Exploring Underwater Hydration: Do Fish Get Thirsty?

Fish live in the water. Does that mean they never get thirsty?

Mar 31, 2023By Heather Jarek, BA Veterinary Technology
do fish get thirsty

Fish require water for survival. Humans and land animals rely on the gaseous form of oxygen. Fish rely on oxygen that is dissolved in water. Their unique gills are filled with blood capillaries that absorb the oxygen from water. This means that fish cannot survive extended periods outside of water. So, does an animal that needs water to live and breathe get thirsty like we do? To answer that question, we need further discuss the relationship between fish and water.

What is Thirst?

person drinking water

What does it mean to be thirsty? The term “thirst” means the urge to drink water. As a human, you are probably very familiar with the feeling of being thirsty. You may notice suddenly feeling thirsty after a long day of work and head straight to the fridge for a bottle of water. Or maybe you just finished a workout and are overcome with the need to down a bottle of ice-cold water. Sometimes the urge to drink water is minimal, while other times, you feel as if you could drink an entire barrel of water. This driving force to drink water is your body’s way of keeping you from becoming dehydrated. Humans, and other land animals, need to actively seek out water to stay appropriately hydrated. Fish live in water. Therefore, they don’t need the conscience thought to drink water simply because it is all around them. Living in water means there is no threat of becoming dehydrated.

Do Fish Drink Water?

goldfish jumping out fishbowl

Fish have gills that absorb oxygen from water. Does this also mean they don’t need to drink water as we do? The answer to this question is not “yes” or “no,” but depends on the type of fish we are discussing. Freshwater fish don’t need to actively drink water. These fish have a higher salt concentration in their bloodstream than the water they swim in. The amount of water they take in through their gills is enough to keep them appropriately hydrated. If a freshwater fish were to drink water, it would cause its blood to become too diluted, damaging its bodily organs. Saltwater fish run the risk of losing water to their salty environment. This is because seawater has a higher salt content than the fish swimming in it. This requires saltwater fish to process water and salt through their gills and drink water through their mouths to stay hydrated. This regulation of water and salt balance for saltwater and freshwater fish is called osmoregulation.

What is Osmoregulation?

colorful tropical fish

Osmoregulation is the process that regulates the osmotic pressure and electrolyte balance in humans and other animals. Simply put, it maintains the correct amount of salt and water in the body to keep all organs running efficiently. For fish, their gills exchange water and salt with their environment, while their kidneys play an essential role in maintaining the salt levels within the body. This process of osmoregulation happens subconsciously. Meaning humans and animals do this without having to think about it. The process of osmoregulation in saltwater and freshwater fish is slightly different. This is due to the difference in their environments.

Osmoregulation in Freshwater Fish

group fresh water fish

Osmosis is the flow of water from low salt-concentrated areas to areas of high salt concentrations. For freshwater fish, this means water is constantly trying to move into the fish’s higher salinity body through the fish’s thin skin. Too much water would severely dilute the fish’s blood, causing organ failure and death. This is why freshwater fish don’t drink water and have a unique process of osmoregulation to avoid taking in too much water. The kidneys of a freshwater fish are incredibly efficient and can excrete water at a fast rate. They also reabsorb the salt from the fish’s urine to meet the body’s demands. While the kidneys work hard to pump out the excess water, the fish’s gills are constantly working to pump salts back into the body.

Osmoregulation in Saltwater Fish

saltwater moorish idol fish

Saltwater fish face the opposite problem as freshwater fish. Saltwater fish need to use osmoregulation to keep water in their body rather than losing it to their salty environment. To achieve this goal, a saltwater fish must drink water through its mouth to keep its body balanced and hydrated. A saltwater fish’s kidneys work to remove salt and conserve water. Their gills have specialized salt cells that pump salt into the water and away from the body. A saltwater fish will urinate very little compared to a freshwater fish. This is to conserve as much water as possible.

What About Fish Living in Salt and Freshwater?

salmon swimming upstream

Anadromous fish are fish that migrate from the sea into rivers to spawn. These fish are born in freshwater and live most of their lives in saltwater. They will return to freshwater only to spawn. Salmon is a typical example of an anadromous fish. Salmon and other anadromous fish have the unique ability to move salt in the direction they need it to go. They do this by using what is called NKA pumps. These pumps are cell enzymes that move salt in and out of the body’s cell walls. These fish spend time in a “staging area” before completing their migration. This staging area is a mixture of salt and freshwater and gives the fish time to adapt and switch their NKA pumps in the appropriate direction. Once they are ready, they will continue to swim upstream into fresher waters.

So, do Fish Get Thirsty?

school fish swimming together

To get thirsty, you need to feel the urge to drink water. Saltwater fish drink water only because they constantly use osmoregulation to balance their bodies’ salt and water levels, not necessarily because they are thirsty. Freshwater fish constantly absorb water through their gills and skin due to osmosis, so it’s doubtful they ever feel thirsty. It’s impossible to know if fish feel genuine thirst as humans and animals do. This is because we can never honestly know what a fish thinks or feels. However, most experts agree that due to a fish’s water-filled environment, it’s doubtful they get thirsty.

Heather Jarek
By Heather JarekBA Veterinary Technology

Heather is a lover of all animals, big and small. She graduated with a bachelor’s degree in veterinary technology in 2014. She has been working as a licensed veterinary technician for the last eight years. Her favorite hobby is horseback riding, and she has been riding horses since the age of eight. She enjoys spending time with her family at the lake with their golden retriever Calista in her free time.