Changes in River Herring Spawning and Juvenile Emigration Phenology in Response to Environmental Conditions
The adult landward spawning migration in spring and the seaward juvenile emigration in fall are two key life history events in the life cycle of the diadromous river herring. The timing of the juvenile emigration appears to be influenced by variation in temperature and river flow, while the timing of the spawning migration is temperature dependent. This environmental dependence may lead to inter-annual variation in the timing of spawning migrations and juvenile out-migrations, and in a potential mismatch with optimal conditions in the receiving marine (for juveniles) or freshwater environment (for spawners), leading to reduced productivity. Using generalized additive models we examined temporal variation in the date of migration of adult river herring from the Damariscotta, Androscoggin, Connecticut, and Monument rivers and in the date of emigration of juveniles from the Hudson and Connecticut rivers, and their relationship to environmental conditions. We demonstrate that initiation of both the spawning migration and juvenile emigration has advanced in recent decades. The influence of air temperature, river flow, and sea surface temperature on the timing of migration is also discussed.