Cooperative Fish and Wildlife Research Units Program: Utah
Education, Research and Technical Assistance for Managing Our Natural Resources

Physical and biotic processes affecting trout habitat extent and quality often vary differentially across scales with the net effect oftentimes uncertain. For example, submerged macrophytes have conflicting effects on overall trout habitat by reducing preferred habitat extent within a river channel while simultaneously increasing a river channel’s productivity and growth potential. In the Henrys Fork River, Idaho, USA, we hypothesized higher submerged macrophyte coverage at the reach scale would exert a negative influence on the proportion of preferred trout habitats—defined as “habitat extent”—but would improve biotic habitat quality as measured by reach-scale models of Net Rate of Energy Intake (NREI). We therefore hypothesized a neutral or positive relationship between reach-scale submerged macrophyte coverage and a measure of net trout habitat suitability. Through random habitat sampling, aerial imagery, and snorkel surveys, we found macrophyte coverage resulted in a complementary riverscape of five habitat types. Macrophyte clumps created geomorphic forces that gave rise to intermacrophyte spaces and contributed to the formation of the remaining habitat types: gravel bars, emergent margins, and the thalweg. When testing the relationship between submerged macrophyte coverage and trout habitat, we found our hypothesized models of how habitat metrics interact with submerged macrophyte coverage appeared to fit the data well. Habitat extent declined quadratically with increasing submerged macrophyte coverage, whereas habitat quality increased exponentially. As a result, weighted trout habitat suitability changed little. As our study only featured a moderate range of macrophyte coverage (from 32 to 94% available surface area), future testing across a wider range of coverages may provide a more thorough test of our hypotheses. Nonetheless, our results highlight the importance of submerged macrophyte coverage to trout habitat via geomorphic effects. Our results also highlight the importance of trout habitat plasticity as well as habitat quality in determining overall habitat suitability.