The Atlantic salmon, known scientifically as Salmo salar, inhabits the northern Atlantic Ocean and the rivers that flow into it. While often called the “king of fish” and historically a commercial food source, it is distinct from the several species of Pacific salmon (Oncorhynchus genus). Today, the vast majority of Atlantic salmon consumed worldwide comes from aquaculture rather than wild populations.
The Defining Anadromous Life Cycle
The Atlantic salmon’s life cycle is defined by an anadromous migration pattern: hatching in freshwater, migrating to the ocean to mature, and returning to the river of birth to reproduce. This process begins in autumn when the female digs a nest, called a redd, in the riverbed gravel, where eggs are deposited and fertilized. Following hatching in early spring, the initial stage is the alevin, a tiny, translucent fish that remains hidden in the gravel, surviving entirely on its attached yolk sac for several weeks.
Once the yolk sac is absorbed, the alevin emerges as a fry, feeding on microscopic life and aquatic insects. The fish then develops into the parr stage, characterized by dark, vertical markings called parr marks, which provide camouflage against the river bottom. Parr remain in the river for one to six years, depending on water temperature and food availability, before undergoing a physiological transformation known as smoltification.
Smoltification prepares the salmon for saltwater life by changing its appearance and internal biology. The parr marks fade, and the fish develops a silvery sheen. Internally, the gills transform to manage the osmotic challenge of a marine environment, converting the tissue from salt-absorbing to salt-secreting. The resulting smolt migrates downstream to the ocean, having imprinted the unique chemical signature of its natal river into its memory for its eventual return.
Distinguishing Wild Salmon from Farmed Salmon
Wild and farmed Atlantic salmon, though the same species, develop into distinct biological entities due to their different environments. Wild populations maintain high genetic diversity, with local stocks adapted to the specific conditions of their home rivers. Farmed salmon originate from a limited number of wild stocks and have been selectively bred for traits like rapid growth rate, resulting in a more uniform genetic profile.
A notable difference is the color of the flesh, which in wild salmon results from their natural diet. Wild salmon consume crustaceans rich in the carotenoid astaxanthin, giving the flesh its deep pink-orange hue. Farmed salmon are raised on commercial pellets and require synthetic astaxanthin to be supplemented into their feed to achieve the color consumers expect. Physical characteristics also differ: wild salmon are leaner and more muscular from open-ocean migration, while farmed fish, raised in confined net pens, may exhibit physical wear like damaged fins and scales.
The cultivation process creates a divide in the life history of the fish. Farmed Atlantic salmon are raised in high-density aquaculture operations, typically in net pens submerged in coastal waters. This differs from the wild life cycle, where fish undergo long-distance ocean migration before returning to freshwater spawning grounds. Today, over 94% of all adult Atlantic salmon biomass is found within aquaculture, a stark contrast to declining wild populations.
Major Threats to Population Survival
Wild Salmo salar populations face human-induced pressures across their life cycle, causing a global decline of over 23% between 2006 and 2020. A primary barrier to successful spawning is the presence of physical obstructions like dams and culverts, which block access to historical upstream spawning and feeding grounds. Even with fish passage structures installed, these barriers can still reduce the accessibility and quality of available habitat.
Habitat degradation in freshwater environments further compromises juvenile survival. Pollution from agricultural and industrial runoff introduces siltation, which clogs the gravel beds and suffocates developing eggs and alevin by reducing oxygen flow. Overfishing continues to remove too many adult fish before they can complete their spawning migration, leading to fewer eggs laid and less resilient populations.
The rise of commercial aquaculture introduces risks to wild stocks. Escaped farmed salmon can interbreed with wild fish, leading to genetic dilution that weakens the fitness of the local population. Furthermore, the high-density environment of net pens acts as a reservoir for diseases and parasites, such as sea lice. These can spread to wild smolts migrating past the farms, contributing to increased marine mortality.