Big Drinkers

The guy who came up with the expression “drinking like a fish” knew what he was talking about. Although it sounds a little kooky, saltwater fish have to guzzle saltwater to help them survive in a saltwater environment.

A fish has to maintain the proper amount of sodium, chloride, and other dissolved minerals — the “salt” in saltwater — in its tissues.

But the water around the fish doesn’t want to cooperate. It tries to even out the distribution of freshwater and saltwater by passing through a fish’s skin and gills.

For a freshwater fish, water enters the body, so there’s no need for the fish to drink. In fact, it’s just the opposite: the fish has to urinate often and in copious amounts to get rid of the water and keep it from puffing up like a balloon — up to a third of its body weight every day.

But a bony saltwater fish — like tuna, cod, or halibut — urinates very little. Instead, it must take in water. Its body fluids contain a lower concentration of salts than the water around it, so the fluids pass outward through the skin and gills. To make up for the loss, the fish must drink a lot of water — up to about a quarter of its body weight per day. To take in a comparable amount of water, a 150-pound person would have to drink more than 80 glasses a day.

By taking in so much water, the fish also takes in a lot of salt. Special glands in its gills pump that excess salt back out.

So for a fish, “drinking like a fish” isn’t a bad thing — it’s a matter of survival.