Evidence of River Herring Life History Variation in Penobscot River Estuary, Maine USA

Alewife and Blueback Herring, collectively River Herring, are two diadromous species at or near historically low abundance levels across their range. River Herring use of estuarine habitat is poorly understood, but it has been characterized as transitional habitat for migrants rather than a significant nursery area.  In the Penobscot River, these species are currently being enhanced by aggressive stocking efforts and large-scale dam removal.  We surveyed the lower Penobscot River and Estuary from April through October 2012 and 2013 with pelagic trawl gear.  We found multiple year classes of River Herring throughout the period including adults (>age 3) and juveniles (ages 0-2), with juveniles accounting for about 40% of the pelagic estuarine biomass.  Juvenile River Herring abundance was variable but was greater in spring and fall and was concentrated at moderate salinities (5-20ppt).  These findings provide a baseline for characterizing the dynamics of pelagic estuary habitat and both species' role in this dynamic area.  Finally, we discuss prolonged estuarine residence in the context of trophic dynamics within the system and explore the implications of life history variation for these imperiled species.