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.
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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See also
- How Do Fish Know When to Migrate?
- How Do Fish Know Which Way to Swim?
- How Do Fish Sleep Without Closing Their Eyes?
- How Do Fish Sleep Without Going to Bed?
- How Do Fish Sleep Without Falling to the Bottom of the Ocean?
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