Trout fishing is understandably a popular sport. Trout takes some skill to catch and put up a good fight when you hook into one. They also live in some of the most spectacular places on earth, allowing for a wide range of very enjoyable fishing experiences. But have you ever wondered if these predominantly freshwater fish have the ability to live in seawater?
Some trout species have anadromous forms that live in saltwater while returning to freshwater to spawn. These fish have the ability to survive in both saltwater and freshwater habitats. To do this, their bodies have to go through a gradual adaptation. Otherwise, they couldn’t survive the change in the environment.
Freshwater or saltwater fish cannot survive if their environment’s salinity falls outside a specific range.
In general, fish are highly sensitive to their surroundings’ salt content and are adapted to use the salt in the water around them to maintain homeostasis. It may seem counter-intuitive, but it could become dehydrated and die if a fish is not adapted to saltwater conditions.
Can Trout Survive in Saltwater?
The fish we know of as “trout” actually belong to three different genera of the subfamily Salmoninae, meaning that they are not necessarily closely related. Inherently, they are quite diverse in their traits, adaptations, and home habitats. In general, however, there are two types of fish:
- Euryhaline: This refers to fish adapted to live in environments with a wide range of salinity levels
- Stenohaline: This refers to fish that cannot survive notable changes in salinity levels.
Several species of trout can survive in saltwater, making them euryhaline by definition, but the conditions are important. These types of trout would need time to slowly acclimate to the changes in salinity- if you were to simply throw a freshwater trout in the ocean, it would likely die.
Trout and Osmoregulation
Fish, including trout, are in a homeostasis state when their internal salinity is balanced with their external environment. They maintain this balance through the process of osmoregulation. (This video does a fantastic job of explaining osmoregulation in less than three minutes.)
Put simply; osmoregulation dictates whether a fish needs to take in water to maintain proper hydration or release excess water. That is determined by the relative salt concentration of the water around them.
Osmoregulation in Saltwater Trout
You’ve heard the idiom, “to drink like a fish,” but did you know saltwater fish actually drink water for a reason?
Because saltwater has a higher salinity externally, saltwater trout constantly loses freshwater from their cells to their environment. To survive, they have to constantly drink water and filter out the salt to strike homeostasis.
They have impressive kidneys to filter the salt out of the water they drink. Still, because they are continually fighting a battle against dehydration, they have no physical mechanism for letting out excess water.
This means that if saltwater trout were placed into freshwater, their cells would continue to drink in more water than would exit their cells, eventually causing their cells to become over-hydrated and burst, killing them.
Osmoregulation in Freshwater Trout
On the other hand, freshwater trout are continually taking freshwater from their surroundings because their internal salinity is higher than their environment. So, to prevent their cells from exploding, freshwater trout are constantly secreting.
While these fish do “drink” some water (simply because it flows in when they open their mouths), they do not have the physical adaptations to filter salt out of the water they drink.
Therefore, if a freshwater trout were to suddenly find itself in saltwater, it would quickly lose all the water from its cells to its outside environments. And with no mechanism to replenish its water supply, the fish will become dehydrated and die.
Osmoregulation in Euryhaline Trout
You may recall from above that euryhaline trout can survive in both salt water and fresh water environments. This is because they are adapted to take in water, filter it, and excrete excess, depending on their environment.
These adaptations show up at different stages of their life cycle in trout species, which we will explore in more detail below. But suffice it to say, the only fish that can survive in both fresh water and saltwater environments have the capability to both drink and excrete.
Trout and Salt Water
Stenohaline refers to aquatic organisms that cannot tolerate salinity changes and are usually not well adapted to extreme water temperature changes. Some trout can adapt to salinity changes at some point in their lives, making them euryhaline trout that are mostly acclimated to freshwater or saltwater.
Most trout are euryhaline but spend most of their lives in either freshwater or saltwater.
- Freshwater Trout: These trout live most of their lives in a freshwater environment such as rivers or lakes. An example of freshwater trout is the brown trout. There are populations of these types of trout, however, that spend a lot of time in oceans, too.
- Saltwater Trout: These trout live most of their lives in saltwater environments, existing most commonly in the coastal oceans. Some examples of a saltwater trout includes the Steelhead Rainbow Trout, The sea run Brown Trout or Sea Trout ,and some populations of Coastal Cutthroat Trout.
- Many people (understandably) believe that Spotted Trout, Speckled Trout, and the Weakfish trout are also examples of saltwater trout; however, these fish are called trout but belong to an entirely separate family of fish called drum Sciaenidae
Euryhaline Trout and Salt Water
Euryhaline refers to any aquatic organism that is adapted to tolerate changes in salinity and water temperature. Applied to trout, this means the fish can be adapted to live in both rivers and oceans.
Trout adapted to live in both freshwater and saltwater are known for migrating between the two during different life stages. In general, there are two types of Euryhaline fish that go carry out different states of their life cycles in different habitats; they are:
- Anadromous fish: These fish spawn in freshwater but otherwise spend the majority their lives in saltwater, only migrating back to freshwater to reproduce (like the coastal Rainbow Trout.)
- Catadromous fish: These fish spawn in saltwater but otherwise spend their lives in freshwater, only migrating back to saltwater to reproduce.
The Life Cycles of Catadromous fish Trout
One great example of an anadromous trout is the Rainbow trout. Some types of rainbow trout, like the Steelhead designation, spend their lives in the ocean before returning to freshwater to spawn.
Here we will focus on the Steelhead trout as an example. The Steelhead trout is one member of the Rainbow trout species that lives an anadromous lifestyle and has developed unique adaptations:
- The Steelhead spawns in freshwater streams, where the conditions are suitable for nesting.
- Once the juvenile steelhead hatch, they will remain in the freshwater habitat for up to two years while they develop. This is because they are not born with the ability to survive in saltwater. They must first smolt.
Smolting refers to the physiological changes that adolescent steelheads experience that allow them to survive in saltwater conditions. Once that happens, they can migrate back and forth between the ocean and freshwater rivers several times throughout their lives.
It is advantageous from an evolutionary perspective to be able to migrate because the freshwater habitats are better protected for spawning, while the ocean habitats offer more food.
As mentioned at the beginning of the article, however, you cannot suddenly transfer a Steelhead from the ocean into a freshwater habitat, as the fish takes cues from its life cycle to know when they’re able to venture into different environments.
Trout live in diverse habitats, being found in both saltwater and freshwater habitats. But despite this, they cannot simply be plucked out of a saltwater environment and plunged into a freshwater habitat.
Trout are highly sensitive to their surroundings’ salinity, and even the slightest deviations can cause them to become dehydrated or overhydrated and cause them to die. Even the trout adapted to survive in both habitats cannot be forced to respond to salinity changes quickly.
Therefore, the only trout that can survive in saltwater are anadromous saltwater trout like the Steelhead Rainbow Trout, Sea Run Brown Trout, and Coastal Cutthroat Trout.