Efficacy of suppressing non-native lake trout in an isolated backcountry lake in Glacier National Park

Abstract

Prior to the recent invasion of non-native lake trout Salvelinus namaycush, Glacier National Park (GNP) supported approximately one-third of the remaining natural lake habitat supporting threatened bull trout Salvelinus confluentus. However, bull trout populations have recently declined and are at high risk of extirpation in several lakes in western GNP due to the establishment of lake trout. In 2009, the U.S. Geological Survey and the National Park Service began suppressing lake trout in Quartz Lake (352 ha) to reduce impacts to native bull trout. The objectives of this study were to: (1) describe the demography of the lake trout population during the suppression program (2009-2013); (2) identify the timing and location of lake trout spawning; (3) determine the most efficient combination of gill net mesh color and twine diameter to capture juvenile lake trout (age 2 to age 4); (4) assess the effects of suppression on the growth rate of the lake trout population and use this information to model harvest scenarios; and (5) determine whether suppression negatively impacted bull trout. Lake trout exhibited slower growth, lower condition, and lower fecundity relative to other populations. Spawning locations were identified on cobble and boulder substrates (depths 2-20 m) near the base of two avalanche chutes where adults began aggregating between 1 and 9 October prior to thermal destratification (11-12 C°). Catch rates of spawning (ripe) adults were highest 12-25 October when temperatures declined to below 10 C°. Gill nets with 0.1 mm twine thickness and green color increased catchability of juvenile lake trout. Although density dependent parameters were not included, population simulation models indicated the population was growing exponentially and would likely reach carrying capacity within ten years without suppression. Suppression resulted in declining population growth rates (lambda) from 1.23 prior to suppression to 0.61-0.79 during suppression. Bull trout redd abundances remained stable throughout the suppression period. My results indicate targeted suppression successfully reduced lake trout abundance and that continued suppression at or above observed exploitation levels is needed to ensure continued population declines and to avoid impacts to the bull trout population.