Asked by: Angela Cobb, Leicester
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.