Invasive predatory lake trout Salvelinus namaycush were discovered in Yellowstone Lake in 1994 and caused a precipitous decrease in abundance of native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri. Suppression efforts (primarily gillnetting) initiated in 1995 did not curtail lake trout population growth or lakewide expansion. An adaptive management strategy was developed in 2010 that specified desired conditions indicative of ecosystem recovery. Population modeling was used to estimate effects of suppression efforts on the lake trout and establish effort benchmarks to achieve negative population growth (λ < 1). Partnerships enhanced funding support, and a scientific review panel provided guidance to increase suppression gillnetting effort to >46,800 100-m net nights; this effort level was achieved in 2012 and led to a reduction in lake trout biomass. Total lake trout biomass declined from 432,017 kg in 2012 to 196,675 kg in 2019, primarily because of a 79% reduction in adults. Total abundance declined from 925,208 in 2012 to 673,983 in 2019 but was highly variable because of recruitment of age-2 fish. Overall, 3.35 million lake trout were killed by suppression efforts from 1995 to 2019. Cutthroat trout abundance remained below target levels, but relative condition increased, large individuals (> 400 mm) became more abundant, and individual weights doubled, probably because of reduced density. Continued actions to suppress lake trout will facilitate further recovery of the cutthroat trout population and integrity of the Yellowstone Lake ecosystem.

The potential to provide inferences about fish abundance from environmental (e)DNA samples has generated great interest. However, the accuracy of these abundance estimates is often low and variable across species and space. A plausible refinement is the use of common aquatic habitat monitoring data to account for attributes that influence eDNA dynamics. We therefore evaluated the relationships between eDNA concentration and abundance of bull trout (Salvelinus confluentus), westslope cutthroat trout (Oncorhynchus clarkii lewisi) and rainbow trout (Oncorhynchus mykiss) at 42 stream sites in the Intermountain West (USA and Canada) and tested whether accounting for site-specific habitat attributes improved the accuracy of fish abundance estimates. eDNA concentrations were positively associated with fish abundance, but these relationships varied by species and site, and there was still considerable variation unaccounted for. Random site-level differences explained much of this variation, but specific habitat attributes of those sites explained relatively small amounts of this variation. Our results underscore that either eDNA sampling or environmental characterization will require further refinement before eDNA can be used reliably to estimate fish abundance in streams.

The nesting ecology of Apalone spinifera in large North American rivers is largely unknown despite the wide distribution of the species in these naturally dynamic ecosystems. We describe the nesting locations, timing, behavior, and habitat of A. spinifera in relation to natural and anthropogenic factors in the Missouri River. Nesting followed annual peak river stage, mostly occurred in the afternoon when air temperatures were 25°–30°C, and did not occur when human activity was nearby. Apalone spinifera nested in June in a year with average discharge (2012), but nested 20 d later in a year with a large flood event (2011). During the average discharge year, 90% of nests were found on islands, but similar proportions of nests were found on island and mainland habitats during the flood year because many islands were inundated. Nests were mostly in mixed-gravel substrates where vegetation cover was sparse or absent. Depredation occurred only after the emergence of hatchlings (∼ 60 d after nesting) and...

Preventing the interbasin transfer of aquatic invasive species is a high priority for natural resource managers. Such transfers can be made by humans or can occur by dispersal through connected waterways. A natural surface water connection between the Atlantic and Pacific drainages in North America exists at Two Ocean Pass south of Yellowstone National Park. Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri used this route to cross the Continental Divide and colonize the Yellowstone River from ancestral sources in the Snake River following glacial recession 14,000 bp. Nonnative lake trout Salvelinus namaycush were stocked into lakes in the Snake River headwaters in 1890 and quickly dispersed downstream. Lake trout were discovered in Yellowstone Lake in 1994 and were assumed to have been illegally introduced. Recently, lake trout have demonstrated their ability to move widely through river systems and invade headwater lakes in Glacier National Park. Our objective was to determine if lake trout and other nonnative fish were present in the connected waters near Two Ocean Pass and could thereby colonize the Yellowstone River basin in the past or future. We used environmental DNA (eDNA), electrofishing, and angling to survey for lake trout and other fishes. Yellowstone cutthroat trout were detected at nearly all sites on both sides of the Continental Divide. Lake trout and invasive brook trout S. fontinalis were detected in Pacific Creek near its confluence with the Snake River. We conclude that invasive movements by lake trout from the Snake River over Two Ocean Pass may have resulted in their colonization of Yellowstone Lake. Moreover, Yellowstone Lake may be vulnerable to additional invasions because several other nonnative fish inhabit the upper Snake River. In the future, eDNA collected across smaller spatial intervals in Pacific Creek during flow conditions more conducive to lake trout movement may provide further insight into the extent of non-native fish invasions in this stream.