A life cycle simulation model for exploring causes of population change in Alewife (Alosa pseudoharengus)

Over the last two decades, major changes in abundance and population characteristics of Alewife (Alosa pseudoharengus), an anadromous herring species, have been observed along the US Atlantic coast. Loss of spawning habitat, bycatch mortality in the directed pelagic fisheries, increased predation mortality by rebounding predators such as Striped Bass, changes in water flow and temperature affecting recruitment success, changes in ocean thermal habitat and direct and indirect effects of changes in zooplankton community have been expounded by different researchers as putative hypotheses for population changes in Alewife. Unfortunately, long-term, concurrently-measured time series of regional factors and direct measures of biological processes needed to elucidate underlying causes are severely lacking for Alewife. Therefore, we developed, calibrated and validated a mechanistic, spatially-explicit, full life-cycle simulation model that can be used to explore population responses of Alewife to various exogeneous drivers. Daily processes such as spawning, recruitment, mortality, exploitation, predation and movements are generated by using empirically-derived deterministic and stochastic relationships and time-series of environmental data linked to specific life stages. We demonstrate the use of the model as an investigative tool by simulating three hypotheses and comparing model results to observed trends in Alewife populations from southern New England.