The upstream migration of fish is a mass movement involving certain species swimming from the ocean, where they spend their adult lives, back into freshwater rivers and streams. This life cycle is known as anadromy, meaning the fish live in saltwater but reproduce in freshwater. This multi-stage journey requires physical adaptation and precise navigational skills to overcome the current. While many fish species migrate, salmon are recognized for the scale and dramatic nature of their upstream travel.
The Most Famous Upstream Traveler
The most widely known upstream travelers are the various species of salmon. Pacific salmon include the Chinook (King), Coho (Silver), Sockeye (Red), Pink (Humpback), and Chum (Dog) species, alongside the anadromous form of Rainbow Trout, known as Steelhead. The Atlantic Salmon also undertakes this return journey from the sea.
These migrations can cover vast distances, with some populations traveling over 1,500 kilometers to their high-elevation spawning grounds. The fish cease feeding upon entering freshwater and must rely entirely on stored body fat and protein. This reliance is necessary because the migration can deplete between 50% and over 90% of a Pacific salmon’s total energy.
The Biological Drive for Migration
The reason for this upstream migration is reproduction. Anadromous species mature in the nutrient-rich environment of the ocean, allowing them to grow large enough to produce a high number of eggs. The adult fish then return to the specific freshwater location where they were born, known as the natal stream, to spawn.
The headwaters and smaller tributaries offer conditions suited for the survival of eggs and newly hatched fry. These upper reaches feature cold, highly oxygenated water and gravel stream beds that provide protected pockets for the eggs to develop. This environment shields the vulnerable offspring from the high salinity of the ocean, the turbulence of large rivers, and the majority of ocean predators.
How Fish Navigate the Current
Completing this long and complex route requires a layered system of sensory and physical adaptations. During the long oceanic phase, salmon are believed to use the Earth’s geomagnetic field as a navigational map to get within the vicinity of their home coast. This initial, broad-scale navigation ensures they arrive at the correct general region before switching to more precise cues.
Upon reaching the coast, fish rely on their highly developed sense of smell, or olfaction, to identify their natal stream’s chemical signature. They imprinted on this unique mix of organic and mineral molecules as juveniles before migrating to the sea. This olfactory homing allows them to distinguish their specific river and navigate through the maze of tributaries to the exact spot where they began their lives.
Physically, the fish use a behavior called positive rheotaxis, which is the instinct to orient and swim against a current. They utilize the lateral line sensory system to detect minute changes in water pressure and flow, helping them to maintain their position and conserve energy. This powerful swimming ability allows them to execute burst swimming necessary for leaping over natural barriers like waterfalls.
Other Fish That Migrate Upstream
While salmon are the most celebrated, the upstream migration strategy is utilized by a variety of other fish families. The Striped Bass, a popular sport fish, migrates from the Atlantic Ocean into rivers to spawn in the freshwater reaches. American Shad and river herrings, members of the herring family, also undertake anadromous migrations to lay their eggs far upriver.
Lampreys are also anadromous, swimming upriver to reproduce in gravel nests before the adults die. Sturgeon, large bottom-dwelling fish, include several anadromous species, such as the Atlantic Sturgeon and Green Sturgeon. This pattern of growing in the ocean and reproducing in freshwater contrasts with the catadromous migration, exemplified by the American Eel, which lives in freshwater but migrates downriver to the ocean to spawn.