A New Paradigm in Alewife Migration: Oscillations between Spawning Grounds and Estuarine Habitats

The current paradigm in Alewife Alosa pseudoharengus migrations is shifting from a single reproductive event and rapid movement in and out of freshwater to one that recognizes more nuanced movements and asynchronous batch spawning. In this study, Alewives (n = 72) were acoustically tagged in the Carmans River, Long Island, New York, a small coastal river with restored fish passage. Our objective was to examine fine-scale spatial movements in the tidal and reconnected habitats to identify spawning behavior patterns, including habitat use and spawning periodicity, that can be applied to fishway restoration and species management. Alewives that completed their spawning migration (n = 27) exhibited variability in posttagging trajectories, residency, passage rate, and behavior. The current paradigm consisting of a single upstream migration followed by a downstream migration was only seen in 26% (n = 7) of fish, while 41% (n = 11) deviated from the paradigm and made multiple reversals and numerous movements on and off the spawning grounds. Oscillations spanned from the estuary into the reconnected habitat and resulted in a proportionally large amount of time spent in the estuarine and brackish habitats. Some oscillations also consisted of multiple fishway passage events within a season. Oscillation behavior is likely a naturally occurring spawning strategy that has gone undetected. Consequently, the proportion of individuals of a population that display single versus multiple passage events could result in fishway counts that bias run size estimations and thus population assessments.