PARENT SESSION
Contributed Oral Session 62: Fish Predation and Morphology
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 518 C, Level 5, Palais des congrès de Montréal
Top predator removal leads to alternative states in brook trout sport fisheries.
Browne, David*,1, Rasmussen, Joseph2, 1 Department of Biology, Montreal, Quebec, Canada2 Department of Biological Sciences, Lethbridge, Alberta, Canada
ABSTRACT- Using archival scale samples, historical creel records, and modern comparative studies we tested if top-predator removal from a system with strong intraguild predation resulted in alternative states in the dominant sport fish population. The study system consisted of brook trout, lake trout, yellow perch, white sucker, and cyprinid species. Brook trout and yellow perch exhibited an intraguild predation interaction with brook trout as the intraguild predator. Lake trout were a second predator on yellow perch. Results of a modern comparative study indicated that in the presence of lake trout, brook trout showed no ontogenetic shift in resource use and fed primarily on littoral derived resources. In the absence of lake trout, brook trout exhibited a significant ontogenetic shift from littoral to pelagic derived resource use due to competition with perch for zoobenthos. Historical records indicated that lake trout had been extirpated from one of the study lakes in the 1980s. Creel records indicated that following the extirpation of the top-predator, lake trout, maximum CUE of brook trout declined by over 50% suggesting the fishery had shifted into a low-density brook trout state. We tested for the occurrence of alternative food web states in this altered system by reconstructing ontogenetic diet shifts of individual brook trout from the isotopic signature of scale samples collected before and after lake trout extirpation. Historical scale material showed no significant shift in carbon isotope signature between ages 2 and 3. However, scales collected after lake trout extirpation showed a significant negative shift in carbon isotopic signature consistent with increased competition for zoobenthos and a shift to pelagic resource use. Our results support the hypothesis that top predators may cultivate optimal conditions for the survival of juvenile intraguild predators by preying down potential competitors for shared resources.
Key words: intraguild predation, food webs, alternative stable states, stable isotopes