The question of whether salmon is a freshwater fish does not have a simple “yes” or “no” answer. Salmon are unique among many fish species because they are programmed to live in two completely different aquatic environments during their lifetime. This requires the fish to transition between the low-salinity environment of rivers and the high-salinity environment of the ocean. This dual residency means that salmon are neither exclusively freshwater nor exclusively saltwater fish, but rather a species defined by its movement between the two.
Defining the Anadromous Lifestyle
The specific life pattern of the salmon is formally known as anadromy, a term derived from Greek roots meaning “running upward.” An anadromous fish is one that spends most of its adult life feeding and growing in the ocean before migrating up into freshwater rivers to reproduce. This strategy takes advantage of the marine environment’s vast food resources to achieve large body size, while utilizing the protected upper reaches of rivers for spawning and early development.
This migratory pattern is the opposite of the catadromous lifestyle, exemplified by the freshwater eel. Salmon’s anadromous existence classifies them as diadromous, meaning they regularly travel between fresh and salt water. This classification highlights the biological necessity of both environments for completing the species’ full reproductive cycle.
The Journey: Salmon Life Stages and Habitats
A salmon’s habitat shifts dramatically over its lifespan. The cycle begins in freshwater, where eggs are laid in gravel nests called redds, and hatch into alevin and then fry. For the next one to eight years, depending on the species and location, the juvenile salmon, now called parr, remain in their natal river or stream, feeding on aquatic insects and developing dark camouflage spots.
The next stage involves a shift to the estuary and then the ocean. The parr undergo a physiological change and become smolts, preparing to migrate downstream toward the sea. Once they reach the ocean, they spend between one and six years as adults, growing rapidly in the marine environment to reach their full size. The vast majority of a salmon’s feeding and growth occurs in this saltwater phase.
When mature, the adult salmon use their acute sense of smell and the Earth’s magnetic field to navigate back to the exact freshwater system where they hatched. Upon re-entering the river, they cease feeding and use stored energy to swim upstream to their spawning grounds. After reproduction, Pacific salmon species typically die, while Atlantic salmon may survive to return to the ocean and spawn multiple times.
The Transition: How Salmon Adapt to Different Waters
The successful migration between fresh and saltwater is made possible by osmoregulation, which manages the fish’s internal water and salt balance. In freshwater, a fish’s body has a higher concentration of salt than the surrounding water, causing water to constantly rush in and salt to leach out. To counter this, a freshwater fish rarely drinks, excretes large volumes of dilute urine, and uses specialized cells in its gills to actively absorb salt ions from the water.
When the salmon transforms into a smolt and enters the ocean, this entire system must flip to handle the opposite challenge. Saltwater has a much higher salt concentration than the fish’s body, causing the salmon to constantly lose water and gain excess salt. The smolt begins to drink large amounts of seawater, stops producing dilute urine, and its gills switch function.
The change is driven by the \(\text{Na}^+/\text{K}^+\)-ATPase enzyme, a molecular pump in the gill cells. In freshwater, this pump helps take in salt, but during smoltification, its activity significantly increases and is relocated to actively pump excess sodium and chloride ions out of the body. This physiological transformation, which typically occurs over a few weeks, is what allows the salmon to survive the transition from a river to the sea.
Exceptions and Variations in Salmon Species
While the anadromous life cycle is characteristic of most salmon, some specific populations have evolved to complete their entire life cycle without ever entering the ocean. These are referred to as landlocked salmon, and they are true freshwater fish. A notable example is the Kokanee salmon, which is a naturally occurring landlocked variant of the Sockeye salmon species.
These populations are found in large freshwater lakes across North America and Europe. Instead of migrating to the sea, the young fish migrate from the river where they hatched into the lake, which serves as their primary feeding and growth habitat. When mature, they undertake a spawning run, migrating up the tributary streams or to suitable lake shores to reproduce.