How can some Fish adapt to both Freshwater and Saltwater?

Fish are classified as either freshwater or saltwater based on their habitat, and this is the key difference between saltwater and freshwater fish. But there are odd fish called euryhaline fish. Examples of these euryhaline fish are salmon and eels. They have the ability to tolerate huge changes in outside salinity, either because of their life cycle. They are capable of switching from one kind of osmoregulation to another.

Following are the reasons why fishes survive in seawater and freshwater constantly.

  • Most fish species can only survive in one or the other based on their salinity tolerance, or how much salt their bodies can handle.
  • Euryhaline organisms are able to adapt to a wide range of salinities, an example of a euryhaline fish is the molly (Poecilia sphenops) which can live in freshwater, brackish water, and or salt water.
  • Diadromous fishes have evolved mechanisms to transition between seawater and freshwater environments.
  • All fish, whether they live in saltwater or freshwater, 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.

Can a freshwater fish adapt to saltwater?

No, freshwater fish can't survive or adapt to saltwater due to dehydration. In freshwater fish water inside their bodies would flow out of their cells and they will die due to dehydration because seawater is too salty for freshwater fish.

Is there any fish that can survive in both saltwater as well as freshwater?

Yes, there are some fish species that can live in both saltwater as well as freshwater. These species are called euryhaline fish.

Check out the list of fish that can survive in both saltwater and freshwater:-

Gulf sturgeon

Gulf sturgeon are fish that migrate into brackish and salt water during the fall and feed there throughout the winter months, and on the other hand in the spring, they migrate into freshwater and remain there through the summer months. Gulf sturgeon can grow to be 9 feet in length and weigh over 300 pounds. Females grow larger than males, and they inhabit both saltwater as well as freshwater habitats.

Green sawfish

Green sawfish live in Marine and in freshwater habitats. The green sawfish is the largest species of sawfish and grows 24 feet in length. All sawfish are harmless if left undisturbed. They are found in the Indian and western Pacific oceans, sawfish are actually a Species of ray.

Atlantic Stingray

The Atlantic Stingray has been known to enter freshwater and can tolerate varying salinities. They prefer water temperatures between 59-86°F. They feed on benthic invertebrates, Marine worms, crabs, etc.


Barramundi lives in both freshwater and saltwater. They eat almost anything, including other barramundi, and can consume prey up to 60% of their own length. They can grow up to 200 cm in length and 60 kg. These fish during their lifecycle can change sex from male to female.

Sockeye salmon

These species are considered "anadromous" which means they live in both freshwater as well as saltwater. They are born in freshwater where they spend a few months to a few years (depending on the species) before moving out to the Ocean. At the time of release or deposit of eggs, they head back to freshwater.

Bull shark

Bull sharks are fish species that can thrive in both salt and freshwater. They are known to travel far up rivers. Bull sharks are normal prey, but will eat almost anything, but their diet consists mainly of fish. Bull sharks even eat other Sharks. They hunt during the day and at night. Normally sharks must keep salt in their bodies to survive, and most can live only in saltwater. But bull sharks have developed special adaptations.