Five species of Pacific salmon return each summer to breed in Alaskan streams, rivers, and lakes. I will admit salmon are confusing fish. Not only do they have some of nature’s most complicated life cycles, but each species of Pacific salmon is known by two different common names. Are Atlantic and Pacific salmon the same fish? Which species of Pacific salmon is worth the most money to commercial fishermen? Do the various species taste different? Which species of salmon freezes the best? Why do some species grow larger than others, and why do salmon jump when they return to spawn?
Over the next few weeks, I hope to answer the above questions while I profile each of the five species. In this post, I will give you a generic overview of salmon and describe the life cycle of a Pacific salmon.
Pacific salmon and Atlantic salmon belong to the same family but not to the same genus. Atlantic salmon are more closely related to some species of trout than they are to Pacific salmon. One big difference between Pacific and Atlantic salmon is Pacific salmon only breed once, and then they die. Atlantic salmon return to freshwater to breed many times before they die.
The five species of Pacific salmon are pink salmon, also known as humpies; chum salmon, also known as dog salmon; red salmon, also known as sockeyes; silver salmon, also known as coho; and king salmon, also known as chinook. The five species look very similar to each other in their marine ocean phase, but once they enter fresh water, salmon go through significant physical changes, and each species has distinctive markings.
Fertilized salmon eggs incubate in the gravel of a river or lake bed for a length of time that varies depending on the species as well as other factors. Once the egg hatches, it is called an alevin. An alevin is small and has a relatively large, orange yolk sac attached to its body. The alevin receives its nutrients from the yolk sac and remains hidden from predators in the safety of the gravel bottom of the stream or lake. As the alevin grows, it depletes the nutrients in the yolk sac and begins to develop mouth parts.
Once the yolk sac is depleted, the young salmon leaves the safety of the gravel bed and must search for its food. At this point in its life cycle, the fish is called a fry. Except for pink salmon, a fry has parr marks along each side of its body. These marks provide camouflage to protect the fry from predators. Fry eat food such as insect larvae and plankton.
This is where the life cycle begins to get complicated. Fry remain in fresh water for a length of time which not only varies between species but may also vary between populations of the same species. Sockeye and silver salmon usually remain in fresh water for one or two years, while pink and chum salmon migrate to sea soon after they emerge from the gravel. King salmon fry usually stay in fresh water for one year.
Before salmon migrate to the ocean, they lose their parr marks and turn silver in color. At this stage of their lifecycle, they are called smolt. Once smolt leave their freshwater stream, they spend a great deal of time in brackish water where freshwater streams flow into the ocean. They feed and grow in the brackish water until they reach a certain size, and then they migrate to the ocean. Once they enter the marine phase of their lifecycle, they are considered adult salmon.
Adult salmon remain in the ocean for a variable amount of time, depending on the species and the population. King salmon can stay in the ocean for as long as six years, but pink salmon return to freshwater to spawn when they are only two-years-old. Once adult salmon return to freshwater, they undergo a dramatic physical change. Sockeye salmon, king salmon, and silver salmon turn dark red, while chum salmon develop calico bands on each side of their bodies. Pink salmon turn dark, and males develop a hooked jaw and a large hump on their back.
Salmon return to the stream or lake where they were born to spawn and die. At this point in their lifecycle, they are called spawners. Once the salmon reach their spawning grounds, a male and female form pair bond. The female digs a bed, called a redd, for the eggs in the gravel. She deposits her eggs in the redd, and the male swims over the eggs and fertilizes them with his sperm. She then brushes a light coating of gravel over the eggs. Once they spawn, all species of Pacific salmon slowly deteriorate and die, their bodies left to fertilize the stream or lake where they were born, ensuring the birthing grounds will remain rich in nutrients for future generations.
Every year, I watch salmon return to their natal streams to spawn. This summer, we had a huge return of pink salmon to the many streams on Kodiak Island, and at times when I sat on our boat, salmon surrounded me, jumping out of the water as far as I could see, reminding me of popcorn. I am always amazed by the incredible life cycles of Pacific salmon and how the many animals and plants on Kodiak Island depend on salmon to survive and thrive. As they return from the ocean, salmon are chased by humans, bears, eagles, seals, sea lions, sharks, and any fish big enough and fast enough to catch them. It is a wonder any salmon survives the gauntlet it must swim to reach its birth area and reproduce. Once it spawns and dies, the body of a salmon provides nutrients for the stream or lake bed and the plants and trees growing in the vicinity. It is impossible to imagine a Kodiak riparian ecosystem without salmon.
Next week, I’ll write about pink salmon, the smallest but one of the most important of the five Pacific salmon species.
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