Atlantic salmon are an anadromous species, a term describing fish that are born in freshwater, migrate to saltwater to grow, and then return to freshwater to reproduce. This life strategy means they inhabit two vastly different environments over their lifetime. Their existence is defined by a lengthy and demanding migration, covering thousands of miles from the streams of their birth to the open ocean and back again.
The Downstream Journey to the Ocean
Atlantic salmon begin their lives in the cold, clear water of freshwater streams. In their juvenile stage, they are known as parr, distinguished by dark vertical markings that provide camouflage from predators. They spend between one and four years in these rivers, with the duration depending on water temperature and the availability of food, primarily aquatic insects.
As they prepare for their journey to the sea, the young salmon undergo a physiological transformation called smoltification. During this process, they lose their parr markings and develop a silvery color to camouflage them for life in the ocean. Their body chemistry also changes, adapting their gills and kidneys to excrete salt to survive in a saltwater environment. Once this change is complete, the fish, now called smolts, are carried by river currents downstream toward the estuaries.
The migration of smolts to the ocean is timed to coincide with spring river conditions that aid their travel. They actively swim and are also transported by the current, moving towards the coast. This downstream journey is a period of high risk, as the smolts must navigate through new environments and avoid predators, especially in the hazardous estuaries where mortality rates can be high.
Life at Sea and the Return Upstream
Upon entering the Atlantic Ocean, the salmon’s primary objective is to feed and grow. They migrate towards nutrient-rich feeding grounds, with major destinations being the waters off the coasts of Greenland and the Faroe Islands. In these marine environments, they feast on a diet of smaller fish and crustaceans, gaining weight rapidly. This period at sea can last from one to several years.
Salmon that return to their home rivers after just one winter at sea are called “grilse” and weigh between 2 and 3 kilograms. Those that remain in the ocean for two or more winters are larger, often weighing between 3 and 10 kilograms, though some can reach 20 kilograms or more. This extended time allows for greater growth, which provides more energy for the upstream journey and more eggs for females.
Once they reach sexual maturity, an internal trigger prompts them to begin the long journey back to the exact river where they were born. To prepare for the upstream battle against strong currents, they stop feeding and rely entirely on the fat reserves they accumulated at sea. Males often develop a hooked jaw, known as a kype, and both sexes take on darker, more vibrant colors as they approach their spawning grounds.
Navigational Abilities
The ability of Atlantic salmon to navigate from distant ocean feeding grounds back to their specific home river is a notable biological feat. Their primary tool for this homing migration is their sense of smell, or olfactory memory. Each river has a unique chemical signature created by the blend of minerals, vegetation, and organic matter from its surrounding watershed. Smolts are believed to imprint on this specific scent profile before they migrate to the ocean.
As adult salmon approach the coastline on their return journey, they are thought to use this imprinted olfactory information to find their natal river system. They can detect the faint chemical cues of their home stream from miles away, guiding them into the correct estuary and river mouth. This “smell map” becomes increasingly detailed as they move further upstream, allowing them to make the correct turns at river junctions.
While the sense of smell is the main guidance system for the final phase of their journey, it does not fully explain how they navigate across thousands of miles of open ocean. A leading hypothesis is that salmon use the Earth’s magnetic field as a large-scale compass. This magnetic sense may help them determine their latitude and guide them from their feeding grounds back to the general coastal area of their home river.
Obstacles During Migration
The migration of Atlantic salmon is fraught with challenges, both natural and human-made. Natural obstacles include waterfalls and strong rapids that test their strength and leaping ability. Predators also pose a threat throughout their migration, from birds and other fish in freshwater to seals, larger fish, and seabirds in the ocean.
Human activities have introduced a host of additional barriers that impede their journey. Dams and culverts are among the most significant physical obstacles, blocking access to historical spawning grounds and fragmenting river habitats. These structures can delay or completely halt migration. While some dams are equipped with fish ladders, these are not always effective.
Water quality also presents a challenge. Pollution from industrial discharge, agriculture, and urban runoff can degrade river habitats, making them unsuitable for salmon. Changes in water temperature and flow rates, often linked to climate change and land use, can disrupt the timing of migration and create stressful or even lethal conditions for the fish.