Use of Telemetry Methods for Understanding Catchability of Lake Trout in Gillnets

The catchability of lake trout (Salvelinus namaycush) in gillnets largely depends on two factors, the probability of a fish encountering a net and, once encountered, the probability of the fish becoming entangled. Although the probability of entanglement can be predicted quite well using physical models relating body size to mesh size, the encounter probability depends largely upon fish behaviour traits such as movement rate and depth preference. Recent telemetry studies have demonstrated that differences in lake trout summer movement patterns are related to the availability of cold-water forage. In southern lakes, the dominant growth form of lake trout in a lake is also largely determined by the availability of cold-water forage; large-bodied lake trout occurring in lakes with cold-water forage and small-bodied populations in lakes without. To study behaviour differences in large and small-bodied growth forms of lake trout and to determine how these differences could effect gillnet catchability we recently initiated an automated telemetry study on Lac Maganisipi using a Vemco Positioning System (VPS) array. Lac Maganisipi is a relatively unexploited lake trout lake which has no cold-water forage. The dominant growth form of lake trout in the lake is small-bodied, however it also contains a significant population of large-bodied lake trout which feed primarily on smaller lake trout. Using mark-recapture and North American benthic gillnetting methods we have observed significant differences in gillnet catchability of large and small-bodied lake trout in the lake with large-bodied fish being more catchable. Pressure sensitive acoustic transmitters were implanted in 14 small-bodied and 6 large-bodied lake trout which were passively monitored witharray of 30 VR2W receivers. Although movement rates were similar between the two growth forms, the small-bodied fish spent significantly more time suspended >3m above bottom. Our results suggest that observed differences in catchability between small and large-bodied lake trout in Lac Maganisipi can be attributed more to depth preference than to activity rates.