Salmon are a fascinating group of fish, and the question of whether they live in freshwater or saltwater often arises. The answer is not a simple one, as their biology involves a remarkable transition between both environments. This unique life strategy allows them to utilize the resources of vastly different aquatic habitats throughout their lifespan.
The Anadromous Nature of Salmon
Salmon are not exclusively freshwater or saltwater fish; they are classified as anadromous. This term describes fish that hatch in freshwater, migrate to the ocean to mature, and then return to freshwater to reproduce. The word “anadromous” originates from Greek words meaning “upward running,” referring to their migration upstream from the sea. This behavior is a defining characteristic of most salmon populations, setting them apart from many other fish species that spend their entire lives in one type of water.
The Salmon Life Cycle: A Journey Between Worlds
The salmon life cycle begins with eggs laid in gravel nests, called “redds,” in freshwater rivers and streams. After hatching, the tiny fish, known as alevins, remain in the gravel, absorbing nutrients from a yolk sac. Once the yolk sac is absorbed, they emerge as fry, feeding on small insects and plankton in the freshwater environment.
As they grow into parr, they develop camouflage markings and continue to mature in their natal freshwater for a period that varies by species. The next stage, smoltification, involves significant physiological changes that prepare them for life in saltwater. Smolts then embark on a downstream migration, often spending time in estuaries to gradually acclimate to increasing salinity before entering the ocean.
Salmon spend between one and eight years maturing in the ocean, feeding and growing considerably in the nutrient-rich marine environment. When the time comes to spawn, mature salmon undertake an arduous upstream migration, returning to the freshwater streams where they were born. Most Pacific salmon species die shortly after spawning, their bodies providing nutrients back to the freshwater ecosystem.
Adapting to Different Waters
The ability of salmon to survive in both freshwater and saltwater is due to a biological process called osmoregulation. This process allows them to maintain the balance of salt and water, despite the differences in salinity between the two environments. In freshwater, a salmon’s body tends to absorb water and lose salt, so their kidneys produce a large volume of dilute urine, and specialized cells in their gills absorb salt.
Conversely, in the ocean, a salmon’s body tends to lose water and take on excess salt. To counteract this, they drink large quantities of saltwater and excrete highly concentrated urine, minimizing water loss. Their gills play a role by actively pumping out excess salt ions. These physiological adaptations enable salmon to make transitions, with changes in gill function and kidney activity being important for their survival in varying salinities.
Why the Confusion?
Confusion about whether salmon are freshwater or saltwater fish stems from observing them at different stages of their life cycle. People might see them in rivers during their spawning runs, leading to the assumption that they are exclusively freshwater species. However, this freshwater presence is typically for a specific, temporary purpose. Many also associate “fish” with either a purely freshwater or purely saltwater habitat, making salmon’s dual existence less intuitive.
Some salmon populations can become “landlocked” and complete their entire life cycle within freshwater lakes, behaving similarly to their ocean-going counterparts by migrating into rivers to spawn. For example, sockeye salmon that live entirely in freshwater lakes are sometimes called Kokanee salmon. This variation contributes to the perception that they might be solely freshwater inhabitants.