Salmon undertake one of nature’s most remarkable journeys, a challenging upstream migration. This feat is not exclusive to one sex; both male and female salmon participate in this arduous swim. Their determined ascent against powerful currents is a fundamental part of their life cycle, driven by instinct to ensure the continuation of their species.
Who Swims Upstream?
Both male and female salmon embark on the upstream migration from the ocean to freshwater environments. These fish are classified as anadromous, a term derived from Greek meaning “to go, or run, uphill.” This signifies their life cycle: they hatch in freshwater, mature in saltwater, and then return to freshwater to reproduce. This behavior is common for most salmon species, including those in the Pacific and Atlantic oceans.
The Purpose of the Upstream Migration
The primary purpose of the salmon’s challenging upstream journey is reproduction. After living in the ocean for several years, adult salmon return to their natal rivers to spawn. This instinct guides them, sometimes over thousands of miles, to gravel beds suitable for laying and fertilizing eggs. The freshwater environments upstream provide ideal conditions for egg incubation and the early development of young salmon, offering protection from predators and suitable water temperatures.
Navigating the Ascent
The upstream migration presents salmon with immense physical challenges. They must battle strong currents, navigate rapids, and leap over natural obstacles such as waterfalls. This demanding journey depletes their energy reserves, as salmon stop feeding once they enter freshwater. Beyond natural barriers, they also face predators like bears and eagles, and human-made obstructions such as dams. To prepare for this taxing journey, salmon undergo significant physiological changes, with males developing hooked jaws (a kype) and sometimes humps on their backs.
The End of the Journey
Upon reaching their spawning grounds, female salmon construct nests, called redds, by using their tails to clear depressions in the gravel. These redds provide a protected environment for the eggs. Simultaneously, one or more males release milt, fertilizing the eggs as they settle into the prepared nest. The female then covers the eggs with gravel, often moving upstream to create additional redds.
Post-Spawning Fate
For most Pacific salmon species, including Chinook, Coho, and Sockeye, this act of reproduction marks the end of their life cycle; they die shortly after spawning, returning vital nutrients to the freshwater ecosystem. In contrast, Atlantic salmon can survive and return to the ocean, potentially spawning multiple times.