How Do Fish Know When to Swim Upstream?

Fish use a special kind of clock inside their bodies, like a tiny, living timer that helps them know when it's time to swim upstream. This clock is set by the day and night cycle, and also by changes in water temperature. When the time comes, they just know it’s time to move.

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Examples

A fish in a river feels the water get warmer and starts swimming upstream just like it always does.
The sun sets every day, and that tells the fish it’s time to go on a journey.
When the current changes, the fish know they’re not alone, they all start moving at once.

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Categories: Biology · fish,biology,sensory perception