Limitations of Non‐Volitional Upstream Passage for Alewife (Alosa pseudoharengus) and Blueback Herring (Alosa aestivalis)

Abstract

We used PIT telemetry (n = 10,292 fish tagged) to evaluate upstream passage at a non‐volitional fishway (trap, lift, and truck) that passed approximately 10‐million river herring (alewife Alosa pseudoharengus and blueback herring A. aestivalis) around the lowermost dam in the Wolastoq/Saint John River, NB between 2020 and 2023. Between 26% and 62% of tagged fish reached the fishway crowding pool, while less than 14% were detected passing upstream. River herring experienced considerable passage delays (median = 3 days) after reaching the crowder entrance. The probability of passing on the date of first detection was only 10%, and it was positively correlated with the rate of fishway operation (i.e., fish lifts/unit time). The rate and probability of passage were greater for alewife than blueback herring and increased with total length for both species. Collectively, our results suggest that passage efficiency and duration were limited by the movement capacity and operation frequency of the fishway, and potentially the (high) number of fish attempting to pass at a given time. Ultimately, if the design and operation of non‐volitional fishways do not accommodate the size and behavior (i.e., schooling density and migration time) of target populations, our results indicate that potential consequences may include passage delays, reductions in passage efficiency, and selective pressures (e.g., size and species) on target populations.