Shifts in the Trophic Ecology of Brook Trout Resulting from Interactions with Yellow Perch: an Intraguild Predator-Prey Interaction

In size-structured populations, predator-prey interactions may be preceded by a phase of resource competition earlier in ontogeny, with potential consequences for Population dynamics and resource management. We hypothesized that brook trout "Salvelinus fontinalis" and yellow perch "Perca flavescens" would compete for shared resources and interact as predator and prey. We used stable isotopes and stomach content analysis to compare the trophic ecology of brook trout in lakes with and without yellow perch. Percent littoral resource use by brook trout differed between perch and nonperch systems, ranging from approximately 50% to 100% in nonperch lakes compared with 10-70% in perch lakes. Nonpiscivorous-sized brook trout (fork length [FL] < 25 cm) showed a significant ontogenetic diet shift toward greater pelagic resource use during growth from 15 to 25 cm in sympatry with yellow perch. Nonpiscivorous-sized brook trout in nonperch lakes consumed a mixture of zoobenthos, zooplankton. and littoral prey fish. In contrast, in lakes containing yellow perch, nonpiscivorous-sized brook trout fed primarily on pelagic prey. Brook trout shifted to piscivory at a FL of approximately 25 cm in both perch and nonperch systems. Isotopic results and stomach content analysis indicated that yellow perch were the primary prey of piscivorous-sized brook trout (FL > 25 cm) in perch lakes, occurring in 66-100% of the brook trout sampled and accounting for over 97% of the diet by weight. Overall, the presence of yellow perch resulted in a shift in the energetic basis for brook trout production from primarily littoral to primarily pelagic. The shift in brook trout food web position was associated with a decline in brook trout catch per unit effort. We conclude that brook trout and yellow perch interact as intraguild predator and prey and that the interaction is dominated by yellow perch.