While modern Atlantic and Pacific salmons differ in many ways, they both evolved from a common ancestor. Why Pacific salmons developed the particular life strategy which they did will probably remain a mystery. I don't think the Atlantic salmon's life cycle could be described as going "back in (sic) forth between fresh water and salt year after year". Repeat spawning among Atlantic salmon occurs less frequently than it does among steelhead, about 10 percent of a typical population surviving to spawn a second time.
Along with the Pacific salmons, of course, steelhead and cutthroat are included in the genus Oncorhynchus both of which can be repeat spawners. The Japanese cherry salmon (O. masou) has also been found to occasionally survive to spawn a second time.
Atlantic salmon are so "old world." They didn't get the memo like the progressive Pacific salmon did. Steelhead (rainbow), cutts, browns, and char (all species) are old world too, at least in terms of pre-historic ancestors.
Or, there is Spaz' explation, which is kinda' appealing.
There's a gotta be a biological explanation, or is it really just one of those unanswered ecological mysteries? As I say this, it occurred to me how equally mysterious it is that they can find there way back to their natal spawning beds. No there's no real answer to this phenomenon either.
East Coast Salmon have been trying to relocate to the PNW for generations, have you seen the Hudson River? No fish in it's right mind wants to die there. Now, a nice peaceful stream on Baranof Island, if you gotta go that's the place.
If you consider all the species of animals on the planet, there are quite a few that reproduce once and then die. Given that the biological imperative is reproduction for species' success, then there isn't a biological "need" for parents that don't nurture their young to survive. Just be thankful you're a member of a species that is a repeat spawner!
Although nutrient recycling is an appealing explanation, I don't think it passes a biological or evolutinary test. I don't know.
As for migration, magnetic compassing is the best hypothesis for oceanic migration. I don't know if this is yet proven. Olfactoray sensation has been clearly demonstrated to explain freshwater migration of salmon and steelhead. They litterally "smell" their way back to their natal waters to within a matter of meters.
I spent a lot time fishing for atlantic salmon when I worked in Russia in the early 90's up in the Kola region. Also fished in Finland for them. I used to see carcasses in the river. Asked a friend, who was a a Russian fisheries biologist, if the salmon return every year to spawn as I also heard that they do not die when they spawn. He responded that the majority of atlantic salmon die when they spawn. Mostly females survive to spawn again. Do not know if this is true.
If you ever get the opportunity fish for them take it. There is still a good opportunity to catch one up in E Canada. Friend still does a DYI trip up there every year. They are a really fun fish to fish for. Especially skating a fly on top over a pool holding a large fish.
Although it has never been observed in the wild, it actually has been shown that under ideal conditions stream resident male Chinook are able to spawn more than once. The thing to remember here is that this is laboratory conditions where everything is perfect for survival and only a small percentage was able to survive. This has never been observed in nature and it is doubtable that it does occur. Just an interesting little fact.
Basically the trait of dying after spawning comes down to a list of trade offs. Long migrations into the higher reaches of streams often mean that there are less predators for juveniles to deal with, but require much more energy from adults. Pacific salmon also guard their redds until death, giving their young a greater chance of survival. Beyond this they also put more energy into their reproductive organs than multi-spawning fish like Atlantics and Steelhead. This means more and larger eggs. Larger eggs produce larger offspring, meaning that survival is higher. As has been mentioned before a larger part of this trait also has to do with the fact the streams in the Pacific Northwest area exceptionally nutrient poor and the major influx of nutrients that the salmon provide fuels the ecosystem, ensuring that their offspring will have a food source when the emerge from the gravel.
If you are looking for some good information on why Pacific salmon don't survive to spawn more than once, check out Thomas Quinn's book The Behavior and Ecology of Pacific Salmon. It is a bit heavy on the science end but is an excellent read.
I think it is very difficult to explore the Why of Salmon behavior. We can look into the advantages of certain types of behavior, but don't confuse that query with a Why. After all, maybe the salmon ARE trying to evolove into a steelhead-like life cycle. Maybe someday, one will survive, and thus begin a race of salmon repeat spawners. Maybe, just maybe, a salmon has survived, and we just haven't learned about it yet. Maybe it was a young jack chum spawning close to the ocean who got washing down after spawning, and found a way to reacclimate to the ocean, to re-smoltify.
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