How Does a Fish Know When to Leave the Ocean and Migrate to Rivers?

Fish use special senses to know when it's time to move. Like a fish with a super-smell, they can feel changes in the water and smell chemicals that tell them the river is near. When these signs change, they get excited and start their journey inland.

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Examples

A fish smells a river nearby and starts swimming up it like it's following a trail.
A group of fish feel the water getting less salty and know they are close to their home.
When a fish sees other fish starting their journey, it joins in too.

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Categories: Biology · fish migration,biology,sensory perception