Assessment of an invasive lake trout population in Swan Lake, Montana

Abstract

The recent invasion of lake trout into the Swan River drainage in Northwest Montana threatens one of the last remaining recreational bull trout fisheries in the USA. An inter-agency group is implementing an experimental lake trout suppression program on Swan Lake. The objectives of this study were to establish a baseline data set on the lake trout population in Swan Lake concurrently with the experimental removal effort, simulate alternative management scenarios using matrix models and identify spawning locations of lake trout to target adult fish and embryos. A commercial gill-net sampling effort provided data to estimate abundance, size structure, age structure, growth, condition, maturity, fecundity, and mortality of lake trout in Swan Lake. Lake trout in Swan Lake grew rapidly, attained large sizes, and were in high condition. The size and age structure of lake trout sampled was skewed towards juvenile lake trout, indicating the population was growing rapidly. Matrix-model simulations also indicated the lake trout population would continue to grow with no suppression efforts, but suppression efforts could reduce the population growth rate. Population growth was particularly sensitive to changes in age-0 survival in population models. Elasticity analysis of matrix simulations indicated survival from birth to sexual maturity, followed by survival of adult fish contributed most to population growth. Lake trout spawning locations were identified using ultrasonic telemetry, short-set gill nets, and in-situ egg nets. Spawning locations identified with acoustic telemetry were confirmed by capturing gravid lake trout in gill nets and lake trout eggs in the substrate. These results suggest that the inter-agency group should focus removal efforts on sub-adult and adult lake trout at if extirpation of the population is the goal. Given the uncertainty in the vital rates and the potential bias in exploitation rates used to model suppression scenarios, annual suppression efforts should be increased from the 2008 level to ensure a decline in the lake trout population.