Do fish drink

Asked Mother recently.

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To find out why the sea is salty, click here.

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Q.� Why do fresh and marine fish drink differing amounts

A.� For fish, drinking is a means of balancing the amount of salt in their body with the amount in the water that they live in, water naturally diffuses from the body across the skin, as though attempting to dilute the outside sea.

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Fish have quite leaky skin in comparison to our relatively water tight skin. As a result marine fish loose a lot of fresh water to the sea, while the salt is excreted via the gills. But as they need the water for normal bodily functions, it has to be replaced and so most marine fishes drink constantly.

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In contrast this exchange isn't necessary for fresh water fish as they have more salt in their bodies than in the surrounding water: they don't need to replace water that hasn't been lost.

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Q.� Do fish drink with their mouths

A.� Both types of fish take in water through their mouths, using their gills to trap oxygen and carbon dioxide, however this isn't officially drinking, it's breathing.

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Q.� What about when fresh water fish migrate to salt water

A.� These kinds of fish, called anadromous fish, are born in fresh water, migrate to the salty sea and then back to fresh water to spawn their eggs. This whole process requires physiological adaptations to cope with the switch from coping with the changes in saltiness.

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Anadromous fish need to spend time in the estuary to give their bodies chance to adjust to the increase in salt in the water.

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by Lisa Cardy