So, when a fish is in salt water, it’s kidneys have to work hard to save water and eliminate all the extra salt.. When it freshwater, they have to work hard to save salt and get rid of excess water. Both are functions of the kidney, but are done in different ways. If a fish moves into salt from fresh, or fresh from salt, it basically has to switch polarity on it’s kidneys. This is very stressful, and not instantaneous.

So most fish, wait around in the estuary where fresh and salt mix, and it’s easier for their kidneys to switch. But salmon, oh no, they just keep heading up stream. So their Cortisol (stress hormone) spikes. Spikes up high enough that it shuts down their immune systems, as well their appetite and many other bodily systems. The fish get so stress, that they are stuck in a positive feedback loop, until they permanently burnout their ability to respond to cortisol.

So they become immune to stress at that point. Sounds nice, right? Except their immune system is permanently shot, at their body begins to break down. Often attacked by fungus at this point, that gives some of them the appearance that they are rotting while alive.

Around 5% of Atlantic salmon (genus Salmo) adults are able to avoid the burn out, and break free of the cycle. Swimming back out to sea and living on.

That depends on what you call a salmon.

The five widely recognized species of Pacific salmon: Chinook(King), Coho(Silver), Sockeye(Red), Chum(Dog) and Pink all die shortly after they spawn.

Steelhead, which are genetically identical to a rainbow trout, just one that migrated into saltwater are often considered salmon, and they can spawn multiply times, returning to the ocean each time, the same applies to Cutthroat Trout, Arctic Char and Dolly Varden. These species are Salmonidae that breed in fresh water and feed in the ocean, so they are often considered salmon.

Atlantic salmon also have the ability to spawn multiple times, however that rarely happens.

The fact that salmon die after spawning is very important to coastal ecosystems. If you look at salmon as a species, really what they do is take nutrients from the ocean and bring them into rivers and streams. All the body mass they accumulated through years of feeding in the ocean now gets eaten by wildlife, or decomposes in streams, feeding fish, birds and rodents, as well as future generations of salmon in the beginning of their life cycle. Bears often only eat parts of the salmon and throw the rest into the woods. This is often seen as waste but also serves a very important purpose. These rotting fish fertilize the trees close to the rivers and streams, giving them the ability to grow bigger root networks, which in turn prevent the gravel beds on which salmon spawn from getting silted in. Irresponsible logging practices from the past have damaged much of the salmon’s traditional spawning grounds, resulting in fewer fish being able to spawn and contributing to the decline of salmon stocks.

This is going to be rather lengthy to give an adequate explanation.

Atlantic and Pacific salmon belong to the family Salmonidae, but Pacific salmon belong to the genus Oncorhynchus while Atlantic salmon (Salmo salar) are the largest members of the genus Salmo. Atlantic salmon are more closely related to certain species of trout, such as brown trout (Salmo trutta) than they are to Pacific salmon. Atlantic salmon have large, black spots on their gill covers and back.

The lifecycles of natural populations of Atlantic and Pacific salmon are similar. Atlantic salmon spend one to four years in the ocean before returning to spawn in the freshwater stream where they were born. One big difference, though, is Atlantic salmon don’t always die after they spawn. Pacific salmon are semelparous, meaning they die after they spawn. Atlantic salmon are iteroparous which means they might recover, return to the sea, and repeat the migration and spawning pattern. Spawning takes a huge physiological toll on a salmon, though, and most Atlantic salmon do not survive to spawn a second or third time.

The keywords here are semelparous and iteroparous,

Semelparity and iteroparity are two classes of possible reproductive strategies available to living organisms. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime.

In truly semelparous species, death after reproduction is part of an overall strategy that includes putting all available resources into maximizing reproduction, at the expense of future life.

A classic example of a semelparous organism is Pacific salmon (Oncorhynchus spp.), which lives for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and dying.

Reproduction is costly for Pacific salmon because their life history requires transition from saltwater to freshwater streams and long migrations, which can be physiologically taxing. The transition between cold oceanic water to warm freshwater and steep elevation changes in Northern Pacific rivers could explain the evolution of semelparity because it would be extremely difficult to return back to the ocean. A noticeable difference between semelparous fish and iteroparous salmonids is that egg size varies between the two types of reproductive strategies. Egg number, however shows little variation between semelparous and iteroparous populations.

Pacific salmon

Highly elevated cortisol levels dictate the post-spawning death of semelparous Oncorhynchus Pacific salmon by causing tissue degeneration, suppressing the immune system, and impairing various homeostatic mechanisms.

[2]

After swimming for such a long distance, salmon expend all of their energy on reproduction. One of the key factors in the Pacific salmon’s death is that these fish do not feed during reproduction so body weight is extremely reduced.

In addition to physiological degradation, pacific salmon become more lethargic as mating goes on, which makes some individuals more susceptible to predation because they have less energy to avoid predators. This also increases mortality rates of adults post-mating.

The current hypothesis behind this reason is that iteroparous species reduce the size of their eggs in order to improve the mother's chances survival, since she invests less energy in gamete formation. Semelparous species do not expect to live past one mating season, so females invest a lot more energy in gamete formation resulting in large eggs.

In any iteroparous population there will be some individuals who die between their first and second reproductive episodes, but unless this is part of a syndrome of programmed death after reproduction, this would not be called semelparity.

This distinction is also related to the difference between annual and perennial plants. An annual is a plant that completes its life cycle in a single season, and is usually semelparous. Perennials live for more than one season and are usually (but not always) iteroparous.

Semelparity and iteroparity are not, strictly speaking, alternative strategies, but extremes along a continuum of possible modes of reproduction. Many organisms considered to be semelparous can, under certain conditions, separate their single bout of reproduction into two or more episodes.

Costs of reproduction can explain the correlated evolution of semelparity and egg size: theory and a test with salmon.

This last link (below) gives the trade offs to each method of reproduction.

Semelparity and iteroparity

Footnotes

They hang up their boots. During September British Columbia is rather smelly.

If you are a bald eagle, a crow, a bear or a seagull this looks yummy.

Salmon begin to deteriorate as soon as they enter fresh water. Even before they spawn, many will be unfit to eat. All of them, but particularly the males, undergo a rapid transformation from sleek, silver, and fatty, to hump-backed, hook-jawed, fanged and discolored.

There’s a metaphor in there somewhere for males with only spawning on their mind, but I’ll let it rest.

For example, consider the humble Pink Salmon (also called Humpy)

This:

Becomes this:

Becomes this:

Appetizing, no? Yes, he is (was) still alive.

They get so rotted they can barely swim, and you can catch them easily by hand.

Whoever can create a market for hand-harvested wild Alaskan Salmon pudding will have no shortage of supply, but demand may be a problem…

Worry not-none of them go to waste. They provide a flood of nutrients benefiting the entire local ecosystem.

Imagine you live in your apartment, and your wife stays at home and cooks for you and has sex with you, and you are happy, life is good. In the morning you travel to your office, one mile away, and return after work. Let’s suppose you love your job and you love your wife.

Now imagine that the distance gradually increases between your home and office. Last year it was 1 mile, now it’s 1.5 miles. Will you quit your job that you like? Probably not. Nor will you quit when it becomes 2 or 5 or 18 miles.

That’s how I believe many such events started, for instance birds migrating thousands of miles. As continents moved, distances increased, and those that were able kept making the journey even when it became ridiculously long.

Salmon are driven to spawn in the lakes… if conditions change over time and they are drawn further and further away, they still have it coded in their DNA to do what their ancestors did… spawn in the lakes. Those that are able, will return and reproduce. Those not able will die off, thus evolution favors those that are capable of the long journey.

They are different species (different genus actually) within the same family. Atlantic salmon (Salmo solar) are more closely related to brown trout (Salmo trutta) than they are to Pacific salmon of the Oncorhynchus genus. You could also include white fish, which are another fish in the Salmonidae family, in your question.

I believe in the theory of natural selection. Thus, I believe at some point in the past, there was an evolutionary advantage to Pacific salmon that was greater than the evolutionary advantage of living for the chance to survive to spawn again. Through natural selection several species evolved to die when they spawned because those fish were more likely to pass on their genetics than the ones who lived.

For Atlantic salmon, the chance to spawn more than once provided a great chance to pass on their genetics. Thus, they evolved to survive after spawning.

The native habitats of Pacific and Atlantic salmon are quite different. Pacific salmon spawn in rivers flowing through rugged mountain ranges on the west coast of North America. Altantic spawn in much flatter regions of Europe and the east coast of North America. These differing habitats can explain why it is advantages for one to survive and another to die after spawning. E.g. the dead carcasses of Pacific salmon have been found to provide up to 90% of the nutrients in some rivers. This would not likely be the case for the more fertile rivers of Europe even if the salmon did die.

Pacific salmon have a life style with a reproductive strategy called “semelparous” . That means that an entire generation dies after one mating. The advantage is that they give all their effort into producing offspring so these have the maximum chance themselves at survival. Atlantic salmon, on the other hand, are “iteroparous” and can go through multiple reproductive cycles before dying. This has the advantage that any disadvantage of the offspring from a single reproductive effort is compensated by repeated reproductive cycles. In the end, the total number of offspring that themselves survive to reproduction is about the same.

Perhaps the best example of semelparity is the “century plant”, an agave that lives and grows for many decades, though not necessarily 100 years, and flowers and produces seeds once. Then it quickly dies.

The strategy may seem inefficient or ineffective to us but it really works and an awful lot of plants and animals do that.

There are two species of salmon in the Pacific northwest: northern California, Oregon, Washington, British Columbia, and Alaska. Both species hatch in the spring, and as tiny tots tumble down hundreds of creeks and streams to the ocean, where they stay and grow for 4 years. In the fourth spring, each fish struggles upstream to the places it hatched, and proceeds to lay or fertilize a mass of eggs. All the fish in one species then heads back down to the ocean, returning each of the next 3 to 5 springs. The other species finished laying/fertilizing, exactly where they hatched, and then dies immediately. No second, third, or fourth chances to reproduce. This die-off does nothing for the species. They reproduce once and die. There's something in the DNA that greatly reduces the success of this species; there is no benefit, because newly hatched small fry don't compete with their parents for food, shelter, mates, etc. The other species reproduces for several more years. So only one of the two species dies off immediately. Humans catch salmon in the ocean; I'm not sure what season, but it's not summer, because that's when the boats got repaired and refurbished. I know the Elvekema--our salmon fishing boat--which we had just sold to the guys who usually rented it for the season--went down in the 1969 Alaskan earthquake about December. I find salmon to be a very interesting example of species diversity.

The Pacific salmon die on their own after spawning and the Atlantic salmon, which return back to the sea, have become lean and lanky with very little else to eat but bones.

Atlantic salmon (Salmo salar) before and after spawning.

Atlantic salmon can spawn up to four times during their lives. Pacific salmon spawn just once and die after spawning.

The gist of it is, as they transition in freshwater, from salt water, anadromous fish such as slamon and lamprey because are extremely stressed. They essentially have to reverse how their kidneys function, and put them into overdrive. During this release of cortisol, the stress hormone, their bodies become resistant to it. Eventually, they cease responding to it entirely.

While stress sounds bad, we need some to function. Cortisol is a key hormone is the regulation of many of your bodily functions, including hunger, and indirectly, cell division. When salmon and lamprey cease to respond to cortisol, they cease to eat, and as their energy runs low, their ability to replace cells fades, as well as their ability to keep existing tissue alive. It’s why many spawning salmon have white patches, or chunks. Spots where they have no skin left, and their fat cells are showing through.

But some salmon and lamprey are lucky enough to survive to come back again, around 3–5% in Atlantic salmon. Of course, their are also a handful of males that mature as parr, spawn via sneaking, then go out to sea and come back to do it again a second time.

tldr: They die from stress induced starvation. The cessation of responding to cortisol in salmon is similar to Cushing Syndrome in humsn.

Not very long after salmon hatch, they begin their journey downstream to the ocean where the they feed and grow for a few years. It's at the end of their lives when the instinct to spawn makes them swim back to the spot where they were born. They are old now and they stop eating while on their journey to spawn. Once they reach their destination, they're weakened. They lay and fertilize their eggs. Once done, they continue not to feed instinctively knowing their purpose is done. Since they've already gotten to a weakened state, they die very soon after they've spawned giving way to the new generation.

Well, you have to remember that the “first” breeding season isn’t the first year of life. Depending on the specific species, they can spend several years, often 4–5 on average, out in the ocean before they come back to spawn.

At that point they do die off though, as they spend all their energy moving back up into freshwater to find their spawning grounds, often including stopping eating altogether once they enter freshwater.

That’s one of the reasons salmon are so critical to ecosystems, that mass of dying fish helps provide a large amount of food to other creatures, as well as adding many nutrients to the waters in which they spawn.

Have you ever read ‘The Selfish Gene’ by Richard Dawkins?

The Salmon fish are a good example of what you’re question is about; they will use all their energy to swim upstream of an energetic river just to spawn and die.

But this is, in a way, a perfect system for reproduction of a species; by spending their lives building up enough size and energy to return to the spawning grounds, the Salmons maximise their lives without having the burden of looking after offspring, then at the end of their useful lives they can create the next generation without becoming a burden for their offspring.

(Humans could learn a lot from this lesson).

But I believe the system has evolved so that only the biggest, strongest Salmon are likely to survive the arduous journey up the river, which in turn means that only the genes best suited to their environment are likely to be passed onto the next generation.

The genes don’t care if an individual dies, individuals always die.

The genes of the fittest and luckiest can be immortal…

The forests of North Western America are fertilised by the bodies of dead salmon.

Predators like bear, eagle, wolf, and lots of smaller ones, will take the dead bodies, find some shelter and eat what they want. Sometimes this is hundreds of metres into the forest. What is left rots into the soil, consumed by insects, etc. Sometimes the number of dead salmon is so huge that the predator might just eat the tasty bit, leave the rest and get another one.

So predators disperse the bodies of dead salmon into the forest, finishing a lifecycle that began in the very streams and rivers that flow though the forest.

You may be able to prolong the life of a spawned out pacific salmon with expert care, though it may not feel you’re doing it any favors. It has reached the end of its life, and all of its major systems (digestion, respiration, etc.) are shutting down. In Pacific salmon at least these are not reversible (to my knowledge).

I’ve theorized that there used to be King salmon that were able to survive and return, because we used to catch much larger fish. But, I expect the decreasing size is due more to changes in the ecosystem (predation, food scarcity, etc) than my fanciful subspecies of immortal King Salmon that survive spawning and grow to immense sizes.

However, I reserve the right to make up whatever I like when it comes to fish stories. It’s the sacred right of all fishermen.

On the Atlantic Coast , the hen Salmon spend the winter months in fresh water , and then follow the current downstream to the ocean in the heavy rains of Spring.

There , they might spend a year or three at sea , feeding on krill, ( which is why their flesh is red. They pick up pigments from the shrimp and other crustaceans) , and baitfish , like smelt or capelin , before they make another journey to the river they were born in. They might make many trips over their life span. So , yes , they transition to salt water. However , it is not fresh water , then suddenly salt. Salmon spend time adjusting , both ways, from salt to fresh and fresh to salt , by staying in the river estuary where the water is a mixture of both. After a time , not long , they can move either to the sea ,if they are returning, or upstream to fresh. So , yes they can move from fresh to salt , but they require certain conditions to adapt.

There are a couple of reasons why you would not want to eat a salmon after it has spawned. You could actually eat one but I don’t think that you would enjoy yourself.

Atlantic salmon are a little different than Pacific but I believe that the reasons why not to eat one are similar. My experience is with pacific salmon so I’ll answer using them for an example.

Salmon reach sexual maturity and instinctually begin to migrate back to their home river. Many Salmon swim from the gulf of Alaska to as far south as Northern California to accomplish this. Somewhere along the journey south, typically a short time before they enter freshwater they stop eating. This means that they are expending an enormous amount of energy both swimming long distances and finishing the process of growing eggs or milt (fish sperm) this means that they are relying on their reserves as their only source of energy. By the time salmon have made that journey and successfully spawned there is very little nutritional value in their meat. It is mushy, discolored and if the darker pre-spawn fish I have eaten is any indication, it’s got to taste horrible.

If you have ever seen a spawned out salmon slowly succumbing to the end of its life you wouldn’t have asked the question at all. They are dying. The strain of the journey, the change from saltwater to freshwater and the competition for the best mate or mates have taken their toll. What you see is a fungus and parasite ridden, physically altered, horrific version of the once majestic fish. What is left is ideal for transferring nutrients to insects and plants but has little value and is completely unappealing for human consumption.

The fish Salmon is anadromous and travels from sea to higher reaches of the river for breeding. It does not feed during its breeding migration and as such is tired and exhausted by the time it reaches its breeding ground. So it dies after mating and laying down eggs. However, not all Salmon individuals die, some of them survive and swim back down to the sea and return to the same breeding ground at the higher reaches of the river next year.

The European Eel is Katadromous and travels from the rivers to sea for breeding at the distance of almost 3000 kilometers near Bermuda sea. This fish also does not feed during its breeding migration and after breeding, the breeders die. Here, the return of any breeder to the European rivers is not recorded. The death of breeders after breeding is again because of strain , stress and starvation making them extremely weak.

Salmon on the return trip to the spawning grounds of their birth turn into “zombie fish”. They basically start to rot and die before they actually do so. This happens as soon as the salmon returns back to fresh water. Salmon (Pacific) are semelparous which means they breed only once. Salmon put everything they have into that breeding. There is no second chances. The eggs and sperm take nutrients from the muscles, skin and organs. If you look at salmon eggs they are big and give the developing young a massive amount of energy to grow giving them the best start in life. Feeding stops and the entire digestive system stops using energy. Immunity goes way down and the fish doesn’t even attempt to stop infections. All of this takes energy and the fish cuts all energy usage down to zero except gonad preservation and muscle contraction for swimming. All energy is spent fighting its way up stream to get back to the spawning grounds and competing for mates. Nothing is left on the table. Its breed or die trying.

There are 2 species of salmon on the coast northern California, Oregon, Washington, British Columbia, and Alaska. Both hatch up the hundreds of little creeks and streams along the coast, and come tumbling down to live in the Pacific to grow for four years. Then they swim up to the exact spot where they hatched and spawn. One group swims back down to the ocean, returning each spring for the next several years. The other species spawns and immediately dies. Right there where they hatched. Shortly thereafter, the new generations hatch and tumble down to the sea for 4 years. There is no benefit to the deaths of the one species. It's just a genetic flaw. Neither is landlocked.

You would think that something as powerful as a Pacific Salmon might live for sometime after spawning but that just isn’t the case. Every Salmon, depending on it’s particular species, after being born starts its journey to the sea and returns in 2–6 years to spawn. This journey brings them from the main body of the ocean on a trip to the exact spot they were born where they do the same thing as their parents did, they spawn and will survive for some days to perhaps a couple of weeks then die. They fight their way up rivers, creeks, waterfalls which all require enormous energy resources, they have to fight past hundreds of hungry bears which feed on them by the thousands, fishermen, otters, too etc. Once in their respective spawning grounds the females dig a hole in the gravel, sand, and lay their eggs the males release their sperm over these eggs and the process begins again as it has for millions of years. There is some evidence that the females may remain for up to 2 weeks guarding the nests. Their longevity after all this depends on how beaten up they are from the very difficult journey to the spawning grounds and their particular species but none of them lasts very long after this. This is merely a general outline of their life and death cycle I hope it is useful to you.

No. Even the fortunate few Salmon that survive the trip, and actually spawn, die soon afterwards.

Q: “Why do salmon return to the streams they were born in to spawn?”

When any creature is about to give birth for the first time, it has to decide just where that will take place.

Animals can’t “reason” about it the way that humans do, so they have to take their chances on a throw of the dice.
They don’t know what the best place to give birth is, and they can’t figure it out because their brains aren’t good enough.
But they know one thing anyway:
They know that the place they themselves were born at least was good enough to keep them alive themselves, so it is the best place they know for keeping their own babies alive.

So of all the fresh-water streams pouring into all the oceans of all the world, there is exactly one that is known to the expectant parents to be suitable for spawning.
So they go there.

Many animals return to the place they were born to give birth themselves.
That’s how colonies and flocks get established.
Some plants invest energy in seed dispersal, but others let their seeds fall where the parent has found a good place to live.

I read that human females show a tendency to return to the place of their birth as the “right” place to live after marriage, but I can’t find the citation.