The Atlantic salmon (Salmo salar) is a ray-finned fish native to the North Atlantic Ocean and its tributary rivers. It has an anadromous life cycle, meaning it hatches in freshwater, migrates to the sea to grow, and returns to freshwater to reproduce. This migratory pattern requires complex physiological transformations, allowing the fish to thrive in two vastly different environments. The Atlantic salmon is one of the most studied and economically important fish species in the Northern Hemisphere.
The Freshwater Nursery Stages
The Atlantic salmon’s life begins in the gravel beds of its natal stream, where the female deposits eggs in a depression called a redd. After hatching, the young salmon remains in the redd as an alevin, surviving on its attached yolk sac. Once the yolk is absorbed, the fish emerges and begins the parr stage.
The parr develops vertical bars and spots, known as parr marks, which provide camouflage against the riverbed. During this phase, the juvenile fish is highly territorial, aggressively defending a section of the stream to feed on aquatic insects and invertebrates.
The duration of the parr stage is highly variable, lasting one to seven years depending on water temperature and latitude, which influence growth rates. The parr must reach a critical size before preparing for ocean migration. This extended freshwater residence is crucial for the species’ survival.
The Ocean Migration and Feeding Phase
The physiological shift from parr to ocean-ready smolt is a complex process called smoltification. During this transformation, the fish changes from camouflaged brown to a silvery color, and its body undergoes osmoregulatory reorganization. The gills and kidneys are remodeled to handle the osmotic challenge of salt water, allowing specialized cells to excrete excess salt.
The newly formed smolts migrate downstream in the spring, typically between April and June, moving into the North Atlantic. This extensive feeding phase involves long-distance migrations to rich feeding grounds, such as the Labrador Sea or waters off West Greenland. Salmon often travel thousands of kilometers, with some tracked traveling 2,400 kilometers.
The marine environment provides resources for rapid growth, with salmon feeding primarily on small fish and crustaceans. Salmon are categorized by the number of winters spent at sea: those returning after a single winter are known as grilse (1SW), while those remaining for two or more winters are called multi-sea-winter (MSW) salmon.
The Spawning Journey and Reproduction
After one to three years at sea, mature salmon begin their return migration, guided by a precise homing instinct. This navigation is driven by olfactory cues, as the fish recall the unique chemical signature of their natal river imprinted during smoltification.
As the adults move upriver, they cease feeding and undergo dramatic physical changes, shifting in coloration from silver to a darker, reddish-brown hue. Males develop a hooked jaw structure called a kype, used for dominance displays during spawning. The upstream journey is arduous, requiring the fish to overcome obstacles using only stored energy reserves.
Spawning occurs in the autumn, typically between October and November. The female uses her tail to excavate a depression in the gravel bed (a redd), deposits her eggs, which are fertilized by the male, and then covers them for protection. The spent adults are known as kelts.
Unlike Pacific salmon, Atlantic salmon are iteroparous, meaning some can survive, return to the sea to feed, and migrate back to spawn again. However, this repeat spawning is rare, with less than five percent of kelts currently returning in subsequent years.
Population Status and Conservation Pressures
Despite their complex life cycle, wild Atlantic salmon populations are facing significant declines across their range. The species was globally reclassified as ‘near threatened’ in 2023, and populations in Great Britain were designated ‘endangered’ following a 30-50% decline since 2006. Overall pre-fishery abundance in the North Atlantic fell by more than half between 1983 and 2016, a trend that continues despite reduced commercial fishing.
Multiple stressors in both freshwater and marine environments drive these declines.
Freshwater Stressors
Habitat degradation is a major issue in rivers, where dams and culverts block access to spawning and feeding grounds. Water pollution from agriculture and logging introduces sedimentation that can suffocate eggs and alevins in the redds.
Marine Stressors
Pressures are compounded by climate change and aquaculture. Rising ocean temperatures impact prey availability and alter migration routes, decreasing feeding time and survival. Sea cage salmon farming introduces threats, including the transmission of parasites like sea lice to wild smolts, causing high mortality. Escaped farmed fish also pose a genetic risk by interbreeding with wild populations, weakening the adaptability of native salmon.