You might be tempted to answer this question with "Of course they don't! They're swimming in water." Or maybe you start thinking about how swimming in saltwater makes you thirsty. So, might fish feel the same?
The short answer is we don't know for sure if fish get thirsty. "It's impossible to know what a non-human animal truly experiences," says Tillmann Benfey, professor of fish physiology and aquaculture at the University of New Brunswick, Canada in an email interview.
But you? You know exactly when you're thirsty. You might experience a range of symptoms, perhaps dry mouth, fatigue, reduced urine output, lightheadedness, and weakness, among others. And when you're really, really thirsty – like lost in the desert without a canteen type of thirst – you simply crave water, sometimes to a degree that you can hardly think about anything else. That powerful urge is what keeps our bodies from becoming dangerously dehydrated.
But because we can't put ourselves in a fish's shoes (fins?), there's no way for us to know how fish perceive thirst, or if they do at all. "I think of thirst as a cognitive response to hydration and it's hard to get inside a fish's brain," says Boston College assistant biology professor Christopher Kenaley via email. "However, we do know quite a bit about how fishes regulate water balance."
Regardless of their thirst drive (or lack thereof), these creatures absolutely need hydration to stay alive. They regulate water balance via a process called osmoregulation, which is common to many other vertebrates, including humans.
Ultimately, says Kenaley, osmoregulation maintains the appropriate amount of salts and water in the body, and two main organs facilitate this process. First, the kidneys kick into gear, helping to maintain salt levels, and second, the gills have special cells that exchange water and salt with the environment.
Notably, the process varies quite a lot depending on where the fish lives.
Freshwater Fish Vs. Saltwater Fish
Freshwater fish don't actively drink water because it dilutes their blood and bodily fluids. "The challenge for a freshwater fish is different than a marine fish. For freshwater fishes, the blood and tissues are much saltier than the external environment and thus water follows this osmotic gradient, i.e., the body is a salty sponge," emails Kenaley. "So, the challenge in this case is to keep water from diluting the body. To counter this, the kidney of a freshwater fish expels a lot of water from the blood and creates very dilute urine." Freshwater fish are almost always peeing out this diluted urine, he notes, while their gills are constantly pumping salts back into the body using the specialized salt cells.
On the other hand, saltwater or marine species often drink water through their mouths to keep hydrated. The challenge there is to avoid losing water to the much saltier environment – and to keep excess salt out. "Their kidneys remove salt and conserve water while the salt cells in their gills pump salt into the water. Using these different directions of passing salt and water, the bodies of marine and freshwater fish are equally hydrated and salty," says Kenaley.
Then You've Got the Salmon....
But what about anadromous fishes, like salmon, which swim through both freshwater and saltwater? "When adult salmon migrate into freshwater in order to reproduce, there is often a 'staging area' where the salmon hang out before completing their migration," says Rebecca Asch, an assistant professor of fisheries biology at East Carolina University via email. "This staging area is located so that the fish are exposed to some brackish (fresher) water so that they can gradually gain osmotic competency before migrating into their freshwater spawning grounds."
And as climate change rapidly warms the world's waters, fish like salmon may experience fast-changing conditions regarding temperature stability in water columns around the globe. This can dramatically alter the way fish adjust to the water's characteristics.
Asch says that in some cases where warming causes sea ice to break off and float freely in a salmon staging area, "there will not be a lot of mixing between fresh and saltwater because the sea ice produces a large amount of freshwater as it melts and this serves as a barrier preventing mixing." In that scenario, this barrier prevents salmon from being exposed to brackish water, which delays their ability to adapt to varying salt content in their environment.
Aren't We All Fishes?
If all of this sounds fishy to you, it shouldn't – our own bodies share similarities with fish. Because humans share evolutionary history with fish, says Christopher Kenaley, "you can even make the case that we're a type of fish. After all, we descend from them. Thus, we have retained many of the same osmoregulatory mechanisms our fishy ancestors had, and modern fishes still use."
That doesn't mean you should put your fishy credentials to the test, especially when it comes to saltwater. All animals can ingest a bit of seawater (which is about 3.5 percent salt by weight), but it won't sate your thirst. Instead, you'll get thirstier by the minute as your body uses water to eliminate excess salt from your bloodstream. Freshwater fish that wind up in seawater will have a similar experience, eventually dying as their bodies become overly dehydrated.