The annual salmon run is a natural event where various salmon species journey from the ocean back to their freshwater birthplaces. This arduous migration is a spectacle of endurance, as fish navigate upstream against currents, often leaping over obstacles. Beyond its biological importance, the salmon run is a significant ecological process, providing nutrients to freshwater ecosystems and supporting diverse wildlife.
Understanding the Salmon Run Phenomenon
Salmon exhibit an anadromous life cycle: they begin in freshwater, migrate to the ocean to mature, and then return to freshwater to reproduce. This upstream migration is known as the “salmon run.” After years in the marine environment, adult salmon return to their natal rivers to spawn, ensuring the survival of their offspring.
Freshwater environments, especially upper river reaches, offer oxygenated, clean, and cold water ideal for egg development and young salmon survival. Spawning in these protected areas also reduces predation compared to the open ocean. Most Pacific salmon species, and many Atlantic salmon, die shortly after spawning, transferring marine nutrients back to the freshwater ecosystem.
Seasonal Timing and Geographic Variations
Salmon runs generally occur from late summer through fall in the Northern Hemisphere. In regions like the Pacific Northwest, Alaska, and parts of Canada, these migrations are a prominent seasonal event. For example, Alaska’s commercial salmon season typically spans from May to October. The precise timing, however, varies considerably by geographic location, specific river systems, and species.
Species-Specific Run Schedules
Different salmon species adhere to distinct run schedules, though there can be overlap depending on the region.
Chinook Salmon
Chinook salmon, often called King salmon, are typically among the first to arrive, with runs beginning in May in some systems. Chinook runs can be categorized into spring, summer, fall, and even winter populations, named for when most fish enter freshwater. Fall-run Chinook are common on the West Coast, migrating from late July, peaking in September, and spawning from October through December.
Sockeye Salmon
Sockeye salmon, also known as Red salmon, generally begin their major migrations in June and peak in July. For instance, in Alaska, the first sockeye run occurs from late May through early July, with a second run from mid-July to mid-August.
Coho Salmon
Coho salmon, or Silver salmon, are later arrivals, with runs typically starting in late summer and continuing into the fall. In Alaska, Coho runs begin in August and can extend through October. In California, adult Coho enter freshwater from September through January, with spawning mainly from November to January.
Pink Salmon
Pink salmon, often called Humpy salmon, have a very concentrated season, usually peaking from mid-July through mid-August. Notably, Pink salmon runs occur in significant numbers primarily during even-numbered years in many Alaskan rivers.
Chum Salmon
Chum salmon, or Dog salmon, generally run later than pinks, with their peak occurring from mid-July through mid-August in some Alaskan areas. In the Puget Sound, Chum runs peak from October through December, while Southeast Alaska can see summer runs peaking mid-July to mid-August and fall runs peaking in September or later.
Atlantic Salmon
Atlantic salmon typically migrate upstream to their native rivers between February and October.
Environmental Factors Affecting Runs
Several environmental conditions influence the timing and success of salmon runs. Water temperature is a significant factor, as salmon prefer cooler waters for migration and spawning. Temperatures exceeding 18°C can affect migration speed, cause delays, and alter salmon distribution, while sustained temperatures above 21-24°C can be lethal.
River flow levels, influenced by rainfall and snowmelt, also play a role. Higher flows can facilitate upstream migration, while low flows can impede it.
Dams present a physical barrier to salmon migration, blocking access to spawning and rearing habitats. Even dams with fish passage facilities can impact fish survival due to turbine mortality, increased water temperatures in reservoirs, and predation.
Climate change is exerting increasing pressure on salmon runs, contributing to warmer water temperatures, altered precipitation patterns, and changes in snowpack. These changes can lead to earlier spring melts and reduced summer flows, creating conditions unsuitable for salmon. The timing of salmon migration is closely linked to the ecosystem’s food web, and climate-induced shifts can create a mismatch between the salmon’s arrival and the availability of their food sources.