Rainbow versus cutthroat trout effects on predatory invertebrate assemblages

Authors: Amber Bell

Year: 2019

Abstract

Input and output subsides link aquatic and terrestrial ecosystems. Non-native trout occur at higher densities in streams where they replace native trout and can disrupt key ecosystem functions that cross ecosystem boundaries, such as aquatic-insect emergence. The increased biomass and behavioral differences of non-native trout can influence other aquatic predators such as predatory stoneflies (Perlidae) and water striders (Gerridae), as well as riparian predators such as spiders (Tetragnathidae). Our study sites were in six streams along the northern Wasatch Front, Utah. We hypothesized three streams with non-native rainbow trout, Oncorhynchus mykiss, would have fewer aquatic and riparian predators than three others with native Bonneville cutthroat trout, Oncorhynchus clarki Utah. We found mean rainbow trout biomass (598.96 ± 277.2 SE mm/m^3 ) was higher than cutthroat trout biomass (202.09 ± 87.5 SE mm/m^3 ). Water strider densities were higher in cutthroat streams than in rainbow streams (1.07 ± 1.02 SE m^3, 0.18 ± 0.13 SE m^3, respectively), as were perlid stonefly densities (0.25 ± 0.14 SE m^3, 0.12 ± 0.05 SE m^3, respectively). In contrast, tetragnathid spider horizontal-web density was lower in cutthroat streams than in rainbow streams (1.53 ± 1.20 SE m^3, 3.07 ± 1.30 SE m^3, respectively). Differences in habitat could account for some of our findings. Rainbow streams were wider with higher aerial-wood and submerged-wood biomass. Aerial wood could provide web-building sites for tetragnathid spiders and submerged wood could elevate in-stream productivity. On the other hand, cutthroat streams were deeper and had larger substrates, possibly better habitat for perlid stoneflies. In any case, evidence suggests non-native rainbow trout do interact differently with other predators compared to native Bonneville cutthroat trout.