fitting into Food Webs: Behavioral and Functional Response of Young Lake Trout ( Salvelinus namaycush) to an Introduced Prey, the Spiny Cladoceran ( Bythotrephes cederstroemi)

Abstract

Functional response experiments with alternative prey demonstrate how an exotic zooplankter co-exists with salmonid fish in Lake Superior. Young lake trout ( Salvelinus namaycush) response to Daphnia, a typical prey genus, and to Bythotrephes, a spined invertebrate predator that invaded North America in the mid-1980s, is examined in a variety of laboratory experiments that span various prey densities, frequencies, experience, and consumer size. Bythotrephes’ caudal spine protects the animal from small fish predation. At intermediate densities, the spiny cladoceran also disrupts foraging behavior of young-of-year fish. Lake trout response to Bythotrephes is dependent on the length of the spine and fish size. The degree to which lake trout are able to discriminate between prey and resume their prior attack rate on Daphnia depends on the absolute density of Daphnia and the frequency with which fish encounter Bythotrephes. For both experienced and naïve fish, aversive behavior to Bythotrephes occurs after a certain threshold of encounters. Under conditions of high encounter rates, once aversive behavior is established in YOY fish, foraging efficiency on Daphnia improves because Bythotrephes is recognized and ignored. The density-dependent behavioral and functional responses resemble classical predator reactions to unpalatable prey.