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Why do salmon die?

dead_salmon_in_spawning_season1

A spawned out salmon/Peter Forsyth, Wiki Strategies

 

This is the season of death along the Alaska coast. In rivers, creeks and streams from near the Arctic Circle to the southern tip of the Inside Passage, salmon are dying or already dead by the millions.

 

No one knows exactly why they die , although there are indications the fish might be trying to tell us something about the dangers of stress.

The death of the salmon is natural enough. Everyone knows that. Salmon have been returning to their natal streams to spawn and die since before the first humans set foot in Alaska.

But the mechanism of predetermined death is still not fully understood.

No one call really answer a fundamentally simple question: Why exactly are Pacific salmon destined to their fate?

The old belief was that the fish starved to death because they stopped feeding when they entered fresh water. But the case is clearly not that simple as illustrated by at least two  experiments in which scientists have rehabilitated spawning salmon.

In one case, sockeye that returned to a Canadian river “were held in captivity in fresh water without spawning well beyond the time when they would normally have done so and died.” Feeding was then begun. Some of the salmon survived.

In a later experiment in Oregon, Chinook salmon were spawned, “reconditioned” and spawned again a year later. 

Given the ability of humans to save spawned-out Pacific salmon by artificial means, it is rather baffling that none of them have ever been documented to survive spawning in their natural environment, although small numbers of both Atlantic salmon and steelhead trout, a cousin of the salmon, do survive spawning to do it again.

The stress question

Could it be that some Atlantic salmon and steelhead are just better at dealing with stress than their compatriots and their Pacific salmon relatives? The idea has been indirectly floated by a few scientists.

“The die-off of Pacific salmon shortly after they spawn is one of the most dramatic but poorly understood phenomena in biology,” Terence Barry of the University of Wisconsin and colleagues observed in a 2012 paper published in the Journal of General and Comparative Endocrinology. “Cortisol excess of unknown etiology mediates this programmed death by causing tissue degeneration, suppressing the immune system, and impairing various homeostatic mechanisms.”

The jargon there might be a little hard to wrap your head around, but that doesn’t matter. The key word is a simple one “cortisol.”

Cortisol is the so-called “stress hormone.” “Psychology Today” magazine has labeled it “public health enemy number one” among humans.

“Scientists have known for years that elevated cortisol levels interfere with learning and memory, lower immune function and bone density, increase weight gain, blood pressure, cholesterol, heart disease… The list goes on and on,” Christopher Bergland writes there. 

“Chronic stress and elevated cortisol levels also increase risk for depression, mental illness, and lower life expectancy. ”

A salmon art form

Pacific salmon appear to have the lower-life expectancy issue nailed. They make it back to the stream of their birth for one shot at sex, and then they die. Some scientists have suggested the salmon are somehow cellularly programmed for this fate.

That’s a difficult hypothesis to prove.

What can be shown is that they suffer from a cortisol build up that proves deadly. It is not impossible it is related purely to the stress of spawning which starts with the difficult, physiological process of adapting from a saltwater organism to a freshwater organism before making a challenging run to the spawning ground through gangs of predators – whales, sea lions, seals, fishermen, bears and predatory birds of several sorts – only to battle for mates before the exertion of spawning.

Barry and his colleagues have theorized that something in this activity triggers production of an enzyme that blocks another “gatekeeper” enzyme that normally regulates cortisol metabolism to protectthe fish from dangerous cortisol buildups.

“Cortisol excess that is not associated with spawning (e.g., during stress) does not result in tissue degeneration and death, and (2) the death of salmon only occurs after spawning,” they write.

Yes, it’s complicated, as much in nature once you get past the fundamentally simple:

Salmon spawn and die and their carcasses fertilize the waters in which their young will grow.

The strategy works great for them as a species. It might not work so well for humans who are generally advised to try to moderate their coritsol production. The Mayo Clinic offers a whole list of ways to help do that and adds that “the payoff for learning to manage stress is peace of mind and — perhaps — a longer, healthier life.”

 

 

 

 

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