Fish capable of living in both saltwater and freshwater possess a biological adaptability, allowing them to thrive in environments with vastly different salt concentrations. This ability to transition between habitats provides access to varied food sources and breeding grounds.
The Science of Water Adaptation
Fish living in environments with fluctuating salinity, known as euryhaline species, employ complex internal processes to maintain their body’s water and salt balance. This regulation is called osmoregulation, a process that ensures their cells do not become too diluted or too concentrated. Unlike stenohaline fish, which are restricted to either salt or freshwater, euryhaline fish can manage wide salinity ranges.
The gills, kidneys, and intestines play integrated roles in this balance. In freshwater, fish actively absorb salts through specialized cells in their gills to counter salt loss, while their kidneys produce large volumes of dilute urine to excrete excess water absorbed by osmosis. Conversely, in saltwater, these fish must prevent water loss and excrete excess salt. They achieve this by drinking seawater, absorbing water and salts in their intestines, and actively secreting concentrated salt through their gills. The kidneys also produce small amounts of concentrated urine to conserve water.
Types of Migratory Fish
Many fish species migrate between fresh and saltwater, often for reproduction. These migrations fall into three main categories based on their direction and purpose.
Anadromous fish
Anadromous fish are born in freshwater, migrate to the ocean to mature, and then return to freshwater to spawn. Young fish hatch and grow in rivers or streams, adapting to saltwater before migrating to the ocean. After years in the marine environment, adults journey upstream back to their birthplaces to breed.
Catadromous fish
Catadromous fish follow the opposite pattern: born in saltwater, they migrate into freshwater to grow and mature, then return to the ocean to spawn. Juveniles move from marine spawning grounds into rivers, residing there for years. Upon maturity, they migrate downstream back to the sea to reproduce, often dying after spawning.
Amphidromous fish
Amphidromous fish migrate between fresh and saltwater, but not for breeding. Eggs hatch in freshwater or estuaries, and larvae drift to the ocean to grow. After developing in marine waters, juveniles return to freshwater to live as adults.
Notable Examples of Water-Adapting Fish
Salmon
Salmon are anadromous fish, beginning their lives in freshwater streams. Young salmon, called smolts, transform to prepare for saltwater before migrating to the ocean to grow. They later use a strong homing instinct, guided by cues like the Earth’s magnetic field, to return to their birth rivers to spawn.
Atlantic sturgeon
Atlantic sturgeon are anadromous, born in freshwater rivers along the Atlantic Coast. Juveniles may remain in their natal rivers for several years before moving to coastal ocean waters. Mature sturgeon then migrate back to freshwater to spawn.
Striped bass
Striped bass are anadromous fish that spawn in freshwater rivers and estuaries. After spawning, adults often disperse into coastal ocean waters, undertaking seasonal migrations along the coastline.
European and American eels
European and American eels exemplify catadromous migration. Both species are born in the Sargasso Sea, where their eggs hatch into tiny, transparent larvae. These larvae drift on ocean currents, transforming into “glass eels” and then “elvers” as they enter coastal waters and migrate into freshwater rivers and lakes. They spend many years maturing in freshwater before embarking on a long journey back to the Sargasso Sea to reproduce and die.
Bull sharks
Bull sharks are notable among sharks for their ability to move between marine and freshwater, making them a prominent euryhaline species. They possess specialized physiological adaptations, including their kidneys, gills, and rectal glands, which enable them to adjust their internal salt levels to varying salinities. While they frequent coastal and brackish waters, bull sharks can travel far up rivers, even thousands of miles inland.
Goby species
Some goby species are amphidromous. These fish reproduce in freshwater, and their newly hatched larvae are carried downstream to the ocean. After a period of development in marine waters, the juvenile gobies return to freshwater streams to reach their adult habitats.