Effects of Gill-Net Trauma, Barotrauma, and Deep Release on Post-Release Mortality of Lake Trout

Post-release mortality violates assumptions of many fisheries studies, thereby biasing parameter estimates and reducing efficiency.  We evaluated effects of gill-net trauma, barotrauma, and deep-release treatment on post-release mortality of Lake Trout Salvelinus namaycush.  Lake Trout were captured at depth (maximum, 60 m) with gill nets in Priest Lake, Idaho and held in a large enclosure for 10–12 days.  Post-release mortality was the same for control and deep-release treated fish (41%).  Mixed-effects logistic regression models were used to evaluate effects of intrinsic and environmental factors on the probability of mortality.  Presence of gill-net trauma and degree of barotrauma were positively related to the probability of post-release mortality.  Smaller fish were more likely to suffer post-release mortality.  On average, deep-release treatment did not reduce post-release mortality, but effectiveness of treatment increased with fish length.  Of the environmental factors evaluated, only lift time was significantly related to post-release mortality.  Longer lift times were associated with lower post-release mortality rates.  Our study suggests that post-release mortality may be higher than previously assumed for Lake Trout because mortality continues after 48 hours.  In future studies, post-release mortality could be reduced by increasing gill-net lift times and targeting larger fish.