The “salmon run” describes the period when mature salmon migrate from the ocean back into freshwater rivers and streams to reach their natal spawning grounds and reproduce. This annual migration is a significant natural event, characterized by the sheer numbers of fish undertaking an arduous upstream journey.
The Salmon Life Cycle and the Purpose of the Run
Salmon are anadromous fish, meaning they hatch in freshwater, migrate to the ocean to mature, and then return to freshwater to spawn. Young salmon, known as fry, spend varying amounts of time in freshwater before transforming into smolts and migrating to the ocean. They grow and mature in the marine environment, often for several years.
The “run” is the final, often strenuous, leg of their life cycle. Driven by instinct, adult salmon navigate back to the precise freshwater streams where they were born. Their bodies undergo physiological changes to adapt from saltwater to freshwater, and they cease feeding during this migration, relying on stored energy reserves. Once they reach their spawning grounds, females deposit eggs in gravel nests, called redds, which are then fertilized by males. Most Pacific salmon species die shortly after spawning, completing their life cycle and contributing nutrients back to the freshwater ecosystem.
Key Species and Their Typical Run Timings
Salmon runs occur at different times depending on the species and geographical location. Chinook salmon (King salmon) are often among the first to migrate, with runs typically beginning in May and extending through July in many Alaskan rivers. Some populations, like fall-run Chinook, begin their journey in late July and peak in September.
Sockeye salmon (Red salmon) generally follow, with runs intensifying from June through August. In some areas, early sockeye runs occur from late May to early July. Peak activity often occurs in July.
Coho salmon (Silver salmon) typically begin their runs in late July and continue through October, with peak activity from August through September. Pink salmon (Humpy salmon) are distinctive for their biennial runs, occurring primarily in even-numbered years. Their migration usually occurs from late July through mid-August.
Chum salmon (Dog salmon) have varied run timings, with summer runs peaking from mid-July to mid-August and fall runs peaking in September or later.
Factors Influencing Run Timing
The precise timing of salmon runs is not fixed and can vary significantly due to several environmental factors. Water temperature is a primary influence, as salmon require specific ranges for successful migration and spawning. Unsuitable temperatures can delay or accelerate their upstream movement. River flow rates also play a crucial role, with higher flows, often from rainfall or snowmelt, providing easier passage and triggering migration.
Rainfall events increase water levels, creating favorable conditions for salmon to ascend rivers. Snowmelt contributes to river volume, facilitating earlier runs. Tide cycles also influence migration, particularly in coastal rivers, where salmon may time their entry with high tides to reduce energy expenditure. Geographical location is another factor, with run timings differing even for the same species across distinct river systems, reflecting localized environmental conditions and genetic adaptations.
Ecological Importance of Salmon Runs
Salmon runs are ecologically significant events that extend their influence far beyond the immediate aquatic environment. Salmon are considered a keystone species because their migrations transfer substantial marine-derived nutrients from the ocean to freshwater and terrestrial ecosystems. As salmon return to their natal streams and die after spawning, their carcasses decompose, releasing nitrogen, phosphorus, and other elements into the surrounding environment.
These nutrients enrich riverbeds, supporting aquatic insects and juvenile fish, and are also absorbed by riparian vegetation along the riverbanks. Wildlife such as bears, eagles, and various birds and mammals depend on salmon as a seasonal food source, further distributing marine nutrients into the forest. The presence of salmon can be an indicator of the health and integrity of both aquatic and terrestrial ecosystems.