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

Abstract

Expansion of an invasive Lake Trout Salvelinus namaycush population in Swan Lake, Montana, threatens a core area population of Bull Trout S. confluentus. Given the recent development of novel suppression methods, such as use of carcass analog pellets to cause high mortality of embryos, there was a need to quantify spawning season aggregation sites, site use, and spawning habitat for Lake Trout in Swan Lake. Acoustic tags were implanted in 85 Lake Trout during the summer in 2018 and 2019. Nightly tracking efforts during autumn in both years resulted in 1,744 relocations for 49 individual Lake Trout. Kernel density analysis was used to evaluate Lake Trout aggregation sites, identifying 10 distinct sites. All spawning sites were located in the littoral zone along areas of steep bathymetric relief, and these sites composed 48% of total relocations during both spawning seasons. In 2019, side‐scan sonar imaging was used to classify and quantify the total area of spawning substrate, which constituted 12.8% of the total surface area estimated for spawning sites 1, 6, and 9 and 11.4% of the total surface area for aggregation sites 2–5, 7, 8, and 10. Simultaneous treatment of all spawning sites would require 205,709 ± 86 kg of carcass analog pellet material, resulting in 370.4 ± 0.2 kg of phosphorus inputs and 7,487.9 ± 3.1 kg of nitrogen inputs to Swan Lake. Thus, pellet treatment would increase the Carlson's trophic state index (TSI) values from 20.8 to 27.7 for total phosphorus and from 22.1 to 26.2 for total nitrogen. Based on a TSI threshold of less than 40 for an oligotrophic lake, the use of carcass analog pellets could be feasible for supplementing the gill‐netting suppression of Lake Trout in Swan Lake.

Aerial Application of Organic Pellets Eliminates Lake Trout Recruitment from a Primary Spawning Reef in Yellowstone Lake

“…Assumptions of our study were that the timing of Lake Trout fry dispersal from the Carrington Island spawning reef did not vary greatly among years and that the absence of fry production on the Carrington Island reef in 2020 and 2021 was not caused by confounding factors other than the pellet treatments. Lake Trout in Yellowstone Lake spawn earlier than other invasive (Dux et al 2011;Siemiantkowski et al 2022) and native (Marsden et al 2005) populations at similar latitudes, and water temperatures at most known Yellowstone Lake spawning areas could facilitate hatching (Koel et al 2020a), emergence, and potentially dispersal months earlier than these events would occur within the native range of the species (Krueger and Ihssen 1995;Ladago et al 2016). Yellowstone Lake fry contained more food, dispersed later, and achieved a greater maximum length before dispersal than did fry in Lake Champlain, Vermont, within the Lake Trout's native range (Simard et al 2020).…”

“…2016; Dow 2018; Siemiantkowski et al. 2022) and then can be specifically targeted to increase prerecruit suppression efficiency.…”

“…These microhabitats can be located by fine-scale telemetry of spawning adults (Binder et al 2018) or visually by scuba divers or remotely operated vehicles (Farha et al 2020). The spatial extent of these microhabitats can be delineated using side-scan sonar (Richter et al 2016;Dow 2018;Siemiantkowski et al 2022) and then can be specifically targeted to increase prerecruit suppression efficiency.…”

“…Free-swimming juvenile and adult life stages are far more widespread and challenging to remove, especially when occurring in sympatry with desired species (Britton et al 2011;Hansen et al 2019b). The strategy requires locating spawning areas and documenting their relative use, which for Lake Trout can be accomplished by acoustic telemetry of adults during autumn (Flavelle et al 2002;Dux et al 2011;Siemiantkowski et al 2022;Williams et al 2022). In addition, microhabitats consisting of the most suitable (and productive) substrate are typically located within large spawning areas (Marsden et al 1995;Muir et al 2012).…”

“…2011; Siemiantkowski et al. 2022) and native (Marsden et al. 2005) populations at similar latitudes, and water temperatures at most known Yellowstone Lake spawning areas could facilitate hatching (Koel et al.…”