Processes Contributing to Variability in Regional Patterns of Juvenile River Herring Abundance across Small Coastal Systems

Abstract

Many populations of anadromous herring, (e.g., alewives Alosa pseudoharengus and blueback herring A. aestivalis, collectively referred to as river herring) are in decline. To help understand the various processes influencing their relative abundance, we studied juvenile river herring populations in 11 small, coastal Massachusetts systems. We examined diel and seasonal movements, variation in patterns of abundance, and relationships between juvenile river herring numbers and seven abiotic and biotic factors (stream discharge, pond temperature, habitat availability, pond transparency, pH, food availability, and spawning stock size). Seasonally, juvenile downstream migration peaked in early summer, and most juvenile river herring emigrated between 1200 and 1600 hours. Little or no emigration occurred in late summer when stream channels were often dewatered, although several streams experienced a smaller, more variable emigration peak in the fall. In univariate regressions, stream discharge, pond volume, surface area, depth, transparency, and pH were significantly related to variation in juvenile abundance across systems. Multiple regression models that integrated discharge, volume, and transparency, as well as multivariate models that included abiotic and biotic influences, trophic effects, and system size, explained 32–82% of the variability in juvenile abundance across systems. Thus, stream discharge, pond volume, transparency, pH, trophic effects, and system size contribute to heterogeneity across systems and may influence the abundance of these fish during freshwater residence. Increased understanding of the sources of heterogeneity in movement patterns and causes of variability in abundance across systems can help to implement more effective monitoring protocols, more informed land-use decisions, and improved management of river herring.