Estimating Fish Passage Rates from Count Data at Consecutive Dams

Knowledge of the rate that fish pass migratory barriers is important in determining the impact of barriers on the population dynamics of anadromous fishes.  Fish passage criteria at hydroelectric projects are evolving from requirements to pass absolute numbers of fish towards criteria to pass fish at specified rates (e.g. percent passed per unit time). Passage rates can be estimated via telemetry studies, but such studies are often lacking in systems where immediate management questions loom.   The objective of this analysis was to develop a model that used daily counts of American Shad passing consecutive dams on the Susquehanna River to estimate passage rates.  The model estimated two parameters: the probability of fish “surviving” from one day to the next and remaining available to pass, and the probability of fish passing in a day given that it is available to pass.  Daily survival for fish attempting to pass was 87% for Holtwood Dam and 91% for Safe Harbor Dam.  Daily passage was 7% for Holtwood Dam and 36% for Safe Harbor Dam.     These estimates can serve as interim measures of fish passage performance until direct estimates of passage rates can be made via radio telemetry studies.