Fish Assemblage Relationships with Physical Habitat in Wadeable Iowa Streams

Abstract Fish assemblages play a key role in stream ecosystems and are influenced by physical habitat. We analyzed fish assemblages and physical habitat at 93 randomly selected sites on second- through fifth-order wadeable Iowa streams to explore fish assemblage relationships with reach-scale physical habitat in this agriculturally dominated landscape. Sites were sampled using DC electrofishing and the wadeable streams physical habitat protocol of the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program. In all, 82 species were collected, with species richness at sites averaging 14. Over 80% of the sites had fish assemblages rated as fair (53%) or poor (32%) based on a fish index of biotic integrity (FIBI). Ordination separated sites from the two major river drainages along an axis of impairment, with sites in the Missouri River drainage exhibiting lower FIBI scores than sites in the Mississippi River drainage. Physical habitat at most sites exhibited fine substrates, eroding banks, and low-gradient, nonmeandering channel and was dominated by glides. Thirty physical habitat variables describing channel morphology, channel cross section and bank morphology, fish cover, human disturbance, large woody debris, relative bed stability, residual pool, riparian vegetation, and substrate differed significantly between sites with FIBI scores rated as poor and those with FIBI scores rated as good or excellent. Eighteen physical habitat variables were significant predictors of fish assemblage metrics and FIBI in multiple linear regression models, with adjusted R2 values ranging from 0.12 to 0.58. Seventy percent of the model coefficients reflected substrate (40%), residual pool (21%), and fish cover (9%) variables. Fish assemblages in wadeable Iowa streams are strongly associated with the quality of physical habitat. Thus, understanding and addressing the determinants of physical habitat are crucial for managing streams in Iowa and other agricultural regions.