Organic Pellet Decomposition Induces Mortality of Lake Trout Embryos in Yellowstone Lake

Abstract

Yellowstone Lake is the site of actions to suppress invasive Lake Trout Salvelinus namaycush and restore native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and natural ecosystem function. Although gill netting is effective (Lake Trout λ ≤ 0.6 from 2012 through 2018), the effort costs more than US$2 million annually and only targets Lake Trout age 2 and older. To increase suppression efficiency, we developed an alternative method using organic (soy and wheat) pellets to increase mortality of Lake Trout embryos on spawning sites. Decomposition of pellets during two in situ experiments caused dissolved oxygen (DO) concentrations to temporarily decline to lethal levels (<3.4 mg/L) within days of application. Embryo mortalities during the first exposure period (days 1-6 following application) were high at all treatment levels (1.75-28.0 kg/m 2 ) at the substrate surface and within interstices 20 cm below the surface, varying from 97 ± 1.8% (mean ± SE) to 100 ± 0.0%, but may have been enhanced by a handling effect (exposure to sunlight). Embryo mortalities during the second exposure period (days 11-22) were highest 20 cm below the surface, varying from 78 ± 9.7% to 100 ± 0.0%. Almost all (98 ± 3.1%) Lake Trout embryos died after exposure to DO < 3.4 mg/L for >200 h during the second period. Pellets caused lethal DO for several weeks below the substrate surface, despite largely dissolving and dissipating from the surface of treated areas by day 39. Broad-scale application of pellets at 1.75 kg/m 2 following the spawning period in autumn may reduce Lake Trout recruitment and enhance population suppression because the area of 14 verified spawning sites is only 11.4 ha (0.03% of lake surface area). Pellet application may be useful in other similar systems as part of an integrated pest management approach targeting multiple life stages of invasive freshwater fish.

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

“…However, the use of carcass material would redistribute nutrients sequestered within living Lake Trout to BENTHIC SUFFOCATION OF INVASIVE LAKE TROUT the lower food web. Treatment of the estimated 11.4 ha of Lake Trout spawning habitat in Yellowstone Lake with carcass material is limited by spawning season catch rates (Koel et al 2020b). Current spawning season catch rates could not support treating 11.4 ha at the recommended 7-14-kg/m 2 rate for successful embryo suppression.…”

“…Pre‐positioned sentinel incubators, dissolved oxygen concentration data loggers, or both could be used to evaluate the efficacy of large‐scale treatments. Maintenance of dissolved oxygen concentrations below 3.4 mg/L for 200 h can be considered a reliable predictor of embryo mortality (Koel et al 2020b). These techniques could be used alongside emergent fry trapping to evaluate the success of embryo suppression activities at treated spawning sites (Marsden et al 1988).…”

“…Demographic studies have demonstrated that Lake Trout population growth rates are most sensitive to age‐0 survival rates (Ferreri et al 1995; Cox et al 2013; Ng et al 2016; Fredenberg et al 2017), indicating that this stage is most likely to influence population dynamics. Moreover, Lake Trout are broadcast spawners that typically deposit embryos over clean, porous cobble substrate with deep interstitial spaces (Gunn 1995; Marsden et al 1995; Callaghan et al 2016) at depths ranging from less than 1–30 m (Koel et al 2020b). Such substrate is often spatially limited (Cox 2010; Fredenberg et al 2017); in Yellowstone Lake, it represents only 0.03% of the surface water area (Koel et al 2020b).…”

“…Moreover, Lake Trout are broadcast spawners that typically deposit embryos over clean, porous cobble substrate with deep interstitial spaces (Gunn 1995; Marsden et al 1995; Callaghan et al 2016) at depths ranging from less than 1–30 m (Koel et al 2020b). Such substrate is often spatially limited (Cox 2010; Fredenberg et al 2017); in Yellowstone Lake, it represents only 0.03% of the surface water area (Koel et al 2020b).…”

“…Therefore, intentional degradation of interstitial water quality at spawning sites through the application of sediment and organic matter may efficiently reduce embryo survival. Hypoxic conditions after decomposition of whole fish carcasses and organic pellets at treated spawning sites in Yellowstone Lake caused high Lake Trout embryo mortality (Thomas et al 2019; Koel et al 2020b). Accordingly, we conducted controlled in situ experiments to evaluate the effects of sediment deposition and ground fish carcass deposition on Lake Trout embryo mortality.…”