Do fish drink water?
“Water everywhere and not a drop to drink.” Talking about the sea, of course. Each 100 pounds of seawater contains almost 4 pounds of salt. You can’t drink that. What do marine mammals such as dolphins, manatees, orcas, seals and whales drink?
Marine mammals do not actually drink. They get all the water they need from the fish that they eat. The fish they eat do drink the sea water – lots of it – which is processed through diffusion and osmosis.
Marine mammals in captivity that for some reason are not eating are fed freshwater from a pipe or bottle.
Freshwater fish absorb most of the water they need through their skin via osmosis, not all through their gills.
Quoting from Ocean Conservancy: As the fish opens its mouth, water runs over the gills, and blood in the capillaries picks up oxygen that’s dissolved in the water. Then the blood moves through the fish’s body to deliver the oxygen, just like in humans.
Fish gills are responsible for a number of critical functions in addition to respiration: osmoregulation, excretion of nitrogenous waste, pH regulation, and hormone production. See more at Frontiers in Marine Science.