Do Fish Drink Water? Freshwater vs Saltwater Fish and Sharks

A largemouth bass under water with its mouth slightly open — Photo: Robert Pos (CC BY 2.0)

Freshwater fish barely drink at all; saltwater fish drink all day long. That sounds backward, but it follows from one rule: water always moves toward the saltier side. A bass in a lake is saltier than the lake, so water pushes into it through its gills and skin without a single sip. A cod in the ocean is less salty than the sea, so water is pulled out of it, and it has to drink to keep up.

A resting rainbow trout makes about 2.3 ml of urine per kg of body weight every hour. Over a day that is about 5% of its weight, roughly an ounce (25 ml) from a 1 lb (0.45 kg) trout (Curtis and Wood 1992, Journal of Experimental Biology).

Why doesn’t a freshwater fish need to drink?

Because water gets in without it. Fish blood carries salt, more than a lake or river does. Water moves through thin skin and gill membranes toward the salty side, a process called osmosis. So a bluegill, a bass or a catfish soaks up water every second of its life, whether it wants to or not. Drinking would only make the flood worse.

The fish handles the flood two ways. Its kidneys make a lot of very watery urine. In the rainbow trout study by Curtis and Wood, resting trout made about 2.3 ml per kg of fish every hour, which is where the 5% a day comes from. FISHBIO puts the top end for freshwater fish at up to a third of their body weight a day. At the same time, special cells in the gills grab salt from the passing water and pull it back into the blood.

Doesn’t a fish swallow water every time it opens its mouth? No: that water is for breathing. A freshwater fish sends all the water that comes into its mouth out through its gills, while a saltwater fish directs some of it into its gut (McGill Office for Science and Society). A freshwater fish does swallow some water with its food. That is eating, not drinking.

Why does a saltwater fish drink all day?

Turn the rule around. Seawater is saltier than fish blood, so water flows out of the fish. A snapper or a striped bass in the ocean is always drying out from the inside. To replace what it loses, it drinks seawater constantly.

Seawater brings salt with it, and the fish has to get rid of that. Cells in the gills called chloride cells pump salt back out into the sea. The kidneys make only a small amount of concentrated urine, so little water is wasted. In short, freshwater fish fight to keep salt in and water out; saltwater fish fight to keep water in and salt out.

How is a shark different?

Sharks found a third way: they keep a lot of urea, a waste product most animals flush out, in their blood. That makes their blood about as salty as the sea, so water drifts in gently instead of being pulled out, and a shark does not have to drink like a bony fish. The salt that still leaks in leaves through a small organ at the end of the gut, the rectal gland. The same urea is why a dead shark gradually starts to smell strongly of ammonia: bacteria break the urea down into ammonia. The bull shark is the odd one out: it can change how its kidneys work to dump large amounts of urea, which lets it live in fresh water.

	Freshwater fish	Saltwater fish	Shark
Drinks?	Almost never; water floods in on its own	Yes, constantly	Little; urea in the blood lets water drift in
Urine	Lots, very dilute; about 2.3 ml per kg an hour in resting trout	Very little, concentrated	Little; urea is kept, not flushed
Salt	Pulled in through the gills from the water	Pumped out through gill chloride cells	Shed through the rectal gland
Example	Largemouth bass, channel catfish, rainbow trout	Red snapper, cod, striped bass in the sea	Blacktip shark, tiger shark

Do fish get thirsty?

Fish have a drinking switch that works much like thirst; whether it feels like thirst, nobody knows. There is no published study of thirst as a feeling in any fish. In people, thirst is the brain’s alarm that the blood is getting too concentrated. What has been measured in fish is the urge to drink, and it turns on and off on cue.

Tetsuya Hirano (1974) fitted Japanese eels with a tube in the gullet so every swallow could be measured:

A freshwater eel that suddenly lost blood started drinking at once.
A freshwater eel with salt dripped slowly into its blood drank steadily.
Moved to sea water, a freshwater eel started drinking immediately, and it stopped immediately when it went back into fresh water. The chloride in sea water was the trigger.
A seawater eel drank less when its stomach or gut was stretched full.

Hormones carry the signal. Takei, Hirano and Kobayashi (1979) studied angiotensin and water intake in the same eel. Angiotensin II is known as the most important drinking hormone throughout backboned animals, and in eels it makes the fish drink when it acts inside the brain. From the blood, a different hormone, isotocin, is the stronger trigger (Nobata and Takei 2022).

So a fish drinks when it loses blood or its blood gets saltier, the same jobs thirst does for you. Whether flipping that switch feels like anything to the fish is unknown. The same gap turns up in whether fish sleep: plenty of behavior, few measurements of what it feels like.

Do fish pee?

Yes, and a freshwater fish pees far more than you would expect for its size. It does not dribble: resting trout in the Curtis and Wood study held urine in the bladder and let it go in bursts of about 0.85 ml per kg, about every 25 minutes. At the trout rate, a 1 lb (0.45 kg) fish passes about an ounce (25 ml) a day. A 5 lb (2.3 kg) bass at the same rate would pass about 4 oz (125 ml), though nobody has measured a bass the way Curtis and Wood measured trout. A saltwater fish makes only a trickle.

Can a freshwater fish live in the sea?

No. Drop a bluegill in seawater and osmosis works against it at once. Water leaves through its gills and skin, salt pours in, and its gill cells are wired to pull salt in, not push it out. The fish dehydrates while its blood gets saltier, and it dies. A saltwater fish in fresh water has the opposite problem: it swells with water it cannot pump out fast enough.

A few fish can switch worlds. Salmon, striped bass and eels move between rivers and the sea by rebuilding their gills under the control of hormones such as cortisol, growth hormone and prolactin, which change the number and size of the salt-handling cells (FISHBIO). A sudden swap kills an ordinary fish.

What does it mean at the water?

Bait fish die in the wrong water. Freshwater shiners in a saltwater livewell, or saltwater pilchards in lake water, are dying from the moment they hit it. Their kidneys and gills are built for one world. Ask what water bought bait lived in and keep it there. For catfish, the best catfish bait guide covers which live baits are worth the trouble.

A fish out of water dries from the gills first. Gills are thousands of paper-thin folds that only work spread apart in water. In air those folds collapse and stick together, and oxygen uptake stops. A gasping fish is not thirsty; it is suffocating with its gills glued shut.

How long can a fish go without water? For a bass, trout or panfish, seconds count. Ferguson and Tufts (1992) chased rainbow trout to exhaustion, the state of a fish after a hard fight, then held some in air. Twelve hours later 88% of the trout with no air time were alive, 62% after 30 seconds in air and 28% after 60 seconds; do fish drown has the rest of that study. A 2015 review of air exposure (Cook and colleagues) is cited for a limit of about 10 seconds, and the catch and release page has the rest of a clean release. A few air-breathing fish are built for it: the arapaima of South America can survive out of the water for up to 24 hours, and lungfish ride out droughts burrowed in mud (Discover Wildlife).

Keeping bait fish alive

Match the water. Fill the bucket from the water the bait came from. Never freshwater bait in saltwater or the reverse.
Skip the hose. Tap water is a different world again. Use lake, river or sea water only.
Keep it cool and steady. The AFTCO Bass Care 101 targets come from a Mississippi State University study of largemouth bass funded by ten state fisheries agencies; use them as a guide for bait: 84 °F (29 °C) or less, no jump of more than 7 °F (4 °C) at once.
Put air in it. A battery aerator keeps dissolved oxygen at the 5.5 ppm AFTCO asks for. Bait hanging at the surface with its mouth open is short of oxygen, not thirsty.
Do not crowd. Crowded bait uses up oxygen and fouls the water fast. Fewer fish live longer.
Change a third of the water at a time, from the same source, so nothing changes suddenly.

For more questions like this one, start at the fish facts hub.

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