I would think that salmon repopulating rivers after ice ages would suggest that not all salmon return to their native river. Dave or Steve?
Found this one
http://www.dfo-mpo.gc.ca/science/publications/uww-msm/articles/pacificsalmon-saumonpacifique-eng.htm
I was helping tag coho last year on the island....
A number of the coho I tagged in the Englishman (quite a ways up too ~15-20 km), ended up in rivers like the Chase or Millstone. We were also catching a tonne of hatchery fish, which were most likely Little Q or Big Q strays.
In short, the fish don't need to be native to the river to go up it. They will simply join a school and head up the river. Some may go down like the ones I tagged that ended up 50+ km away, but I'm sure closer to spawning a bunch stay.
Another food for thought.... if you go look at a lot of rivers with "record returns," I'd say at the mouth of each river there is a slough of 100,000 pinks sitting there, not really doing anything other than colouring up. I'd say you can't use the low water excuse (personally), just because in those same rivers, fresh coho are shooting past the pinks right up into the system (plus a number of the pinks are already in as well). Maybe the pinks are just lost and can't bounce around any more due to their condition, so they'll just spawn in the estuaries.
Can\t wait to see what happens with coho fishing this year.. I have a feeling it'll be a treat