The fresh water kills them actually. Adult salmon are physiologically adapted to life in salt water - spending any time in a fresh water environment kills them. Their cells are unable to adjust to the change in external ionic concentration (or, more accurately, not able to adjust again as they have already made the adjustment once - in the other direction).
Also, the decomposing bodies of adult salmon assist in providing nutrients to the developing eggs in the very first stages of development.
It seems that they don’t eat the entire time they head upstream, and 90-95% of them just die of exhaustion after (or, before, if they don’t make it) spawning.
As to the adaptive benefit … a fish that doesn’t stop to eat makes it upstream faster, possibly making it less likely that it’ll be caught (by bears, people, birds) as it’s foraging for food, and also getting it up to the spawning grounds earlier so it’s genetic material has a better shot of fertilizing/being fertilized.
And once it’s taken care of business, what genetic advantage is there for it to stick around? The only reason we live so long is that our infants can’t care for themselves, and we have so few of them. If women could drop several thousand babies at once that could care for themselves within a relatively short period of time, you’d probably see us dying shortly after first spawning.
Salmon expend a lot of energy returning to their spawning grounds and many do not make it (not even counting predators). From an evolutionary standpoint this ensures only the strongest (with a fair amount of luck thrown in) reproduce. Most salmon stop eating as well once back in fresh water. When they are done with the arduous journey and spawning they have nothing left for a trip back to the ocean. That said salmon do not generally just spawn and die on the spot. They may spawn multiple times…death is near once they start spawning but it is not an instant light switch that turns out the lights the instant the fish is done.
That link I posted seems to imply that the “freshwater kills” hypothesis is not true.
To further muddy the clear mountain stream:
But that does not indicate why the survivors make it–whether they, somehow, are better able to withstand the freshwater than their cohorts, or if they just manage to have enough food reserves to get them back to sea and back to full health.
It’s a successful trait because it means that without needing to hold back reserves for future survival, they can devote more resources to the production of offspring - in practical terms, they can produce more eggs (and sperm, I suppose) sacrificing their own body tissues in the process, but they can also take bigger risks on the way upstream, because minor injuries won’t have a chance to get infected and become fatal.
Salmon aren’t the only animals on the planet to implement this sort of strategy, in fact it is rather common - some insects don’t even have functioning mouthparts in the adult form, because the sole purpose of the adult is to reproduce.
It’s also very common in plants (where the term is monocarpic - most common in annuals and biennials) - the entire energy reserve of the plant is spent on reproduction; so the parent dies, but achieves a greater chance of producing successful progeny than if it had held back some energy reserves for continued survival.
Because (according to evolutionary theory) this strategy maximizes the number of offspring they can produce during their lifetime.
Salmon put everything they’ve got into one reproductive attempt. It takes a huge amount of effort and energy for them to migrate from the sea back to the river headwaters where they were spawned. There they use up all their remaining fat and energy in making eggs and spawning. They then die because they basically have nothing left.
If an individual salmon held back enough reserves, it might be able to make it back to the sea to fatten up again and make another spawing run. However, its odds of falling victim to predators in the intervening years before it could do so is pretty high. Odds of survival are evidently sufficiently low that it’s much better to spend all they’ve got on one attempt, rather than hold something back for the slim possibility they might be able to have another go.
Many other kinds of plants and animals are what is know as “big bang” reproducers (mayflies and many other insects, century plants, bamboo) that put everything they’ve got into a single reproductive attempt and then die. At the other end of the spectrum are reproductive strategies in which the plant or animal “hedges its bets,” producing only a few young at a time but doing so repeatedly over the course of its life. Technically these are known respecitively as semelparity and* iteroparity*.
The determination of the factors that affect the use of these contrasting strategies is an active field of research in life-history strategies.
OK, it wasn’t salmon in my biology book, but, here’s what it says:
It also mentions the same thing happening in cephalopods:
“Biology–The Science of Life” (1981) Goodyear Publishing. Robert A. Wallace, University of Florida; Jack L. King, University of California, Santa Barbara; Gerald P. Sanders, Sciene writer and chief consultant, Goodyear Publishing Company
AFAIK the only mammal that dies after breeding is the carnivorous marsupial “mouse” Antechinus, in which all males die after copulation. (Females survive to care for the young and may breeed a second season.)
My mistake - I wasn’t clear I was referring specifically to Pacific salmon as opposed to Atlantic salmon which can survive to mate a second time.
Pacific salmon never survive the mating experience - even if captured on their way upstream and held in aquariums they will die within a few days.
They also begin to decompose on the trip upstream - by the time they reach the spawning beds they’re fairly far gone.
Atlantic salmon and Pacific salmon are entirely different species (Atlantic are of the family Salmo and Pacific are of the family Oncorhynchus) and correspondingly have very, very different life cycles.