Stream invertebrate responses to trout introductions: Results from large-scale studies in the central Sierra Nevada and Yosemite National Park

Global introductions of non-native trout have had clear impacts on lake communities but uncertain and seemingly more limited impacts in streams. In this research, we used large-scale surveys to examine relationships between trout introductions and stream invertebrate populations in the Sierra Nevada mountains of California, USA. The Sierra Nevada historically contained no fish in nearly all areas above 1800 m (6000 ft) elevation, but trout have been widely introduced and now occupy most perennial streams in this area. We used two types of surveys in and around Yosemite National Park to simultaneously isolate the effects of trout introductions and eliminate confounding explanations for differences in invertebrate populations among streams and stream reaches. First, we sampled above and below barrier waterfalls on a series of streams to determine whether invertebrate populations change coincident with the onset of trout distributions. Second, we selected 21 stream pairs (a fishless stream matched with a stream containing introduced trout) to determine if differences existed in the size of invertebrate populations in trout versus fishless streams. The combined results indicated that trout introductions are related to decreased abundance in at least 7 invertebrate groups: the mayflies Ameletus, Edmundsius agilis, and Paraleptophlebia; the stonefly Doroneuria baumanni; the caddisflies Anagapetus and Neothremma; and the Tricladida flatworms. Although there were also suggestive patterns for a number of other invertebrate groups, most stream invertebrates occurred at similar frequencies and in similar abundances in fishless and trout streams. Multivariate analyses likewise found no distinction between fishless and trout streams in Yosemite National Park. The most severely affected taxa (Edmundsius agilis, Neothremma, Tricladida) contain primarily endemic species in the Sierra Nevada mountains, suggesting that a lack of evolutionary exposure to trout and other fishes may increase the likelihood and severity of effects from these introduced fishes. Because the strongest effects from trout fell primarily on endemic species, and because the broader invertebrate assemblage was not strongly affected by trout, we recommend that managers focus on endemic invertebrates by conducting more extensive surveys of their current distribution and, when necessary, eradicating trout to increase and recover populations of threatened endemic invertebrates.