Barriers to fish passage and barriers to fish passage assessments: the impact of assessment methods and assumptions on barrier identification and quantification of watershed connectivity

Barriers (culverts and dams) can impede fish passage and affect the overall habitat connectivity of rivers. However, a challenge lies in how to conceptualize and adequately measure passability at barriers. We hypothesize that estimates of barrier and watershed connectivity are dependent on assumptions about the nature of passability, and how it is measured. Specifically, we compare passability estimates in Terra Nova National Park, Canada for individual barriers for two barrier assessment methods (a rapid assessment and one based on FishXing software), two salmonid species, different fish sizes and swimming speeds, and varying hydrological conditions. Watershed connectivity was calculated using the Dendritic Connectivity Index (DCI). Lastly, we test to see what the impact of the various factors is on the practical goal: prioritizing barriers for restoration. Our results show that barrier passability estimates can vary drastically for some barriers (0-100%). In general, the rapid field-based assessment tended to give more conservative estimates of passability than those based on FishXing. Estimates of watershed connectivity were not as sensitive to the assumptions and methods used (DCI: 40-83). Fish size had the greatest effect on DCI. Importantly, variation in DCI had little impact on the restoration priorities. The same barrier was retained as the top priority > 96% of the time. Thus, managers wishing to assess barriers for restoration need to carefully consider how passability is to be measured, but can reduce the impact of these decisions by considering barriers in their watershed context using a connectivity index such as the DCI.