How Does The Salmon Solve Its Osmoregulatory Problems?
Fortunately, the salmon has some remarkable adaptations, both behavioral and physiological, that allow it to thrive in both fresh and salt water habitats. To offset the dehydrating effects of salt water, the salmon drinks copiously (several litres per day). But in fresh water (where water loading is the problem) the salmon doesn’t drink at all. The only water it consumes is that which necessarily goes down its gullet when it feeds. Of course, when an ocean-dwelling salmon drinks, it takes in a lot of NaCl, which exacerbates the salt-loading problem. Kidney function also differs between the two habitats. In fresh water, the salmon’s kidneys produce large volumes of dilute urine (to cope with all of the water that’s diffusing into the salmon’s body fluids), while in the ocean environment, the kidneys’ urine production rates drop dramatically and the urine is as concentrated as the kidneys can make it. The result of this is that the salmon is using relatively little water to get rid of all of the excess ions it can.