Spatial distribution and ecological relationships of stream fishes in Iowa\u27s Driftless Area

In the Midwestern United States, landscape and stream channel modification have caused habitat homogenization, erosional deposition, and increased nutrient loads. Concurrently, declines of native stream fishes, especially those intolerant to ecological disturbances or habitat degradation, have been observed. The Driftless Area ecoregion is a karst landscape that spans across the four states of Iowa, Minnesota, Wisconsin, and Illinois, and exhibits coldwater rivers and streams capable of supporting unique fish assemblages. Similar anthropogenic alterations threaten native fishes in the Driftless Area as seen elsewhere in Midwestern streams and rivers. However, the extensive network of coldwater habitats has allowed for the proliferation of a non-native introduced piscivore, the Brown Trout (Salmo trutta). We collected fish assemblage and physical habitat data at 138 stream segments throughout Iowa’s portion of the Driftless Area to assess the spatial distribution and environmental associates of fishes, as well as the potential negative effects of non-native Brown Trout on multiple stream fish species of greatest conservation need (SGCN). Occupancy analysis of native Brook Trout (Salvelinus fontinalis) and Brown Trout revealed that watershed level factors were effective at predicting both trout species’ occupancy. However, estimates were improved by the inclusion of local habitat parameters. Habitat relationships with non-game SGCN were also evaluated using occupancy modelling. Environmental variables deemed important for occupancy varied by species, but were often a combination of environmental conditions at the instream (e.g., stream temperature, substrate, and velocity) and watershed level (e.g., forest land cover, catchment area, and elevation). Brown Trout length and CPUE displayed different relationships with the occupancy and relative abundance of sculpin (Mottled Sculpin Cottus bairdii and Slimy Sculpin Cottus cognatus) compared to Southern Redbelly Dace (Chrosomus erythrogaster) and Longnose Dace (Rhinicthys cataractae). Additionally, we detected no negative effects of Brown Trout occurrence on the size structure of SGCN. Our results suggest that environmental factors at multiple spatial scales can be relevant for determining species’ occupancy, but the relative effects of characteristics at each scale likely depends on specific habitat requirements. Additionally, biotic interactions can influence the distribution and demography of stream fishes but are unique among species and may depend on abiotic conditions. Combined, results of this study can assist fisheries managers select effective conservation actions to benefit native stream fishes