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

Yellowstone Lake has been the site of intensive efforts to conserve native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and restore natural ecological function since invasive Lake Trout Salvelinus namaycush were first discovered there in 1994. Gillnetting was implemented in 1995 to suppress the Lake Trout population, but despite annual increases in gillnetting effort, the population expanded throughout Yellowstone Lake and increased in abundance until 2012, when the effort became large enough to curtail population growth. The Lake Trout population of Yellowstone Lake is highly resilient to gillnetting, probably because of high early life history survival. Interstitial embryo predators, which are a common source of embryo mortality in the native range of Lake Trout, do not inhabit Yellowstone Lake. Because Lake Trout population growth rates are most sensitive to changes in age-0 survival, alternative methods are being developed to reduce prerecruit survival, with an overall goal of increasing suppression efficiency while reducing long-term costs.

We sought to intentionally degrade interstitial water quality at Lake Trout spawning sites in Yellowstone Lake because salmonid embryos are highly susceptible to sedimentation and degraded water quality. Rapid mortality of Lake Trout embryos occurs when dissolved oxygen (DO) concentrations are below 3.4 mg/L. Whole and ground Lake Trout carcasses caused 100% mortality of Lake Trout embryos at the substrate surface and at a depth of 20 cm in the substrate at treated spawning sites. Biological oxygen demand of the decomposing carcasses caused DO to decline to 0mg/L soon after treatments and caused the high embryo mortality. Although these previous studies suggested that Lake Trout carcasses may be a useful embryo suppression tool, the Lake Trout spawning period peaks during the last week of September, allowing only 2–3 weeks to treat spawning sites prior to the end of gillnetting (which limits carcass availability) and onset of dangerous winter conditions on Yellowstone Lake. Lake Trout carcasses are also difficult to transport in large quantities in boats, time-consuming to apply, and prone to drift or dispersal by wildlife from spawning sites. We therefore sought an analogous organic material that would alleviate these constraints. Pelletized carcass-analogs have been used to restore nutrients and ecosystem function in areas of the Pacific Northwest, where anadromous salmonids historically contributed to stream productivity through carcass decomposition. Fish-based and plant-based organic pellet formulations induced high mortality of embryos in laboratory bioassays by decreasing DO, increasing ammonia, and increasing hydrogen sulfide concentrations. These experiments provided proof of concept for application on Yellowstone Lake.

Fourteen Lake Trout spawning sites were identified in Yellowstone Lake over the past two decades by gillnetting spawning fish and locating telemetered fish. Spawning at these sites was verified by locating Lake Trout embryos by snorkeling (shallow sites), scuba diving, remotely operated vehicle imaging, or benthic sled sampling. Depths of Lake Trout spawning sites vary from <1 to 30m, and surface areas of sites where the outer perimeter has been delineated (n = 11) vary from 0.3 to 2.0 ha (total of 11.4 ha). Although some Lake Trout spawning site substrates consist of angular rock, most are embedded cobble, hardened material deposited by geothermal vents, bedrock, or some combination thereof (Table A.1) with little or no interstitial space, resulting in embryos remaining <20 cm deep in the substrate during development and hatching. Additional, as yet unverified, Lake Trout spawning sites probably exist.