One Rung at a Time: Climbing the Ladder Toward Aquatic Connectivity in Eastern South Dakota

Roads and streams are synonymous with their connection to movement, the acquisition of resources, and as pathways to new opportunities. However, the factors that define good roads and ecologically connected streams are rarely without conflict. Road infrastructure at stream-road crossings has the potential to fragment stream networks. Tube culverts, where streams pass under the road through metal pipes, are particularly concerning for stream connectivity. Undersized, aging, or inappropriately installed culverts can develop vertical drops at the outflow due to high velocities and stream bed scouring. The outlet drop can function as a barrier, preventing the upstream movement of fish. Due to the prevalence of tube culverts and their potential to impede passage for small-bodied Great Plains stream fishes, novel cost-effective mitigation strategies and field evaluations are needed to inform barrier inventories and prioritization. In this study, I assessed the in-situ effectiveness of a low-cost Denil-type fish ladder designed to facilitate upstream movement through perched tube culverts. Using a multiple before-after-control-impact (MBACI) design, I deployed passive integrated transponder (PIT) arrays at seven sites to monitor bidirectional movements of non-game, small-bodied stream fishes at impact and control sites before and after ladders were installed at impact locations. Next, I used the pre-ladder installation movement data to evaluate the relationship between Southeastern Aquatic Resource Partnership (SARP) barrier severity estimates and observed passage. I used a combination of mixed-effects logistic regression and multistate models to obtain predicted probability of passage and species-specific transition probability estimates to quantify movement across a gradient of barrier severity. I found that tube culverts acted as barriers to fish movement, with few observed passage events and low estimated passage success and transition probability across sites. The effect of the experimental ladder on upstream movement was temporally variable, species-dependent, and influenced by body size. The data collected during the study is somewhat inconclusive in determining the effectiveness of the ladder in aiding upstream movement in dynamic prairie streams. Moreover, the SARP score was predictive of passage probability, but the mid-range SARP scores did not consistently predict observed passage outcomes, suggesting the protocol may lack sensitivity for small-bodied species. Overall, the result highlights multiple overlapping levels of variability when incorporating biodiverse species, their unique movement capacity, and dynamic structural interaction with stream environments. This emphasizes the need for in-situ passage studies to refine barrier severity assessments and to inform the development of rapidresponse barrier mitigations options, such as the ladder prototype.