Seven Fish Species That Can Survive In Both Fresh And Salt Water

By Scuba Diver Life

All fish, whether they live in salt water or fresh water, must maintain a certain level of salinity in their bloodstream to survive; the process by which they achieve this equilibrium is called osmoregulation. In freshwater species, this involves actively taking in salt from the surrounding environment via the gills, and excreting enough water through the kidneys to prevent that salt from becoming too diluted. In saltwater species, it involves excreting the excess salt ingested when they drink seawater via the gills and kidneys, thereby preventing the water in their system from becoming too salty. Sharks are a little different — they have high urea concentrations in their blood, which helps them absorb water directly from the ocean around them. They also remove excess salt through their urine. 

  • Cont.

    Most fish are restricted to one environment because they cannot change the way they osmoregulate, but some species spend periods of their life in both environments because they are able to switch between the two methods of osmoregulation. Collectively, these species are known as euryhaline fish, a category that can be further split into those fish that spend most of their lives at sea (anadromous fish), and those that spend most of their time in fresh water (catadromous fish). Although many euryhaline fish can tolerate extreme changes in salinity, most require a gradual shift from one environment to the other to allow their bodies to make the necessary adjustments. Here are seven fish that have evolved to take advantage of both environments. 

  • Gulf sturgeon

    This prehistoric-looking fish lives in coastal rivers from Louisiana to Florida during the summer, and in the marine habitats of the Gulf of Mexico during the winter. Adult sturgeons spend most of their lives in salt water, but migrate to the freshwater rivers in which they began their lives to spawn. Their eggs are sticky, and attach themselves to stones in the riverbed. When they hatch, the juvenile sturgeon spend the first two years of their lives sheltering in the river until they’re big enough to survive in open ocean. 

  • Green sawfish

    The critically endangered sawfish is native to the Indo-West Pacific. It prefers shallow marine habitats and frequents estuaries and rivers. The fish has been recorded traveling considerable distances inland; one was found 150 miles from the coast in Australia’s Northern Territory. It is thought that this species gives birth to live young, which spend their early years in freshwater environments in order to avoid being preyed upon by larger marine animals, including sharks and saltwater crocodiles. 

  • Sockeye salmon

    Like most salmon species, sockeyes are born in freshwater rivers but spend most of their lives in the waters of the Pacific Ocean. They return to their natal rivers to spawn only once, and then die. Interestingly, some sockeye populations choose not to undertake this migration, and instead spend their entire lives in the freshwater rivers and lakes in which they were born. These sockeyes are referred to as kokanee, but there is no physical difference between them and their anadromous cousins. 

  • Bull shark

    Perhaps the most famous euryhaline species, bull sharks have been spotted some 2,500 miles up the Amazon River in Peru, and in the waters of Lake Nicaragua. They are found in many major rivers, including the Ganges, the Brahmaputra and the Mississippi. Although it is thought possible for a bull shark to spend the entirety of its life in freshwater, most populations prefer to spend the majority of their adult life in the ocean, where food and potential mates are more plentiful.

  • Barramundi

    Also known as an Asian seabass, the barramundi is a catadromous fish found throughout the Indo-West Pacific. This species spends most of its life in freshwater rivers, but migrates downstream at the start of the monsoon to mate and spawn. Barramundi eggs require the brackish water found in estuaries and tidal flats to develop and hatch. In a reverse phenomenon to that displayed by the kokanee sockeyes, there are rare populations of barramundi that have adapted to live exclusively in a marine environment. 

  • American eel

    American eels begin their lives as eggs, hatching in the middle of the Sargasso Sea. The tiny larvae are then washed by ocean currents towards the coast of the United States, where they migrate inland to freshwater lakes and rivers. This journey can take many years, with some eels traveling distances of up to 3,700 miles. The eels spend their adult lives in freshwater, before returning to the Sargasso Sea to spawn. They do not eat while they are at sea, and die once their life cycle is complete.

  • Atlantic stingray

    These small stingrays are found in the western Atlantic Ocean, along the east coast of the United States and Mexico. They are capable of tolerating a variety of different salinities, with some populations spending their entire lives in the sea while others cross into freshwater rivers and lakes. There is a thriving population of Atlantic stingrays in the St. Johns River system in Florida. The rays living here are the only permanent freshwater elasmobranchs in North America.