Differential Susceptibility of Three Species of Stocked Trout to Bird Predation

Abstract
Piscivorous birds can remove substantial proportions of stocked salmonids from lakes. To determine whether susceptibility to bird predation differs among trout species, 321 brook trout Salvelinus fontinalis, 330 rainbow trout Oncorhynchus mykiss, and 321 splake Salvelinus namaycush × S. fontinalis were stocked in a small lake in Duck Mountain Provincial Park, Manitoba, in spring 1982. Principal avian predators were common loons Gava immer and great blue herons Ardea herodias, which together averaged 1.5 visits per day to the lake from June through October. No poststocking mortality was observed; the primary cause of mortality appeared to be birds. Intensive gillnetting in fall 1982 and spring 1983 produced 41 rainbow trout, 138 brook trout, and 173 splake. The pelagic habits of rainbow trout make them most susceptible to bird predation; brook trout, which stay nearer the substrate, and splake, which prefer deep areas, would be more difficult for birds to see and capture.

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