Historical Data Provide Important Context for Understanding Declines in Cutthroat Trout

Abstract

We used historical stocking and population survey records of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and other salmonids in the North Fork Shoshone River drainage, Wyoming to summarize fish stocking history and population trends. Based on 98 years of historical records, we found that despite extensive stocking of Yellowstone Cutthroat Trout and minimal stocking of nonnative salmonids after about 1950, populations of wild Yellowstone Cutthroat Trout declined relative to those of nonnative salmonid species. The timing of increases in nonnative salmonids (1970s) did not coincide with their period of most intensive stocking (1935–1950). It is plausible that Yellowstone Cutthroat Trout populations persisted because of high levels of supplemental stocking from 1935 to 1965 and declined with reduced stocking efforts in the 1970s, thereby allowing the increase of introduced nonnative salmonids. The establishment of nonnative salmonids likely further reduced stocking success of Yellowstone Cutthroat Trout due to competition and hybridization. This study demonstrates that an understanding of long‐term stocking records and population survey data can be useful for developing and implementing successful management frameworks for the conservation of imperiled fish populations across the United States.

Hybridization decreases native cutthroat trout reproductive fitness

“…However, these findings are not immediately reconcilable with the system‐wide pattern of Yellowstone cutthroat trout population declines. It is surprising to discover greater reproductive success in Yellowstone cutthroat trout given the precipitous decline of this species in the North Fork Shoshone over the past 60 years (Nordberg et al, 2021). Variability in the reproductive success of Yellowstone cutthroat trout across tributaries of the North Fork Shoshone River could potentially explain this pattern; some spawning tributaries may produce a net positive number of Yellowstone cutthroat where others may act as a metapopulation sink.…”

“…This strategy has been successful in Idaho, where weirs have been employed to selectively pass only Yellowstone cutthroat trout into spawning tributaries (High, 2010; McCormick & High, 2020). Stocking of Yellowstone cutthroat trout individuals may have bolstered the population in this system in the past, but may have also unintentionally introduced less‐fit alleles and contributed to Yellowstone cutthroat trout decline (Nordberg et al, 2021). Increasing the number of Yellowstone cutthroat trout juveniles produced each year via rainbow trout exclusion may be a viable substitute that also does not risk replacing locally adapted alleles with those of hatchery origin.…”

“…Yellowstone cutthroat trout are native to the drainage but have experienced declines since the 1960s (Nordberg et al, 2021), which have been largely attributed to the introduction of rainbow trout in the early 1900s. Both rainbow trout and Yellowstone cutthroat trout were heavily stocked into the drainage during the 1900s, but Yellowstone cutthroat trout were stocked for a much longer duration and in larger numbers (Nordberg et al, 2021). Previous studies have found low numbers of unadmixed Yellowstone cutthroat trout (Kruse et al, 2000; Mandeville et al, 2019) and have found no evidence of spatial segregation between Yellowstone cutthroat trout and rainbow trout into upstream and downstream reaches (Kruse et al, 2000).…”

“…The river's hydrology is dominated by snowmelt runoff, and therefore has its highest discharge in late spring. Yellowstone cutthroat trout are native to the drainage but have experienced declines since the 1960s (Nordberg et al, 2021), which have been largely attributed to the introduction of rainbow trout in the early 1900s. Both rainbow trout and Yellowstone cutthroat trout were heavily stocked into the drainage during the 1900s, but Yellowstone cutthroat trout were stocked for a much longer duration and in larger numbers (Nordberg et al, 2021).…”

“…The Yellowstone cutthroat trout population in the North Fork Shoshone River (Wyoming, USA) is one such population. In this system, the native Yellowstone cutthroat trout have been almost entirely replaced by hybrids and rainbow trout after the introduction of rainbow trout 110 years ago (Kruse et al, 2000; Mandeville et al, 2019; Nordberg et al, 2021). However, Mandeville et al (2019) utilized genomic data to identify the persistence of unadmixed Yellowstone cutthroat trout individuals despite predictions of their extirpation two decades ago (Kruse et al, 2000).…”