PARENT SESSION
Oral Session # 3: Fish Ecology.
Presiding: M Ford
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 102.

Changes in ontogenetic diet shifts of brook charr in response to the presence of yellow perch: A possible example of compensation-depensation.

Browne, David^*,1, Rasmussen, Joseph¹, ¹ Department of Biology, Montreal, Quebec, Canda

ABSTRACT- Fish community composition can have a large impact on fish population structure and dynamics. Brook charr show highly flexible life history traits that may vary in response to habitat gradients, resource levels, and predation. In this study we use gut content data and stable isotope analysis to examine the interaction between brook charr and yellow perch. We determined the trophic position and resource use of brook charr in a set of four lakes containing cyprinids and white suckers either with or without yellow perch present. Carbon isotopic signatures of brook charr tissue indicated brook charr shift from reliance on a mixture of benthic and pelagic resources to a mixture of benthic and profundal resources when perch are present in the lake. Gut content analyses were consistent with isotopic results and indicated a high degree of piscivory and a reduced contribution of zooplankton and large zoobenthos to brook charr diet when yellow perch were present in the lake. Brook charr trophic position increased with fork length to a maximum of 3.9 in lakes without yellow perch. When yellow perch were present in the system, the trophic position of brook charr decreased with fork length from small to intermediate size classes then increased with length to a maximum of 4.3 in large size classes. Analysis of brook charr condition factor across size classes in lakes containing perch indicates a bottleneck in late juvenile growth in contrast to non-perch lakes. Results suggest juvenile brook charr suffer competition from yellow perch for food resources while perch are the primary prey item of adult charr. This size-structured intra-guild predation interaction suggests brook charr-yellow perch communities may exhibit compensation-depensation dynamics.

Key words: brook charr, intraguild predation, trophic plasticity, food web