Why Do Salmon Die After Spawning? Atlantic vs Pacific Salmon

Why Pacific salmon die: spawning once and for all

Biologists call animals that breed only once and then die semelparous, from Latin words meaning “once” and “to give birth”. Five species of Pacific salmon in North America (sockeye, Chinook, coho, chum and pink) and the masu salmon of Asia follow this pattern. Every adult that reaches the spawning grounds will die there.

This “big bang” approach makes sense when the journey is very hard. Many Pacific salmon swim hundreds or even more than a thousand kilometers upstream and climb high into the mountains. After such a trip, the chance of getting back to sea and surviving to spawn again is tiny. Natural selection has favored fish that put everything into one breeding season, producing large eggs and fighting hard for the best nest sites, instead of holding energy back for a future that probably will not come.

What happens inside a spawning salmon?

The process starts long before spawning. Salmon stop feeding when they leave the sea, so the journey, the growth of eggs and sperm and the fights over mates all run on reserves built up in the ocean. Sockeye that migrate far inland can use up most of their body fat and a large share of their muscle protein.

At the same time, their bodies change shape and color. Pacific salmon turn red, green, purple or dark brown, males grow hooked jaws and, in sockeye and pink salmon, a humped back. Their stomachs and intestines shrink, because they will never feed again.

Then comes a final collapse. Levels of the stress hormone cortisol rise very high. This helps release the last energy reserves, but it also breaks down tissues, weakens the immune system and damages the kidneys, heart and other organs. Wounds stop healing and fungus spreads over the skin. Within days or weeks of spawning, the fish die.

Experiments in the mid-20th century showed how closely this is tied to breeding. Kokanee, a lake-living form of sockeye, that were prevented from maturing lived well past the age at which their siblings spawned and died.

Why many Atlantic salmon survive

The Atlantic salmon is iteroparous, meaning it can breed more than once. Its rivers are usually shorter and lower than the great Pacific salmon rivers, so the journey costs less energy, and its body does not go through the same programmed shutdown.

Survival is still hard. Spawning salmon lose a great deal of weight, and many die from exhaustion, injury or disease during the winter. The survivors, called kelts, are thin and dark. Some drift back to sea within weeks, while others stay in the river until spring. Once in salt water, kelts start feeding again and can regain their condition surprisingly quickly.

Repeat spawners usually make up only a small part of a run, often under 10%, though some rivers have more. Most repeat spawners breed just twice, and a very few manage three or more times. Females survive more often than males, which spend longer on the spawning grounds fighting rivals. A few other salmon relatives, including steelhead and sea trout, can also spawn more than once.

How dead salmon feed rivers and forests

Salmon carcasses are not wasted. Each fish carries nitrogen, phosphorus and carbon gathered at sea far inland, where these nutrients are often scarce. As the bodies break down, they fertilize the stream and its banks.

The benefits spread through the whole ecosystem. Insects and other invertebrates feed on the carcasses, and young salmon then feed on those insects. Bears, eagles, gulls, ravens, otters and many other animals feast on spawning and dead fish. In Alaska, thousands of bald eagles gather each autumn along the Chilkat River to feed on late runs of chum salmon.

Bears carry many salmon into the forest to eat, leaving remains among the trees. Studies in Alaska have found that close to a quarter of the nitrogen in streamside trees can come from salmon, and some trees grow faster near salmon streams. Fish are such an important seasonal food that, for brown bears along the North Pacific coast, a good salmon run makes a big difference to how much fat they gain before winter, as explained in What do brown bears eat?

Is it the journey upstream that kills salmon?

Not on its own. The journey drains salmon, but Pacific salmon die even when they spawn close to the sea, and pink and chum salmon often spawn just a few kilometers inland. What kills them is the combination of fasting, breeding and a built-in aging program that switches on once they mature.

Atlantic salmon show that another path is possible. Their life story, with returns, rests at sea and second runs, is described in Why do salmon swim upstream?