Sockeye salmon life cycle
By Marisa Repasch
Salmon are vital for the economies of coastal communities, especially those in the Pacific Northwest. Wild salmon also play a critical role in ecosystem heath, from the ocean, to mountain streams and forests.
More than 130 animal species, including the Orca whale and the Grizzly Bear, feed on salmon and reap the benefits of the protein and nutrients contained in the fish. Salmon are also a central part of the human diet, especially for indigenous coastal communities who have fished salmon for thousands of years.
Wild salmon disappeared from many rivers during the twentieth century due to overfishing and habitat change, like dam construction and water pollution. People have been experiencing the negative effects of decreasing salmon populations, which has triggered calls to restore river systems to their natural states, to help salmon return to mountain streams once again.
Why do salmon migrate up rivers?
Seven species of salmon live in the Pacific Ocean, and five of these species live and breed in North American waters. These species are Chinook, Coho, Chum, Sockeye, and Pink salmon. The largest species of salmon lives in the Atlantic Ocean – the Atlantic Salmon.
Of the Pacific salmon, the largest species is the Chinook, which can grow up to 5 feet long and weigh up to 126 pounds. The smallest salmon species is the Pink salmon, which only reach about 2.5 feet long and weigh up to 12 pounds. These important fish depend on the connectivity between oceans and rivers for a key component of their lifecycle.
While salmon spend most of their lives in the ocean, where food and nutrients are abundant, salmon are anadromous fish. This means they swim up the mouths of rivers and breed in upstream reaches where their eggs are protected from marine predators.
Despite the challenge presented by strong currents and rapids, salmon swim tens to hundreds of miles upstream to spawn year after year, in large rivers like the Columbia and the Fraser, and in the many small rivers that reach the coast.
Salmon typically choose their nesting sites in riffles where the water is shallow, and the flow is fast and turbulent. Their nests are called redds, which are small pockets in the gravel river bed, created by the female salmon using her tail.
By fanning her tail over the gravel, she creates the hydraulic conditions to lift gravel out of the river bed, leaving a shallow hole where she can lay her eggs. She then swims upstream of the nest and fans more gravel to cover the eggs, shielding them from predators.
Because of this unique spawning behavior, salmon need coarse river gravel with a diameter up to 4 inches. This size of gravel not only provides protection for the eggs, but also allows water to flow through and deliver oxygen that the eggs need to develop.
Female salmon typically die within a week after laying their eggs because spawning is so taxing. For example, the Pacific salmon lives in the ocean for many years before migrating up rivers to spawn. However, they die after laying their eggs, so they only spawn once in their lifetime.
Because these salmon die in upstream river reaches, the local flora and fauna benefit from the influx of nutrients that the salmon brought from the ocean. In this way, the salmon life cycle supports the diverse ecosystems that lie along the coast of the Pacific Northwest.
Humans play a role in supporting natural salmon habitat
Humans play a role in supporting natural salmon habitat. We have exploited rivers for their natural energy, taken control of their water flow to reduce flooding, and changed water quality through runoff and pollution. The success of salmon spawning depends on several important factors, which are affected by these activities:
1) River water chemistry. Juvenile salmon establish a biological connection with the unique chemical signature of the river water in which they were born. Many salmon will return to their native river for spawning year after year, and they use the distinct chemistry of the river water to navigate back to their spawning grounds.
2) River water temperature. Salmon egg development is highly dependent on the temperature of the river water. Eggs will incubate faster in warmer water, and slower in colder water, and the migratory patterns of the salmon are timed up with the optimal river temperature.
3) River gravel on the stream bed. River gravel is produced by erosion of bedrock in the river’s headwaters, and is then delivered to rivers and streams during extreme rainfall. The river carries the gravel downstream during floods and high river flows, and the gravel falls to the river bed during low flows.
The effect of dams on salmon habitat
Dams disturb the natural supply and transport of river gravel needed for salmon spawning. Dams block the gravel from moving downstream, destroying salmon spawning habitats in downstream reaches. Periodic dam releases also cause the remaining gravel downstream to be flushed out of the system, further diminishing areas for salmon spawning. Environmental advocates are celebrating the recent surge in dam removal across the United States. According to the dam removal database from the American Rivers organization, agencies across the U.S. removed 90 dams in 2019, reconnecting nearly 1000 miles of river to their downstream reaches. Dam removal is particularly important for rivers in the Pacific Northwest where salmon play a critical role in the coastal ecosystems. For this very reason, four hydroelectric dams on the Klamath River along the California-Oregon border are slated for removal this year, representing the largest dam removal project in U.S. history.
To promote salmon habitat restoration, we can advocate for dam removal on important rivers where salmon populations have declined. We can also help re-naturalize salmon spawning habitats by reintroducing native vegetation and cleaning up litter from the river corridor.
Planting native trees and plants in river corridors will reduce stream temperature, improve water quality, and enhance nutrient cycling in rivers. Trees also supply woody debris to rivers, which is important for stabilizing river banks and gravel bars. Knowing that we have the power to restore rivers to their natural states inspires hope that salmon runs will continue for many future generations.
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