Quantifying the Spatial Structure of Invasive Lake Trout in Yellowstone Lake to Improve Suppression Efficacy

Abstract

Invasive Lake Trout Salvelinus namaycush have altered the once-pristine Yellowstone Lake ecosystem through top-down effects by consuming Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. To conserve Yellowstone Cutthroat Trout and restore the ecosystem, a Lake Trout gillnetting program was implemented to suppress the invasive population. We evaluated the spatial structure of Lake Trout in Yellowstone Lake with the intent of increasing suppression efficiency. Specifically, we addressed questions related to adult Lake Trout aggregation and movement during summer and autumn (spawning) periods and how Lake Trout used locations in the context of suppression efforts. We tracked 373 Lake Trout (>500 mm TL) during the summer and autumn of 2016 and 2017. Based on kernel density estimates, Lake Trout were highly aggregated at 9 locations during summer and 22 locations during the spawning period. Using a novel metric, individual days (product of mean individuals per survey and mean length of stay), five summer locations and five spawning locations had at least 30 individual days. These locations are suggested as priority areas for targeting Lake Trout suppression. Lake Trout were less aggregated and moved less during the summer, making them less vulnerable to a passive gear in the summer than during the autumn spawning period. Lake Trout exhibited low spawning site fidelity compared to populations elsewhere, possibly due to decades of intensive gill netting at spawning locations. Given the aggregation and movement patterns observed in Yellowstone Lake, continuing to target adult Lake Trout during the spawning period is the most cost-effective approach to Lake Trout suppression.

Use of otolith microchemistry to identify subbasin natal origin and use by invasive Lake Trout in Yellowstone Lake

“…Telemetry studies have shown that lake trout exhibit extensive seasonal movements and spawn in distinct locations within Yellowstone Lake (Williams 2019;Gutowsky et al 2020); however, we were unable to detect specific natal origins or movement patterns within the lake using otolith microchemistry. Though our negative results were not wholly unexpected, previous studies have shown microchemistry can detect natal origin differences in freshwater and marine fish separated by as little as 5-11 km geographic distance (Kennedy et al 2000;Chittaro & Hogan 2013).…”

“…Novel techniques 4 to increase mortality of lake trout embryos on spawning sites are also being developed (Thomas et al 2019;Koel et al 2020b). A total of 14 spawning sites have been located and verified by the presence of gametes (Figure 1; Koel et al 2020b) with additional sites identified as probable spawning areas (Williams 2019). An understanding of the relative importance of spawning sites in producing pre-recruit lake trout could help direct gillnetting operations and embryo suppression efforts to the most productive sites and increase efficiency of the suppression program.…”

Combination of Acoustic Telemetry and Side‐Scan Sonar Advances Suppression Efforts for Invasive Lake Trout in a Submontane Lake

“…2021; Williams et al. 2022). Lake Trout in our study also aggregated at all spawning sites that were previously identified by Cox (2010) as hosting spawning activity, confirmed in spawning substrate located along Swan Lake's eastern shore, thus re‐affirming the locations as preferred spawning sites.…”

“…Sex and stage of maturity were determined by visual observation of the gonads through the surgical incision (Williams et al. 2022). Lake Trout selected for transmitter implantation had testes or ovaries that were easily identifiable as mature; immature Lake Trout were not tagged.…”

“…2018; Williams et al. 2022), allowing for the use of targeted gill‐net sets and novel embryo suppression techniques.…”