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Paul Hachey, age 11, of Fredericton, N.B., Canada, for his question:

DO FISH DRINK SALT WATER?

The underwater world of the fishes is strange and wonderful, full of rippling sunbeams and shadows. The world's fresh and salt waters are populated with more than 30,000 different fishes  and all of them have their half hidden secrets.

We tend to regard the average fish as an alien creature, not at all like ourselves. We live in different worlds, dry and wet. But fishes and people are alike in several important ways. For example, our bodies need food and supplies of water to stay alive. Fish need a variety of plant and animal food, as we do. Fishes need drinking water, as we do.

This is, of course, no water shortage in the world of fishes. But there are other problems. Basically, water is needed to maintain a proper balance of chemicals in the body fluids. These fluids are somewhat like diluted sea water  and they must be kept just strong enough without being too strong.

The sea is getting saltier all the time, and billions of years ago its concentration of salty chemicals was much weaker. It was about the same as the concentration of chemicals in the body fluids of a modern fish. Today, the salty sea water could cause a fish to shrivel up like a prune. On the other hand, fresh water could cause him to swell up like a balloon.

Both salt  and freshwater fishes need to drink water ¬but they are faced with different problems. A freshwater fish needs to drink very little water, and most of it goes to his kidneys where it helps to wash out his body wastes.

A saltwater fish has a much bigger problem. He must drink lots of water  which is much too salty. The surplus salt must be sifted out and prevented from upsetting the chemical balance of his body fluids. To cope with the problem, he has his own built in desalting system. Some of the surplus salts are sent outside through special cells in his gills. Some pass through his digestive tract, where they are sifted and eliminated with his body wastes.

As he swims along, a fish seems to be gulping great mouthfuls of water all the time. Actually this water washes through his gills, which take out dissolved oxygen. Only a small amount becomes drinking water, and more drinking water may be absorbed through the fish's skin.

 

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