Here's a breakdown:
* Anadromous: These fish are born in freshwater, migrate to the ocean to grow, and then return to freshwater to spawn.
Examples of anadromous fish:
* Salmon: The most famous example, salmon are known for their incredible journeys upstream.
* Trout (some species): Certain trout species like sea trout are anadromous.
* Eels: European eels, for example, are anadromous, spawning in the Sargasso Sea and then migrating to freshwater rivers.
* Sturgeon: Some sturgeon species, like the Atlantic sturgeon, are anadromous.
Why do they swim upstream?
* Spawning: Anadromous fish return to freshwater to lay their eggs in the environment where they were born. This is usually in the headwaters of rivers, where the water is cleaner and less polluted.
* Protection: Freshwater is often a safer place for young fish to grow and develop, away from predators in the ocean.
It's important to note: Not all fish that swim upstream are anadromous. Some species may migrate upstream for feeding purposes, while others might be simply following currents.