Living organisms are classified as either endothermic or ectothermic. 

Yet, the focus of this article is on the fish. 

A lot of us wonder if fish are cold-blooded or not.

Where exactly do they fall in?

And what exactly is the best way to identify them?

After a lot of research on this topic, this article was put together to give you a clearer understanding of the fish, what determines their temperature and how you can use this knowledge to your advantage.

Are Fish Cold-blooded?

An endothermic animal is one that can generate and control its body heat.

This means that even in an environment with extreme temperatures, the system can work up a climate that balances out the environment.

An endothermic animal is also called a warm-blooded animal.

Some warm-blooded animals are; 

  • mammals (humans, cows, dogs, cats, lions, etc.) 
  • birds (pigeons, hens, owls, kites, ducks, etc.)
  • And some species of fish.

For instance, when a dog wanders into a cold environment, the nervous system gets a signal and begins to regulate its temperature from within. This helps protect the dog from the adverse effects of the cold.

This process used to regulate body temperature in mammals is thermal homeostasis absent in cold-blooded animals.

Meanwhile, an ectothermic animal is one whose body temperature depends on that of the environment. 

They are incapable of regulating their body heat, bringing about change in body temperature according to the environmental temperature. These animals are cold-blooded animals.

Some of them include; 

  • amphibians (toads, frogs, salamander, newts)
  • reptiles (lizards, snakes, chameleons, turtles)
  • and other invertebrates (animals without backbones).

Why is Fish Cold-blooded?

Fish are mostly classified as cold-blooded because they are poikilothermic animals, which is a direct opposite of the homeothermic(animals that can maintain a constant body temperature).

you can also read: Are Fish Herbivores Or Carnivores Or Omnivores?

Their poikilothermic attribute means that their temperature depends on the surrounding body of water.

This is the very first feature that qualifies the fish as a cold-blooded animal.

If the water becomes cold, their body temperature reduces; then, if the water becomes warm due to the weather, their body temperature becomes warm too.

They are unable to generate and circulate heat within their system.

Cold-blooded fish cannot survive outside of their natural habitat; being moved from their habitat to a place with unsuitable temperatures will have adverse effects on their well-being.

Are all Fish Cold-blooded?

Apart from the dolphins, whales, and other mammals that live in the ocean, you would hardly find any warm-blooded fish by nature.

Almost all fish are cold-blooded because they do not have the features that warm-blooded animals have. But there’s always an exception to the rule.

The National Ocean Service has recognized the opah, also known as the moonfish, as a fully warm-blooded fish.

It is the only fully warm-blooded fish discovered because it has unique blood vessels around the gills, which circulates warm blood throughout the body.

The opah is a predatory fish, meaning that it feeds on other smaller fish and other prey.

They typically live very deep down in the sea.

Some other fish that are partly warm-blooded is tuna fish and mackerel sharks. They have those unique blood vessels like the opah fish that enable them to circulate warm blood and keep warm.

Theirs is regional endothermy. This means that the circulation of heat within the body doesn’t occur all over. It is limited to a specific part of the body.

The tuna and mackerel sharks can produce body heat and may survive in extreme weather conditions, yet their temperature can not remain constant.

They do not have the capacity for thermal homeostasis.

There may be other warm-blooded fish in existence, but research shows that they are yet to be discovered.

In 2019, the National Geographic Organization stated that humans do not yet explore more than 80% of the ocean.

There is still a lot more to discover and learn as time goes by.

  • Species [examples] of Cold-blooded Fish

Almost every species of fish lies in this category; this is the popular group. There are thousands of fish species in the cold-blooded category, but I’ll list just a few.

Some of the cold-blooded fish are;

  • Catfish
  • Sunfish
  • Trout
  • Shark
  • Salmon
  • Pike
  • Carp
  • Tilapia
  • Buffalo fish
  • Barracuda
  • Herring
  • Eel, etc.
  • Species [examples] of Warm-blooded Fish 

The warm-blooded group is the road less traveled. The least popular of the groups. The only known examples of the warm-blooded fish are:

  • The Opah fish/Moonfish
  • The Tuna fish
  • Mackerel sharks

Difference between Cold and Warm-blooded Animals

Due to the difference in temperature levels, cold-blooded and warm-blooded animals react differently to environmental changes.

Making a table to list the differences between cold-blooded and warm-blooded animals here will help you understand how various fish species are affected by changes in their environment and temperature.

The significant differences between cold and warm-blooded animals are:


The temperature of animal rises or falls with the environmental temperature.

The temperature remains within the range of 35-40 degrees Celsius.

Fluctuating temperature due to environment

Consistent temperature, despite the environment.

Do not have the capacity to survive in extreme weather conditions.

Can adapt to almost any environment due to the ability to regulate heat.

The body temperature of the animal is dependent on the external environment.

Body temperature is independent of the environment.

Change in the environment affects the rate of metabolism.

Change in the environment has no impact on metabolism.

Need a small amount of food to consume a more significant amount of food, to produce more heat.

This group consists of fish, invertebrates, reptiles, and amphibians.

This group consists of mammals, birds, and select species of fish.

If you own an aquarium or a fish pond, or you’re trying to get one, you must understand what temperature of water your fish likes best.

Because their body temperature depends on their surroundings, your aquarium’s temperature level will either be beneficial or harmful to your fish.

It’s not advisable to maintain a particular temperature range in your pond/aquarium for a long time. It would help if you switched up the temperature now and then. 

This is because water that is too warm lacks enough oxygen, and fish may die off. Simultaneously, water that is too cold due to the weather can make your fish sluggish and overweight.

You must also be careful not to change the water temperature too many times in a day as this can be harmful to your fish.

Fish are often sluggish in cold water and more active in warmer temperatures.

This is why you must give your best by ensuring the temperature level is never too high or low. The best temperature for cold-blooded fish is between 24-27°C.

Sort out the fish you intend to keep according to their preferred temperature levels and not keep fish whose temperature requirements vary widely together. This helps you avoid a situation where one fish species is getting more significant due to better treatment, while the other species are dying of inactivity or infections.

I wish you happy fishing!