Timing and environmental drivers of spawning migrations of alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis) in rivers of Chesapeake Bay

— Annual life history events, such as migration and spawning in fish, are often timed to match seasonal fluctuations in environmental conditions. Understanding phenological patterns and their drivers is needed to conserve vulnerable fish populations, particularly as climate change is disrupting seasonal transitions. In this study, we examined the phenology of spring spawning migrations of river herring, the alewife ( Alosa pseudoharengus ) and the blueback herring ( A. aestiva-lis ), in 4 rivers of Chesapeake Bay over 7 years (2013–2019). We used imaging sonar and weekly biological samples to estimate species-specific hourly fish counts in the Choptank River, Deer Creek (a tributary of Susquehanna River), Marshyhope Creek (a tributary of Nanticoke River), and the Patapsco River. Our results indicate that patterns of water temperature in spring drive spawning migrations at seasonal, daily, and hourly scales. The relationship between fish counts and other environmental factors, such as stream-flow, lunar phase, turbidity, wind stress, and tides, were annually inconsistent, and their influences on migrations may be river specific. For both species, distinct diel patterns in movement were observed. Migrations were diurnal in the Choptank River and Marshyhope Creek but nocturnal in Deer Creek and the Patapsco River. Observed interannual and inter-river variation in phenology and the influence of non-temperature factors on migrations highlight the need for long-term, multi-river monitoring of river herring in Chesapeake Bay.