Benthic Suffocation of Invasive Lake Trout Embryos by Fish Carcasses and Sedimentation in Yellowstone Lake

Abstract

Introduced Lake Trout Salvelinus namaycush threaten native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in Yellowstone Lake, Yellowstone National Park, where gill nets have been used to suppress subadult and adult Lake Trout since 1995. However, survival of embryonic and larval life history stages can have profound effects on the population dynamics of Lake Trout. Inducing additional mortality at those stages, especially if used in concert with intensive gillnetting of older fish, could enhance overall suppression efforts. Therefore, we conducted controlled field experiments at Yellowstone Lake to systematically evaluate the effects of sediment deposition and ground Lake Trout carcass deposition on Lake Trout embryos in pre-positioned incubators. Sediment deposition caused dissolved oxygen concentrations to decline below lethal levels for a prolonged overwinter period (92 d). Embryo mortality among overwintering incubators varied from 97.0 ± 5.3% (mean ± SE) at the substrate surface to 100.0 ± 0.0% at 20 cm below the substrate surface. Decomposition of ground carcass material on spawning sites caused dissolved oxygen concentrations to decline to lethal levels (<3.4 mg/L) for about 9 d after biomass application rates of 14 and 28 kg/m 2 in treatment plots. Exposure to ground carcass material resulted in 100.0 ± 0.0% embryo mortality at the substrate surface and within interstices 20 cm below the surface in 14-and 28-kg/m 2 biomass treatments. Embryo mortality was probably caused by hypoxic conditions within substrates in both experiments. The deposition of sediment and ground Lake Trout carcass material on Lake Trout spawning sites in Yellowstone Lake could provide an additional source of mortality in ongoing Lake Trout suppression efforts. These methods may also be beneficial in other systems when incorporated in an integrated pest management approach targeting multiple life history stages of invasive freshwater fish.

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

“…Identification of spawning locations is also needed for implementing novel embryo suppression methods. Novel methods for suppressing Lake Trout embryos are currently being investigated by Yellowstone National Park, and results indicate that they are effective at causing high mortality rates (Thomas et al 2019;Koel et al 2020c;Poole et al 2020). The measure of Lake Trout aggregation size at confirmed spawning locations in this study will assist biologists in identifying areas to be targeted with embryo suppression methods.…”

Fish carcass deposition to suppress invasive lake trout through hypoxia causes limited, non‐target effects on benthic invertebrates in Yellowstone Lake

“…We conducted an in situ experiment to investigate the non‐target effects of a novel strategy to suppress invasive fish embryos on invertebrates. High carcass cover and low DO concentrations indicated effective implementation of the management strategy and the potential to cause high mortality of lake trout embryos (Poole et al., 2020; Thomas et al., 2019). Carcass treatment did not cause changes in populations of highly abundant Gammarus sp.…”

“…The lake trout spawn from late September to early October (Heredia et al., 2021) at sites characterized primarily by cobble and bedrock substrate; confirmed lake trout spawning sites range in size from 0.3 to 2.0 ha and comprise 0.03% of the total area of Yellowstone Lake (Koel, Thomas, et al., 2020). To curtail lake trout recruitment and improve suppression efficiency, the NPS has developed a novel method that deposits carcasses directly on spawning areas in the littoral zone of the lake to induce decomposition and cause mortality in lake trout embryos via oxygen depletion (Poole et al., 2020; Thomas et al., 2019).…”

“…The NPS deposits carcasses from gill netting operations onto lake trout spawning sites in the littoral zone beginning in late summer to discourage egg deposition on preferred spawning sites. Carcass treatment continues through the autumn spawning period (late September to early October) to smother lake trout embryos that have been deposited by female fish (Poole et al., 2020; Thomas et al., 2019). When applied at densities of ≥7 kg wet mass/m 2 , the carcasses cause short‐term (<2 weeks) reductions in dissolved oxygen (DO) concentrations as they decay, causing up to 100% mortality in lake trout embryos (Poole et al., 2020; Thomas et al., 2019).…”

“…Hypoxic or anoxic conditions may result in mortality, or reduction in growth or reproduction, in sensitive invertebrate taxa (Briggs et al., 2021; Connolly et al., 2004; Nebeker 1972). Sessile taxa may be particularly susceptible to mortality in response to carcass treatment due to an inability to move away from unfavourable conditions caused by carcass decay, such as reduced DO and increased fungal and bacterial growth (Fenoglio et al., 2010; Poole et al., 2020). Mobile taxa may emigrate from the area, resulting in declines in density.…”