Most fish are cold-blooded. “Cold-blooded" is a general term used to describe organisms that are unable to regulate their own body temperature. Instead, the organisms body temperature is solely affected by external temperatures, specifically the animals ambient surroundings.

In contrast, warm-blooded animals are able to regulate their body temperature by converting the food they eat into energy and thus, create heat.

Although most people refer to all types of aquatic animals as fish, there are many exceptions. For instance, organisms such as seals, whales and dolphins are not considered fish; these three aquatic animals are also warm-blooded. In addition, certain fish such as salmon sharks, billfish and blueface tuna are partially warm-blooded. They are able to raise their temperature, but only in certain parts of their body such as their eyes and brain.