Connectivity and Conditional Models of Access and Abundance of Species in Stream Networks

Large investments have been made to restore passage for aquatic organisms at stream-road crossings, but often without estimation of population-level benefits.  We describe an approach to quantifying the effectiveness of passage restoration in terms interpretable at population levels, namely numbers of fish and length of stream gained through restoration, by sampling abundance in a study design that accounts for variable biogeographic species pools, variable stream and barrier configurations, and variable probabilities of capture. We modified a zero-inflated negative-binomial model to estimate the probability of site access, abundance conditional on access, and capture probability of individual fish. Results indicated that replacement of barriers with new crossings designed to allow for greater movement was associated with dramatically higher probability of access for all fishes, whereas existing non-replaced crossings negatively impacted fish distributions. When limited ranges of species were considered, the contributions of culvert replacements were just a fraction (< 10%) of the stream length available upstream, and numbers of fish were modest. Although our framework can address the question of effectiveness in a broad array of stream and crossing configurations, much stronger inferences would be possible if future restoration efforts have available information on crossings and species presence prior to restoration.