Diet overlap among non-native trout species and native Cutthroat Trout (Oncorhynchus clarkii) in two U.S. ecoregions

The invasion of freshwater ecosystems by non-native species can constitute
a significant threat to native species and ecosystem health. Non-native
trouts have long been stocked in areas where native trouts occur and have
negatively impacted native trouts through predation, competition, and
hybridization. This study encompassed two seasons of sampling efforts
across two ecoregions of the western United States: The Great Basin in
summer 2016 and the Yellowstone River Basin in summer 2017. We found
significant dietary overlaps among native and non-native trouts within the
Great Basin and Yellowstone River Basin ecoregions. Three orders of
invertebrates (Ephemeroptera, Trichoptera and Diptera) composed the
majority of stomach contents and were responsible for driving the observed
patterns. Great Basin trout had higher body conditions (k) and non-native
Great Basin trout had higher gut fullness values than Yellowstone River
Basin trout, indicating a possible limitation of food in the Yellowstone
River Basin. Native fishes were the least abundant and had the lowest body
condition in each ecoregion. These findings may indicate a negative impact
on native trouts by non-native trouts. We recommend additional monitoring
of native and non-native trout diets, regular invertebrate surveys to
identify the availability of diet items, and reconsidering stocking
efforts that can result in overlap of non-native fishes with native
cutthroat trout.