Fish that can use both freshwater and saltwater are generally called euryhaline fish. Some, such as Atlantic salmon and American eels, migrate between the two during their life cycle. Others, including mullet, mudskippers and red drum, tolerate changing or intermediate salinities for longer periods. This does not mean every individual can survive an abrupt transfer from tap water to seawater: tolerance depends on species, life stage, acclimation, temperature and water chemistry.
Fish known to use both freshwater and saltwater
The examples below represent different strategies. “Anadromous” fish grow mainly in the sea and return to freshwater to reproduce; other euryhaline fish remain in estuaries or shift habitat when conditions change.
| Fish or group | How it uses fresh and salt water | What makes it useful to compare |
|---|---|---|
| Atlantic salmon | Anadromous: born and spawn in freshwater, grow in the ocean | Life-stage migration and smoltification |
| Eulachon | Anadromous smelt moving between marine and fresh waters | Reproductive migration |
| Shad and American eel | Use both environments during their life cycle | Different directions and timing of migration |
| Mullet (Mugil cephalus) and mudskippers (Periophthalmus) | Euryhaline fish associated with estuaries and variable salinity | Broad estuarine tolerance |
| Red drum | Recognized as a euryhaline fish in Texas guidance | Capacity to occupy changing salinities |
| Blue tilapia | Freshwater-origin fish reported to tolerate saltwater | Acclimation and tolerance limits |
| Snook and tarpon | Marine-associated fish that can also live in freshwater | Shift from saltwater toward freshwater habitats |
These categories are not interchangeable. A salmon’s ability to migrate is a programmed life-cycle process, while a resident estuarine fish may regulate its body fluids across a wide salinity range without making a long-distance migration.
What “euryhaline” and related terms mean
Euryhaline
A euryhaline fish can tolerate a broad salinity range that may include freshwater, brackish water and seawater. The width of that range differs by species, population and life stage.
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Anadromous
Anadromous fish spend a major growth stage at sea and return to freshwater to reproduce. NOAA describes Atlantic salmon as anadromous because they live in both fresh and saltwater. Eulachon are another example.
Stenohaline
Stenohaline fish are restricted to a relatively narrow salinity range. Most fish fall into this category and live exclusively in fresh or salt water rather than switching freely.
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Brackish
Brackish water has an intermediate salinity, typical of estuaries where rivers meet the sea. It can serve as a permanent habitat or as a transition zone during migration.
How fish switch between salinities
Fish must keep the concentration of salts and water in their body fluids within safe limits. The surrounding water determines which way water moves by osmosis and which ions must be conserved or expelled.
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In freshwater
- Body fluids are saltier than the surrounding water, so water enters the fish osmotically.
- The fish actively takes up ions through specialized cells in the gills and other tissues.
- Its kidneys produce large amounts of dilute urine to remove excess water while conserving salts.
In seawater
- The fish tends to lose water to the saltier environment.
- It drinks seawater and absorbs water and salts through the gut.
- Gill ionocytes actively secrete excess ions, while the kidneys excrete additional salts.
Euryhaline species adjust these systems rather than using one fixed setting. Reviews describe tissue-level responses triggered by salinity and coordinated endocrine signals that alter ion transport in the gills and other osmoregulatory organs. Mozambique tilapia, for example, can compensate for continuous, marked external salinity changes while keeping key osmoregulatory measurements within narrow ranges. That ability is still condition-dependent, not unlimited.
Why salmon cannot be moved instantly from fresh to salt water
Atlantic salmon illustrate the difference between migration and an instant environmental switch. Young salmon develop in rivers, then undergo smoltification: a physiological preparation that enables them to maintain salt balance after entering the ocean. Adults later return to freshwater to spawn.
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Because this preparation involves coordinated changes in gills, kidneys and hormones, a salmon that is not at the appropriate life stage should not be assumed ready for seawater. The same principle applies to other migratory fish: their natural movement follows timing, development and gradual environmental cues.
What this means for aquariums
A species label alone is not an acclimation plan. Even when a fish is described as euryhaline, tolerance can vary with size, life stage, population, temperature, dissolved oxygen, pH, hardness and the rate of salinity change.
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- Identify the intended habitat. Determine whether the fish is being kept in freshwater, brackish water or marine water, and whether that matches the requirements for its current life stage.
- Change salinity gradually. The cited sources establish physiological adaptation but do not provide one universal aquarium schedule. Follow species-specific husbandry guidance rather than applying a generic timetable.
- Measure salinity. Use a reliable refractometer or calibrated hydrometer and record changes instead of judging by appearance.
- Match the full water chemistry. Salinity is only one stressor; temperature, pH, hardness, oxygen and ammonia also affect survival.
- Watch for stress. Loss of balance, rapid breathing, clamped fins, abnormal swimming or refusal to eat are reasons to stop the change and restore stable conditions appropriate to the fish.
Do not infer that all salmon, eels, mollies or other members of a named group tolerate every salinity. Confirm the requirements for the exact species and life stage before planning a transfer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare a fish’s freshwater–saltwater ability
- Classify the strategy: migration between habitats or continuous tolerance of variable salinity.
- Check the life stage: juveniles, adults and spawning fish may have different limits.
- Find the documented salinity range: “can enter seawater” is not the same as “thrives indefinitely at full marine salinity.”
- Assess acclimation needs: determine how quickly the species can adjust and what environmental conditions are required.
- Account for local conditions: temperature and water chemistry can narrow the practical tolerance range.
How common is year-round intermediate-salinity living?
Virginia Cooperative Extension reported in 2025 that 32 fish species live year-round in the Chesapeake Bay’s intermediate-salinity conditions. Estuaries therefore support more than temporary passage: for some fish they are permanent habitats where salinity fluctuates with tides, rainfall and river flow.
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