To spawn, salmon return to the river in which they were born. Is it because the

Question

To spawn, salmon return to the river in which they were born. Is it because the individuals remember the specific river, or because populations are genetically conditioned to follow specific environmental cues that guide them to the specific river?

In other words, if a salmon is artificially taken to lay eggs in a different, ("wrong") river, will its progeny return, when grown, to the river where they were born, or that where their mother was born?

If salmon that was grown in a fish-farm is taken to the sea, does it return to a river? How does it choose which?

Explanation / Answer

The mechanism for salmon natal homing isn't exactly known, but there are really two good hypotheses out there.

I would say that both memory and "genetic conditioning" could potentially be factors in how these animals perform this natal homing, among other things. However, it is difficult to pinpoint which one if any, because neither hypothesis has yet to be confirmed, and both function on completely different mechanisms. Not to mention that we can't tell what exactly a fish is thinking, and also what chemical we're looking for, if there is one.

Finally, for your personal information, salmon aren't the only animals to do this: sea turtles return to their birthplace as well.

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