As "Alosa" species are generally anadromous, they face various obstacles to survival. They may have to pass through numerous barriers and waters to get to either their spawning grounds or normal habitats (the sea in most cases). Estuaries are a major factor in numerous "Alosa" species' migrations. Estuaries can be highly variable and complex environments contributing to fluctuating biological interactions, with shifts in osmolarity, food sources, predators, etc. Since many adult "Alosa" species die after spawning, only the young generally to migrate to the sea from the spawning grounds. Duration of migration varies among fish, but can greatly affect survival.