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Aquaculture and the Salmon Life Cycle

The five species of Pacific salmon are a foundation of life in Alaska. Learn how aquaculture supplements and protects these harvests.

by | January 7, 2022

The five stages of the salmon life cycle.
The six stages of the salmon life cycle. Illustration by Tatiana Plazas

Coastal communities have historically thrived on the rich harvests of wild salmon. the last four decades, groups like the Cook Inlet Aquaculture Association (CIAA) have supplemented salmon catches and protected local economies from changes in the harvests. To learn about aquaculture and the salmon life cycle, we must start at the moment of birth.

Salmon are anadromous, which means they live in both freshwater and saltwater at different times of their lives. They hatch in lakes and streams, migrate to the sea, and 18 months to eight years later — depending on species — they return to their place of birth.

Aquaculture and the salmon life cycle go hand in hand by protecting eggs and young salmon to give them a better chance of survival. Salmon that originate from CIAA are hatchery born and ocean raised, which means they hatch indoors in raceways and net pens, protected from predators, floods, drought, freezing and other threats. When they are more mature, they get released into the wild.

Eggs: The First Stage of the Salmon Life Cycle

Sockeye eggs, or roe, are the first stage of the salmon life cycle.
Sockeye eggs, or “roe,” on a fine mesh.

In summer, a female salmon returns to her birth stream to spawn. She chooses a patch of gravel where she lays a redd, a nest of thousands of red-orange eggs, each the size of a pea. Male salmon then jockey for position to fertilize the eggs with sperm, or milt. The eggs incubate over the winter, developing into embryos.

CIAA recreates this process through “gamete collection” using eggs and milt from adult salmon returning to various water bodies, such as Port Graham Bay. These returning adult salmon are called broodstock, wild salmon that began their lives in a hatchery. The collection of gametes allows CIAA to fertilize eggs in protected, indoor hatchery buildings. 

Alevins: The Fragile Hatchlings of the Salmon Life Cycle

Pink salmon alevin

In the spring, tiny alevin emerge from the eggs with a yolk sac dangling from its body. Alevins remain in the gravel nest for another month, living from the nutrients contained in the yolk sac. Soon, the sac “buttons up,” or becomes completely absorbed when the alevin is about an inch long. Only about 20 percent of alevin in the wild survive from hatching to becoming fry.

At CIAA hatcheries, alevin remain indoors, safe from predators and abrupt changes in the environment. It takes about 100 days for the alevin to hatch, and they live in an artificial substrate called bio saddles that mimic the gravel nest used by salmon in the wild. The saddles keep alevin apart—otherwise they would pile onto each other and suffocate.

You pronounce alevin, “ALE-vinn.” It comes from the Old French word “alever,” which means “to raise offspring.”

Fry: Juveniles with Parr Marks

Pink fry, Tutka Bay Lagoon
Pink salmon fry at Tutka Bay Lagoon.

When alevins leave their nest and can search for food on their own, they become fry. They are still very small and must use rocks and aquatic plants to hide from birds, fish, and other predators. Fry behave differently depending on their species. Young king salmon can handle faster streams than other species. Silvers prefer streams, sloughs, and swamps—and especially pools created by beaver dams. Sockeye prefer lakes, while pinks and chum head straight out to sea. 

All species except pinks develop dark vertical stripes called parr marks that provide camouflage and protect the fry from predators. When alevin develop into fry, CIAA moves sockeyes and silvers at the Trail Lakes Hatchery into concrete pools called “raceways” where fresh water circulates to provide needed oxygen. Raceways can be indoors or outdoors, but either way, they are cold places with flowing water. This means the fry feel the same temperature changes as their wild counterparts. 

Pink salmon move out to sea immediately after they’ve buttoned up. Aquaculture workers at Tutka Bay Lagoon and Port Graham place young pinks into net pens, anchored cages that immediately expose the fish to open water. In other bodies of water, like Hidden Lake, CIAA releases sockeye and coho fry directly into the wild.

The Life of A Fry

Fry eat fish food until they reach a target weight for release. CIAA keeps an eye on weight, size, and other environmental factors to know when to release fry directly into the wild. For pink salmon fry, the ideal release weight is around a gram—about the same as a paperclip. At Tutka Bay Lagoon, the hatchery releases them before the water gets too warm. Only sockeye remain in pens until they become smolt.

Aquaculture uses both raceways and net pens, closed structures to reinforce the salmon life cycle. They prevent hatchery fry from having contact with their counterparts in the wild. However, both hatchery salmon and wild salmon are virtually identical.

Smolts: When the Parr Marks Fade

The smolt stage of the salmon life cycle
A Resurrection Bay sockeye smolt.

Eventually fry lose their parr marks and become silver-colored smolt. Pink salmon are born with this silver color and swim directly to sea. Chinook salmon remain in freshwater for one or two years. Silvers stay in freshwater up to five years, and sockeye for up to four years. Smolts swim together in schools and begin adjusting to life in saltwater. They explore estuaries, the mouths of rivers as they avoid predators and pursue sources of food. 

During the smolt stage, salmon become more tolerant of salt in the water. Their bodies change color, becoming more silver, and their cells change the way they absorb nutrients. In time, these salmon are ready to swim out to sea—but they have become “imprinted,” which means they will swim back to the same streams where they were born to spawn and reproduce.

Sockeyes and cohos are the only species CIAA keeps penned until the smolt stage. This means they are imprinted with the location of the stream where they are released before they head out to sea.

Adults: When the Life Cycle Moves Out to Sea

The adult stage of the salmon life cycle
Fishermen on both land and sea harvest millions of adult salmon in bays and the mouths of rivers.

Depending on species, salmon spend between six months to six years in estuaries, bays, and the open sea. In these water, salmon fatten themselves up on krill, squid, and smaller fish. No matter how far they range, salmon always return to their birthplace to spawn. Scientists speculate that salmon are guided home by smell, the sun, or the earth’s magnetic field.

As the salmon return, fishermen catch these full-sized adults in bays and the mouths of rivers. Alaska fishermen catch almost all the sockeye and pink salmon harvested in the United States are caught by Alaska fishermen. 

The Alaska Department of Fish and Game (ADF&G) watches commercial salmon fisheries to make sure enough salmon escape fishermen’s nets. These fish need to return to their spawning grounds and produce strong runs for the following five to 10 years. ADF&G can raise fishing limits to avoid having too many adults spawning in the same streams. The state can also restrict limits if returns are too low to meet these sustainable escapement goals.

CIAA maintains a system of weirs, which function like gates for salmon leaving from or returning to certain lakes. Weir workers provide manual counts of outbound smolt and returning adults using a tool called a tally whacker. This helps ADF&G know when it has met its sustainable escapement goals. 

Sockeye spawning during the last stage of the salmon life cycle
Spawning sockeye salmon with their distinctive hooked nose. Photo by Kenny Regan

Spawners: The Salmon Life Cycle Comes Full Circle

As salmon begin to spawn, they change shape and color. Pink salmon grow an enormous fatty ridge on their back, earning them the nickname “humpback” or “humpy.” All pinks return to spawn every two years like clockwork. Sockeye salmon become bright red, with green heads; the male grow long, hooked noses. Chums turn bright green with purple tiger stripes.

Salmon stop eating when they return to freshwater and die, exhausted, once they have spawned. In late summer, riverbanks are lined with the carcasses of spawned-out salmon.

Spawning salmon generally not harvested for food because they don’t have much fat left. Bears eat them, and some Native villages harvest and dry spawners. The carcasses also feed the caddisfly larvae and other aquatic insects that, in turn, sustain the next generation of salmon. Salmon hatchlings feed directly on their parents’ flesh. 

Alaska’s Genetic Policy

In collecting and releasing fish, CIAA carefully follows ADF&G’s genetic policy to protect wild stocks. This includes the following:

  • Only use local broodstock.
  • Genetic diversity maintained by using broodstock collected across an entire run without selecting for size, weight, or other physical traits.
  • Limits on hatchery stocks that come from a single donor stock.
  • No salmon introduced from outside Alaska.
  • No salmon moved between major geographical areas in Alaska — such as the Arctic-Yukon-Kuskokwim region, Bristol Bay, Cook Inlet, Kodiak, Prince WIlliam Sound, and Southeast.

Fascinating Facts About Aquaculture and the Salmon Life Cycle

  • Breakfast trend — Salmon eggs, also known as “roe,” have a reputation as an omega 3 protein–packed superfood. Salmon roe is used in sushi in Japan and consumed as caviar in Russia. TV chefs and food bloggers are waking up to a new breakfast trend: salmon roe in scrambled eggs. 
  • Backward swimmers — Smolts have a strange way of swimming. They move backward with their heads facing upstream — oriented the same way they will be facing when they return to the stream to spawn. Smolts are nocturnal, running at night to avoid predators. This means that CIAA smolt counting crews have to shift their schedules as well— working at night and sleeping during the day.
  • Outside threat — Northern pike were introduced in Southcentral Alaska back in the 1950s, and since then, they’ve been wreaking havoc on native salmon fry. Aquaculture can protect the salmon life cycle by culling these invasive species. CIAA crews suppress northern pike using gillnets and fishing line to restore valuable salmon populations.
  • Recordbreaker — The longest recorded salmon migration was more than 2,000 miles. A 30-pound Chinook salmon swam 30 miles a day up the Yukon River to make it to her spawning grounds. 
  • Freshwater friends — Landlocked salmon, called kokanee, remain in freshwater their whole lives because natural barriers like closed lakes block them from migrating to the ocean. Researchers do not know what causes them to stay behind while most migrate to the oceans before returning to spawn. 
  • Tough life – Depending on the species of salmon a female can lay anywhere from 1,200 to 14,000 eggs! Generally only two percent of those eggs survive to adulthood to spawn. 

Learn More About How Aquaculture Reinforces the Salmon Life Cycle

What can you do to learn more about the salmon life cycle? CIAA offers tours of its hatcheries by appointment. Local schools can also schedule a classroom visit by a CIAA staff member.

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