Do Fish Get Cold? Winter, Ice and the Fish With Antifreeze

Two small fish on a line lifted through a hole in thick lake ice, orange amphipods scattered on the snow — Photo: Brücke-Osteuropa, Wikimedia Commons (public domain)

Key takeaways

Fish are cold-blooded: a fish's body is the temperature of the water around it. It does not shiver; the cold reaches it as slowness, in its muscles, its heart and its digestion.
A cunner stops moving at 45 °F (7.1 °C); a pumpkinseed all but stops at 46 °F (7.9 °C). The savings come from sitting still plus chemistry slowing by itself, not a special sleep.
The Antarctic fish Notothenia swims 20 times less in winter and more than halves its heart rate. Largemouth bass slow down but keep swimming and eat a little.
Polar fish do not freeze because of antifreeze proteins in their blood. The trick evolved twice, in Antarctic notothenioids and in Arctic cod.
For anglers: fish slow, fish small, put the bait on the fish's nose, expect a short fight, and unhook in the water when the air is below freezing. Late winter under ice is hardest.

Do fish get cold? Yes, all the way through: a fish’s body temperature is the temperature of the water around it. When the pond drops to 40 °F (4 °C), the fish is 40 °F too. But it does not shiver, and it does not feel chilly the way you do on a January morning. Cold reaches a fish as slowness: its heart beats less often, its muscles pull more slowly, and its food takes longer to digest.

A big Atlantic bluefin tuna can hold its swimming muscle up to about 38 °F (21 °C) warmer than the sea around it. The opah goes one better: it is the first fish shown to warm its whole body, heart and brain included.

Are fish cold-blooded?

Yes. A fish has no warm inside the way you do. Scientists call fish ectotherms, which means “heat from outside.” A dog or a duck burns food to stay at one steady temperature, whatever the weather. A fish takes the temperature of the water and stays there. Part of the reason is the gills. Every drop of a fish’s blood passes through the gills to pick up oxygen, and the gills sit in the water. Any warmth the body makes leaks straight out into the lake.

A few big ocean fish have found a way around this. Bluefin tuna run their warm blood past their cold blood in a heat exchanger. That tight net of vessels hands the warmth back before it reaches the gills. Carey and Teal showed in 1969 that bluefin even regulate it, holding more of the warmth in cold seas than in warm ones. Tags on big Atlantic bluefin have since read red muscle up to about 38 °F (21 °C) warmer than the water, and even a young Pacific bluefin manages about 20 °F (11 °C). The opah takes it further: Wegner and colleagues showed in 2015 that it warms its whole body, not only its muscle. A pumpkinseed in your local pond can do none of this. Its body is the pond.

What does cold feel like to a fish?

Nobody can ask a fish. But we can measure what cold does to it, and the answer is the same as for honey in a refrigerator: everything moves slower. Its muscles contract more slowly. Its stomach empties more slowly, so it wants less food. A fish at 40 °F (4 °C) is not suffering. It is running on a low idle.

It does notice, though. Fish move toward water that suits them when they can. The water temperature chart shows the band each species prefers, and the bass chart and trout chart put numbers on it.

What do fish do in winter?

Most of them slow down and do very little. A 2022 study of how fish get through winter, by Reeve and colleagues, gives the numbers. The cunner, a small wrasse of the North Atlantic coast, stops moving when the water falls to 45 °F (7.1 °C) or below. The pumpkinseed, a common pond sunfish, all but stops at 46 °F (7.9 °C); it never goes fully still, and a few kept eating a little even in the coldest water tested.

The savings come from two places, and neither is a special winter sleep. First, the fish stops swimming, so it stops paying for swimming. Second, its chemistry slows down by itself, because that is what cold does to chemistry. There is no special torpor, no hibernation switch like a bear’s. A fish holding still in cold water can look asleep, but sleep in fish has its own signs.

Where do they go? Down. Fresh water is heaviest at 39 °F (4 °C), so under the ice the bottom is the warmest layer in the lake, and that is where most fish gather. NOAA’s Ocean Service puts it the same way, and adds that a few, such as koi, burrow into soft mud and go dormant. Why the warm water sits at the bottom in winter is the story of lake turnover.

The champions live under the Antarctic ice. A 2008 study recorded the heart rate and movement of Notothenia coriiceps, a bottom-living Antarctic fish, through the year. In winter the fish swam 20 times less. Its resting metabolism dropped by a third and its heart rate more than halved.

Largemouth bass never get close to that. A 2025 study tracked wild largemouth through fall and winter. Heart rate, activity and metabolism fell as the water cooled, and stayed low. Yet the fish kept swimming slowly all winter, and the researchers’ energy model says they likely ate now and then; most still came out of winter lighter. A winter bass is a slow bass, not a switched-off one.

Species	Stops moving at	Winter behavior	Source
Cunner (North Atlantic wrasse)	45 °F (7.1 °C) or below	Sits still; savings from inactivity plus chemistry slowing on its own	Reeve et al. 2022
Pumpkinseed sunfish	46 °F (7.9 °C) or below (nearly still)	Minimal activity, not zero; feeding fades out by about 39 °F (4 °C), though a few keep nibbling; no special dormancy	Reeve et al. 2022
Notothenia coriiceps (Antarctic)	Never fully; swims 20 times less	Resting metabolism down a third, heart rate more than halved, all winter	Campbell et al. 2008
Largemouth bass	Never fully; slows through fall	Heart rate and activity low and stay low; keeps slow-swimming, likely eats now and then, most lose weight	Reeve et al. 2025
Antarctic notothenioids and Arctic cod	Never; keeps swimming in ice-cold seawater	Antifreeze glycoproteins in the blood stop ice crystals growing	Chen, DeVries and Cheng 1997

Do fish freeze?

A pond fish does not, because it is never colder than the water it swims in. Fresh water turns to ice at 32 °F (0 °C), and under the ice the water stays liquid, so the fish does too.

The sea is a harder place. Salt water can be colder than 32 °F and still not freeze, and a normal fish in it would turn to ice from the inside. Polar fish carry a fix in their blood: antifreeze glycoproteins, molecules that stick to the tiniest ice crystals and stop them growing. Chen, DeVries and Cheng traced where the gene came from in 1997. In the Antarctic notothenioids it grew out of a gene for trypsinogen, a digestive enzyme. Arctic cod, at the other end of the planet, carry almost the same antifreeze but arrived at it by a different route. Nature invented fish antifreeze twice. The strangest of the Antarctic family, the icefish, went further and gave up red blood cells; they are on the do fish have bones page.

Can fish under ice run out of air?

Yes, but ice by itself is not the danger. The water under it holds oxygen, and a slowed-down fish needs less of it. The trouble is snow. When snow piles up on the ice it blocks the light, and the plants and algae below stop making oxygen. Nothing comes in from the air either, because the ice seals the surface. So the oxygen levels fall all winter and are lowest in late winter, just before the ice goes out. In a shallow, weedy pond with a soft bottom, fish suffocate. That is winterkill, the most common fish kill there is, according to Michigan DNR; the dead fish are often found only a month after the ice leaves.

How should you fish cold water?

Fish bite in cold weather. They bite slowly, seldom and close, so the whole game is pace and placement.

Slow down, then slow down again. The fish’s nerves and muscles run at winter speed. A lure that races past is gone before the fish has decided. Move a bait a few inches, then hold it still.
Fish small. A fish running on a fraction of its summer energy wants a meal that costs almost nothing to catch. Small jigs, single baits, thin line.
Put it on the fish’s nose. Winter largemouth keep swimming slowly and eat now and then. They take what arrives in front of them and will not chase.
Expect late winter to be the hardest ice fishing. Oxygen is lowest then, and fish that are short of air eat least. Early ice, with the oxygen still high, is the easy part.
Do not expect a fight. A cold fish’s muscles pull slowly and tire fast, so a bass in 40 °F (4 °C) water comes in like a wet sock. The fish gives up quickly; unhook it quickly too.
Release in the water when the air is below freezing. The fish is at water temperature and its gills and eyes are wet. Freezing air is harder on them than on you. Keep the fish in the hole, wet your hands, take the hook out and let go.
Watch the weather before the water. A cold front knocks the shallows down a few degrees in a night, and the fish slow with it. The day after a winter warm spell is the day to go.

More questions about how fish work are answered in the fish facts collection.

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