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Do fish drink? © Getty Images

Do fish drink?

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Next time you take a dip in the ocean, consider what the fish are drinking and whether you are swimming in urine.

Asked by: Angela Cobb, Leicester

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Depending on where they live, fish either drink a lot or pee a lot. In the sea, a fish’s body is less salty than its surroundings, so it loses water across its skin and through its gills via osmosis. To stop themselves dehydrating, marine fish drink masses of seawater and produce a trickle of concentrated urine.

When migrating fish like trout and salmon move into rivers and lakes, they face the opposite problem and risk absorbing too much water until eventually their cells begin to swell and burst. To avoid this, they switch from being heavy drinkers to plentiful urinators.

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Authors

Dr Helen Scales is a marine biologist, broadcaster and science writer. She is the author of Spirals in Time and The Brilliant Abyss.

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