Salmon navigate both freshwater and saltwater environments throughout their lives. This ability, known as anadromy, distinguishes them from many other fish species. Anadromous fish are born in freshwater, migrate to the ocean to grow and mature, and then return to freshwater to reproduce. This strategy allows salmon to access abundant food sources in the ocean while relying on freshwater for reproduction.
The Anadromous Journey
The salmon life cycle begins with eggs laid in gravel nests, called redds, within the cold, oxygen-rich waters of freshwater rivers and streams. After several weeks to months, eggs hatch into alevins, which remain hidden in the gravel, feeding on their yolk sacs. Once the yolk sac is absorbed, they emerge as fry, actively feeding on aquatic insects and growing in their freshwater nurseries.
As fry grow into parr, characterized by vertical stripes for camouflage, they continue to develop in freshwater for a period ranging from a few months to several years, depending on the species. This freshwater stage prepares them for the transition to saltwater. When they reach a certain size, usually between 10 to 25 cm, they undergo a process called smoltification.
Smoltification involves physiological, morphological, and behavioral changes, transforming the parr into smolts ready for ocean life. Smolts then migrate downstream, often at night, using the ebbing tide to help them reach estuaries where freshwater mixes with saltwater. This estuarine phase allows them to acclimate to increasing salinity before entering the ocean. After spending one to six years in the ocean, adult salmon journey back to their natal freshwater spawning grounds, guided by their acute sense of smell.
How Salmon Adapt to Changing Salinity
Salmon use osmoregulation to maintain their internal salt and water balance despite moving between environments with different salinities. In freshwater, a salmon’s body has a higher salt concentration than the surrounding water, causing water to diffuse into their bodies and salts to leach out. To counteract this, their kidneys produce large volumes of dilute urine to expel excess water, while specialized cells in their gills actively absorb salts from the water.
When salmon migrate to saltwater, the challenge reverses; the ocean’s high salt concentration causes water to leave their bodies and excess salts to enter. In this environment, their kidneys produce small volumes of concentrated urine, and their gills’ specialized cells reverse their function, actively pumping out excess sodium and chloride ions. They also begin drinking large amounts of saltwater to replace lost water.
Hormonal changes play a role in these adaptations, particularly during smoltification. Hormones become elevated, facilitating the development of osmoregulatory capacities in the gills and kidneys. Conversely, other hormone levels decrease, associated with increased seawater tolerance.
Major Salmon Species and Their Habitats
Salmon species are categorized into Pacific and Atlantic salmon, each with distinct geographical distributions and life history patterns. Pacific salmon include species such as Chinook, Sockeye, Coho, Pink, and Chum. These species are primarily found along the coasts of North America and Asia, from California and Alaska to Russia and Japan.
Chinook salmon, also known as King salmon, are often found in large rivers and can migrate thousands of kilometers upstream to spawn in diverse habitats, including small streams, larger rivers, and lake outlets. Sockeye salmon typically hatch in streams connected to lakes, where juveniles may spend one to three years before migrating to the Pacific Ocean, often feeding on zooplankton which gives their flesh a distinctive red color. Coho salmon typically spend one to two years in freshwater before migrating to the ocean, returning to spawn after about 18 months at sea. Pink salmon have a short, two-year life cycle, spending about 18 months in the ocean before returning to spawn. Chum salmon usually return to spawn after three to four years in the ocean, often being the last Pacific salmon to return to freshwater in a season.
Atlantic salmon is the sole species in the Atlantic Ocean, inhabiting rivers that flow into the North Atlantic, from northeastern North America to Western Europe. Unlike most Pacific salmon, Atlantic salmon can survive spawning and return to the sea to reproduce again. While most salmon exhibit anadromy, some populations have become landlocked, completing their entire life cycle in freshwater lakes and rivers.