Individual repeatability in the timing of river entry indicates the strong influence of photoperiod in the spawning migrations of iteroparous twaite shad Alosa fallax

Abstract The timing of entry into freshwater by mature anadromous fishes is an important component their migration phenology. In iteroparous species, identifying the cues that influence freshwater, and annual variability in these timings, assessing extent individual repeatability. Here, passive acoustic telemetry tracked 71 twaite shad Alosa fallax (‘shad’) returning to River Severn’s upper estuary over successive spawning seasons, providing information on river associated cues. (defined as first detection receiver estuary) was consistent between years strongly predicted photoperiod (as increasing day length), whereas water temperatures flow rates were weak predictors highly variable years. Timings also across consecutive migrations, indicating high application this study thus revealed repeatability which cued mainly length that, being seasonally predictable, should ensure with sufficient time arrive areas when in-river environmental conditions are favourable.