Status of Redband Trout and nonnative trout in the Wood River Basin, Idaho

Abstract The Wood River Basin in central Idaho has been isolated from the surrounding Snake River Basin by Malad Gorge Falls for at least 50,000 years, and recent genetic analyses suggest that Redband Trout Oncorhynchus mykiss gairdneri in the basin represent a previously undescribed lineage. To assess their contemporary status, we electrofished 22 study reaches in 2021–2022 previously occupied by Redband Trout when originally surveyed in 2003. Our objective was to assess changes in the occurrence and density of Redband Trout, Brook Trout Salvelinus fontinalis, and Brown Trout Salmo trutta. In 2021–2022, Redband Trout were absent in five of the 22 originally occupied reaches with Brook Trout as the only salmonid species present at all extirpated reaches. Brook Trout have now been extirpated from two reaches previously occupied with Redband Trout as the only salmonid species present at both reaches. Brown Trout colonized one new reach since 2003 and are now present at all study reaches (n = 3) which exceed 10 m average stream wetted width. Average fish density increased from 2003 to 2021–2022 across all study reaches for total trout (all species combined) and at nearly all study reaches for each individual trout species. Changes in Redband Trout density were unrelated to changes in nonnative trout density. On average, Redband Trout composition (of all trout at each reach) across all study reaches decreased by 13% whereas Brook Trout and Brown Trout increased by 8% and 4%, respectively, from 2003 to 2021–2022. Nevertheless, Brook Trout are increasingly present in headwater streams, whereas Brown Trout occupy larger, lower elevation rivers, potentially threatening the long-term conservation of Redband Trout in the Wood River Basin. Although additional data is warranted to more thoroughly understand the distribution of trout, these preliminary results suggest that management activities may be necessary to control the further spread of nonnative salmonids in the basin.