Do Fish Get Thirsty? Do They Ever Need a Drink for Hydration?

Fish may appear to have an unlimited amount of water at their disposal being aquatic organisms. But have you ever wondered if these animals ever feel thirsty?

Thirst: The Body's Cry for Water

In the most basic terms, 'thirst' refers to the urge to drink water. It is just the brain's way of telling that you are dehydrated because of lack of body fluids. It is also a need that must be satisfied by both plants and animals.

According to biologists, thirst is a subjective perception that serves as an important component of the body's survival system. For instance, humans show this desire in varying degrees to make sure that a healthy balance of water and salts is maintained inside their body. Aside from dehydration, other causes of thirst in humans include food, pregnancy, medication, diabetes, and other medical conditions.

All animals living on land are faced by the threat of dehydration which urges them to drink water when they feel thirsty. In our case, we feel a range of symptoms when we feel thirsty such as dry mouth, lightheadedness, fatigue, reduced urine output, and weakness, among others.

Do Fish Get Thirsty?

Fish don't get thirsty like the way it is perceived by humans or other animals, but they also need to regulate water balance in order to survive and maintain general health. How much water a fish consumes depends on the amount of salt present in its surrounding habitat.

As a solvent, water interacts with substances such as salt across cell membranes of living organisms. In a process called osmosis, water flows across a membrane from a region of low solute concentration to a region of high solute concentration until the cell reaches equilibrium with its external environment.

Depending on their surroundings, fish drink some water through their mouths, but they mostly absorb it through their skin and gills via osmosis. Both freshwater and saltwater fish show different behaviors when it comes to their urge to drink water. This need is further related to the anatomy of their bodies.

Freshwater fish do not actively drink water because it could dilute their blood and bodily fluids. This is because their blood contains higher concentration of salt than the water they are surrounded by. To maintain the balance of salt and water inside their bodies, they absorb water through their skin and gills. Then their kidneys expel a lot of water from the blood and create a very dilute urine to remove all of that excess water. Freshwater fish are always peeing out this dilute urine while their gills constantly pump salts back into the body using specialized salt cells.

Meanwhile, saltwater fish often drink water through their mouths to keep hydrated. They face the challenge of preventing water loss to the much saltier environment while keeping excess salt out. Seawater has 4.7 ounces per gallon (35 grams per liter) of dissolved salt, while most fish blood has 1.2 ounces per gallon (9 grams per liter) of salt. This imbalance causes the fish to lose water to the external environment and be invaded by salt into its cells and inside its body. As a result, a saltwater fish is always thirsty and needs to drink water all the time.