River herring are comprised of two nearly identical fish, Alewife (Alosa pseudoharengus), and Blueback herring (Alosa aesitvalis). Alewife are generally longer than Blueback, but the distinction must be made by dissection, They grow to 12.8”. Alewife spawn at 51-80 F degree water, Blueback spawn at 57 F-80 F degree water, so trailing behind the early migration.