Britannica Sources used in this answer

How do salmon and eels control salt levels in their bodies when moving between fresh water and seawater?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram showing ocean zonation by depth and region, with labeled layers: littoral, sublittoral, bathyal, abyssal, and hadal along the seafloor, and epipelagic, mesopelagic, bathypelagic, and abyssalpelagic in open water. The littoral zone is at the high-tide mark. Vertical scale marks photic and aphotic zones from surface to 10,000 meters. — Marine ecosystem Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.

Salmon and eels are euryhaline organisms, meaning they can tolerate a wide range of external salt concentrations as they migrate between fresh water and seawater. To manage these transitions, they must adjust their osmoregulation processes to reverse their osmotic gradients.12

When in seawater, these fish face a constant loss of water and an influx of salts. To compensate, they drink large quantities of seawater and use specialized cells in their gills—known as chloride cells—to actively pump excess salts out of their bodies. Their kidneys also play a role by excreting very little water to ensure hydration is maintained.312

Conversely, in fresh water, these fish face the opposite challenge: water tends to diffuse into their bodies, while they lose essential salts to the environment. To maintain homeostasis, they drink very little water and their kidneys produce large amounts of dilute urine to expel excess water. They also actively absorb salts from their environment through their gills and skin.123

To successfully adapt to these changing conditions, salmon and eels often spend time in brackish water, which acts as an intermediate environment.32