The interactive effects of stream temperature, stream size, and non-native species on Yellowstone cutthroat trout

Climate change and non-native species are considered two of the biggest threats to native salmonids in North America. We evaluated how non-native salmonids and stream temperature and discharge were associated with Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) distribution, abundance, and body size, to gain a more complete understanding of the existing threats to native populations. Allopatric Yellowstone cutthroat trout were distributed across a wide range of average August temperatures (3.2 to 17.7ºC), but occurrence significantly declined at colder temperatures (<10 ºC) with increasing numbers of non-natives. At warmer temperatures occurrence remained high, despite sympatry with non-natives. Yellowstone cutthroat trout relative abundance was significantly reduced with increasing abundance of non-natives, with the greatest impacts at colder temperatures. Body sizes of large Yellowstone cutthroat trout (90th percentile) significantly increased with warming temperatures and larger stream size, highlighting the importance of access to these more productive stream segments. Considering multiple population-level attributes demonstrates the complexities of how native salmonids (such as Yellowstone cutthroat trout) are likely to be affected by shifting climates.

Presence of Microplastics in the Food Web of the Largest High-Elevation Lake in North America

Microplastics have been documented in aquatic and terrestrial ecosystems throughout the world. However, few studies have investigated microplastics in freshwater fish diets. In this study, water samples and three trophic levels of a freshwater food web were investigated for microplastic presence: amphipods (Gammarus lacustris), Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri), and lake trout (Salvelinus namaycush). Microplastics and other anthropogenic materials were documented in water samples, amphipods, and fish, then confirmed using FTIR (Fourier-transform infrared) and Raman spectroscopy. Our findings confirmed the presence of microplastics and other anthropogenic materials in three trophic levels of a freshwater food web in a high-elevation lake in a national park, which corroborates recent studies implicating the global distribution of microplastics. This study further illustrates the need for global action regarding the appropriate manufacturing, use, and disposal of plastics to minimize the effects of plastics on the environment.

Mating systems and predictors of relative reproductive success in a cutthroat trout subspecies of conservation concern

Mating systems and patterns in reproductive success of fishes play an important role in ecology and evolution. While information on the reproductive ecology of many anadromous salmonids (Oncorhynchus spp.) is well-detailed, there is less information for non-anadromous species including the Yellowstone Cutthroat Trout (O. clarkii bouvieri), a species of recreational angling importance and conservation concern. Here, we used data from a parentage-based tagging study to describe the mating system of Yellowstone Cutthroat Trout from a spawning tributary of the South Fork Snake River, Idaho, and identify predictors of relative reproductive success. We detected evidence of monogamy, polygyny, and polyandry and showed that reproductive success was best explained by arrival time at the spawning ground and total length. Specifically, the largest adults arrived earliest in the season and produced a disproportionate number of offspring. Lastly, we estimated the effective number of breeders (N) and effective population size (N) and showed that while Nb was lower than Ne, both are sufficiently high to suggest Yellowstone Cutthroat Trout in Burns Creek represent a genetically stable and diverse population.

Abiotic conditions are unlikely to mediate hybridization between invasive rainbow trout and native Yellowstone cutthroat trout in a high-elevation metapopulation

Understanding factors mediating hybridization between native and invasive species is crucial for conservation. We assessed the spatial distribution of hybridization between invasive rainbow trout (Oncorhynchus mykiss) and native Yellowstone cutthroat trout (Oncorhynchus clarkii bouveri) in the Lamar River of Yellowstone National Park using a paired telemetry and genetic dataset. Spawning populations containing hybrids (15/30) occupied the full spectrum of abiotic conditions in the watershed (stream temperature, stream size, runoff timing), including an intermittent stream that dried completely in late June, and mainstem spawning locations. Hybrids and rainbow trout occupied an entire high-elevation (∼2500–1900 m) tributary where rainbow trout ancestry was highest in headwaters and decreased downstream. Fluvial distance to this ostensible source population was the only covariate included in top hybridization models; effects of abiotic covariates and stocking intensity were relatively weak. In this watershed, abiotic conditions are unlikely to mediate continued hybridization. We conclude that management intervention is important for the persistence of nonhybridized Yellowstone cutthroat trout and highlight the value of pairing telemetry with genetic analysis to identify and characterize populations for hybridization assessments.

Two Ocean Pass: An Alternative Hypothesis for the Invasion of Yellowstone Lake by Lake Trout, and Implications for Future Invasions

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.

Effects of Air Exposure on Survival of Yellowstone Cutthroat Trout Angled from a Stream with Warm Water Temperatures

We evaluated the effects of air exposure on Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri caught and released in a cold-water stream with elevated water temperatures (i.e., &gt; 14°C) in southeastern Idaho. Anglers caught fish in a 2.3-km section of Fall Creek, Idaho, during August 2018. Sampled fish remained underwater while we measured and then tagged them with T-bar anchor tags. We exposed fish to air for 0, 30, or 60 s and then released them at the point of capture. We continuously monitored temperature during the study period. Water temperatures during the study varied from 10.0 to 19.7°C and averaged 14.9°C (SE = 0.08). In total, anglers caught 161 Yellowstone Cutthroat Trout over 10 d. Of those fish, we did not expose 54 to air; we exposed 54 to air for 30 s, and 53 for 60 s. We used electrofishing to recapture tagged fish and estimate relative survival. Relative survival was highest for fish exposed to air for 60 s (0.40 [SE = 0.25]) followed by 0 s (0.35 [SE = 0.25]) and 30 s (0.30 [SE = 0.27]), but differences were not statistically significant. Results from this study are consistent with other air-exposure studies suggesting that air exposure of 60 s or less is not likely a concern in Yellowstone Cutthroat Trout fisheries. Releasing fish as quickly as possible is always encouraged, but management regulations restricting air exposure seem unnecessary given the collective body of field-based research on air exposure. Nevertheless, similar studies on other systems and species are warranted.