Spatiotemporal variation in relative abundance and food habits of juvenile lake trout in Yellowstone Lake

Abstract

Non-native Lake Trout Salvelinus namaycush introduced to Yellowstone Lake precipitated the decline of the native Yellowstone Cutthroat Trout Oncorhynchus virginalis bouvieri. Three decades of suppression have reduced adult Lake Trout, but juveniles remain common. Understanding where juvenile Lake Trout occur and what they consume is essential for improving suppression. Juvenile Lake Trout (< or = 300 mm) distribution and diet were evaluated at ten locations in Yellowstone Lake during the ice-free seasons of 2021 and 2022. Micro-mesh gill nets that selectively sample juvenile fish were used and catch per unit effort was recorded during the pre-stratified, stratified, and post-stratified thermal seasons. Spatial variation in catch per unit effort was assessed with an autocorrelation and cluster analysis. Stomach contents from juveniles were examined, and diets were summarized by frequency of occurrence, proportion by number, and proportion by weight at each site and season. Concurrent zooplankton surveys were collected to assess prey use versus availability. Juvenile Lake Trout were widespread across Yellowstone Lake, but catches were repeatedly higher in specific areas, with hot spots often occurring in West Thumb. Elsewhere, catches were variable by season and mesh size. Diets were consistently dominated by the calanoid copepod (Hesperodiaptomus shoshone). Although H. shoshone species made up a very small fraction of the zooplankton assemblage, it had the highest frequency of occurrence, proportion by number, and contribution by weight in most sites and seasons, indicating strong selective feeding by juvenile Lake Trout. Daphnia spp., amphipods, and midge larvae were secondary prey items. Focusing monitoring and suppression on areas with high juvenile Lake Trout catches should improve the efficiency of Lake Trout suppression in Yellowstone Lake and provide earlier warning of changes in juvenile distribution and diet.