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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?

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

  1. Salmon have an extremely good sense of smell. One hypothesis is that they retain an imprint of their birthplace's odor, and manage to recognize it again at a later time (as explained by this article).
  2. Another hypothesis: the Earth's magnetic field guides the salmon to their birthplace via geomagnetic navigation. They then use chemical cues to recognize the stream in which they were born (supported by this scientific article).

These hypotheses were mentioned in this Wikipedia article. Note that there was a third pheromone-related hypothesis which was disproved (as shown here).

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.

TLDR: If a salmon was displaced right before birth, or birthed in a pool, I would imagine it returning to that particular location at sexual maturity.


References

  • Lohmann, K. J., N. F. Putman, and C. M. F. Lohmann. “Movement Ecology Special Feature: Geomagnetic Imprinting: A Unifying Hypothesis of Long-distance Natal Homing in Salmon and Sea Turtles.” Proceedings of the National Academy of Sciences 105, no. 49 (December 5, 2008): 19096–19101.

  • Dittman, A., and T. Quinn. “Homing in Pacific Salmon: Mechanisms and Ecological Basis.” Journal of Experimental Biology 199, no. 1 (January 1, 1996): 83–91.

  • Black, Geoff, and J. Dempson. “A Test of the Hypothesis of Pheromone Attraction in Salmonid Migration.” Environmental Biology of Fishes 15, no. 3 (1986): 229–235.

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I don't understand how those two hypothesis answer the question. They provide a mechanism for how to find their way back, but it doesn't answer if a given salmon has a genetic predisposition for going towards a certain spawn-site smell or if it remembers it from its' hatching. Both mechanisms could be consistent with either of the OPs interpretations (although the memory version makes more sense), can you explain why you think both these mechanisms operate interdependent of memory or genetic conditioning? –  Artem Kaznatcheev Aug 10 '12 at 23:18
    
Hmm, I can see what you mean. I've edited the answer slightly. Please let me know if I've solved the problem or what else can be changed. :) –  LanceLafontaine Aug 10 '12 at 23:50
    
It would be rather difficult for a released salmon to return to its birthing fish farm. :-) –  Paul A. Clayton Aug 10 '13 at 13:12

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