Do Fish Drink Water?

Many marine fish, i.e., those that live in seawater, drink large quantities of water, while freshwater fish typically drink very little water. This difference is due to the process of osmoregulation—in this case, how a fish maintains its internal balance of water and salts.

Seawater has a higher concentration of salts than what most types of marine fish need in their bodies. This environment causes them to lose water from their bodies through osmosis. To counteract this, many types of marine fish drink large quantities of seawater. Their kidneys excrete the excess salts while retaining as much water as possible. This helps them maintain their internal balance of water and salts, which is crucial for their survival. This is not true of all marine fish, however. Some, such as sharks and rays, have a concentration of salt in their bodies that is similar to that of seawater, and they are able to maintain a proper balance without needing to drink water.

Freshwater fish, on the other hand, live in water with a much lower concentration of salts as compared to what they need in their bodies. This means that water is naturally absorbed into their bodies (through osmosis). Because of this, freshwater fish do not need to drink large amounts of water, though they do drink a little and ingest some with their food. Large amounts of excess water are excreted through their kidneys, which helps maintain the proper balance of water and salts in their bodies.

Some fish, like salmon, are known to travel between freshwater and seawater during their life. These fish have the ability to adjust their physiological processes to cope with the changing conditions. They spend time in brackish water (slightly salty water), which helps them adapt to their new environment.