A salmon run is when salmon migrate from their ocean feeding grounds back to the freshwater rivers and streams where they were born to reproduce. This journey, often spanning vast distances, culminates in a display of biological determination. It represents a phase in the salmon’s life cycle, driven by an instinct to return to their natal waters to lay and fertilize eggs. The annual return of these fish transforms river ecosystems, highlighting a connection between marine and freshwater environments.
The Incredible Journey
Salmon migrate from the ocean to freshwater, a journey that can cover hundreds to thousands of miles. This migration is guided by a homing instinct, where salmon use cues like the Earth’s magnetic field in the open ocean and their acute sense of smell to identify the chemical signature of their natal river as they approach coastal waters. During this upstream travel, salmon face physical challenges, battling strong currents, leaping over waterfalls, and navigating various obstacles.
As salmon transition from saltwater to freshwater, they undergo physiological changes to adapt their bodies for reproduction. Their bodies cease feeding, relying entirely on stored energy reserves accumulated in the ocean. Skin thickens, color changes become pronounced, and males often develop a hooked jaw, known as a kype, all in preparation for spawning. This transformation also includes adjustments in osmoregulation, allowing their bodies to manage the shift from a high-salt marine environment to a low-salt freshwater one.
The Spawning Ritual
Upon reaching their freshwater spawning grounds, female salmon construct nests, known as redds, in gravel streambeds. Using their tails, they create a depression in the gravel, ensuring a clean and oxygenated environment for the eggs. As the female deposits her pea-sized eggs into the redd, a male salmon simultaneously releases milt, or sperm, to fertilize them. The female then covers the fertilized eggs with gravel, providing protection until they hatch.
After spawning, the fate of adult salmon varies by species. Most Pacific salmon species, including Chinook, sockeye, coho, pink, and chum, are semelparous, meaning they reproduce once and then die shortly after. In contrast, Atlantic salmon are iteroparous, with some capable of surviving the spawning process and returning to the ocean to spawn again in subsequent years.
Ecological Significance
The annual salmon run impacts both freshwater and surrounding terrestrial ecosystems. As salmon return from the nutrient-rich ocean, they transport marine-derived nutrients, such as nitrogen and phosphorus, into nutrient-poor freshwater systems. After spawning, the decomposition of salmon carcasses releases these valuable nutrients, enriching the water and stream-side soils. This influx of marine nutrients supports a diverse array of life, fostering the growth of algae, aquatic invertebrates, and riparian vegetation.
The presence of salmon during a run provides a temporary but abundant food source for numerous wildlife species. Bears, eagles, wolves, and other predators and scavengers congregate along rivers to feed on the returning fish, distributing marine nutrients further into the forest landscape. Studies indicate that salmon carcasses can act as a natural fertilizer, contributing to the growth of trees along riverbanks, with some analyses showing a percentage of tree nitrogen derived from salmon. This nutrient transfer creates an ecological link, sustaining the health and productivity of interconnected ecosystems.