The internal body temperature of fish in water varies and generally matches the temperature of their surrounding environment. Fish are ectothermic creatures, commonly known as cold-blooded animals, meaning they cannot regulate their body temperature internally. Instead, they rely on external factors, like the water temperature, to maintain their body temperature.
The body temperature of a fish typically closely mirrors the temperature of the water in which they reside. If the surrounding water temperature changes, the body temperature of the fish will adjust accordingly. This is why fish in cold water environments, like polar regions, tend to have lower body temperatures compared to fish living in warmer waters.
In some specific cases, certain fish species, such as tunas and some sharks, have the ability to elevate their body temperature above that of the surrounding water, termed regional endothermy. They achieve this by having special physiological adaptations that help them generate and retain internal heat, allowing them to maintain higher body temperatures in specific parts of their bodies compared to the surrounding water.