How do anadromous clupeiform fishes use both sea and freshwater habitats?

Anadromous clupeiform fishes, such as certain species of shad and alewife, utilize both sea and freshwater habitats by migrating between them to complete their life cycles, primarily for the purpose of breeding in fresh water after spending their adult lives in the sea.12

This transition requires significant physiological adaptation to manage osmotic pressure, which is the pressure exerted by differences in salt concentrations between the fish's body fluids and the surrounding water. Because the conversion of their internal regulatory systems takes time, these fish typically congregate in estuaries or brackish waters while migrating. This period allows them to adjust their osmoregulation mechanisms—such as altering how their gill cells transport ions and how their kidneys process water—to avoid physiological shock when moving between high-salinity ocean environments and low-salinity freshwater rivers or lakes.23