Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Novel experimental approaches to suppressing lake trout have focused on the early-life stages because lake trout spawning behavior and the physiology of lake trout embryos provide an opportunity for embryo suppression with limited bycatch. Spawning lake trout congregate on rocky shoals broadcasting gametes over angular clean cobble substrate (Binder et al. 2014), and demonstrate spawning site fidelity (Esteve et al. 2008). Lake trout embryos are non-motile and have undeveloped physiological systems with a limited ability to acclimate to environmental perturbations (Pörtner and Farrell 2008; Helvik et al. 2009). Increasing mortality beyond gillnetting and in lieu of gillnetting (the most preferred option) is probably feasible because of the vulnerability of lake trout embryos. For example, the placement of lake trout carcasses on spawning areas after lake trout have spawned resulted in > 95% mortality, when carcasses remained on the site (Thomas 2017). Currently, scientists with the National Park Service and the US Geological Survey, Montana Cooperative Fishery Research Unit are evaluating the efficacy of carcass analogs (pellets made from rainbow trout carcasses and pellets made from plant material) and the results are promising. Much of the consternation in continuing the lake trout suppression effort in Swan Lake is related to cost and associated bycatch of bull trout in a gillnetting approach. Use of novel approaches such as those being evaluated in Yellowstone Lake may provide a tool for use in Swan Lake. However, for these methods to be effective, the exact locations of lake trout spawning sites need to be determined. There is some uncertainty in the spawning locations given that Steed et al. (see Rosenthal et al. 2016) found different 'primary' spawning locations than those defined by Cox (2010). The differences observed may be related to the gillnetting program disrupting lake trout spawning behavior. Thus, the goal of this project is to describe lake trout spawning locations and habitat associated with those areas when gillnetting suppression is not operational. The results from this project will provide locations for embryo suppression if that is deemed feasible in Swan Lake.