Muddying the waters: investigating the generality of silt-resistance in mound-building Nocomis spp. using hornyhead chub (Nocomis biguttatus) and redspot chub (Nocomis asper)

Fishes in the genus Nocomis function as keystone engineers via their mound-building spawning behavior. Furthermore, mound construction grants some Nocomis spp. resistance to excess sedimentation, as it concentrates gravel in an otherwise silty substrate. However, it is unclear whether silt-resistance is a general trait of Nocomis spp. or is instead more species-specific. The objectives of our research were to identify local- and watershed-scale environmental variables that could explain the distribution of hornyhead chub (Nocomis biguttatus) and redspot chub (Nocomis asper) in Kansas, with a particular focus on the impacts of sedimentation and agriculture. We accomplished these objectives by comparing physicochemical variables between sites where Nocomis spp. were present versus absent using analysis of deviance, and by constructing local- and watershed-scale environmental niche models (ENM) for the hornyhead chub using LASSO-regularized logistic regression. We found that streams where Nocomis spp. were present had less silt, clearer water, and more natural upstream watersheds compared to sites where absent. Furthermore, silt coverage was the only important predictor variable in the local-scale ENM, while the percentages of upstream watersheds that were agricultural or reservoirs were most important in the watershed-scale ENM. All three predictor variables exhibited negative relationships with chub occurrence. Our results suggested that neither species were silt-resistant, potentially meaning their ability to function as keystone engineers could be compromised in streams with excess sedimentation. As such, the implementation of land management practices that reduce soil erosion may be necessary to recover populations of these Nocomis spp., ultimately restoring their keystone functionality.