Why are pikes restricted to freshwater throughout their life cycles?

Pikes (family Esocidae) are strictly restricted to freshwater throughout their life cycles because they are physiologically adapted to that environment, unlike some of their relatives in the Salmonidae family that have evolved an anadromous life cycle allowing them to migrate between freshwater and the sea.12

In aquatic environments, fish must maintain a delicate balance of internal salinity to survive. Freshwater fish, such as pikes, face the constant challenge of water diffusing into their bodies and the loss of essential ions to the surrounding environment. To compensate, they have evolved specialized gill cells that actively pump ions into their bodies and produce large quantities of dilute urine to expel excess water. Because pikes lack the physiological mechanisms to regulate their internal osmotic pressure in the high-salinity environment of the ocean, they are unable to survive or thrive in marine habitats. Their broad distribution across the Northern Hemisphere was achieved historically through direct freshwater connections between drainage basins rather than through marine migration.32