What does it mean to warm-blooded?
A living creature which has been classified as being warm-blooded has the ability to maintain its body temperature higher than the environment. A cold-blooded creature has the ability to make its body temperature change in response to the surrounding temperature, whereas a warm-blooded creature will keep its body temperature relatively constant regardless of its surroundings.
Examples of warm-blooded creatures include mammals and birds.
Examples of cold-blooded creatures include reptiles, amphibians, and fish.
Is there a warm-blooded fish?
Alhough scientists like to package creatures into nice easy categories like warm-blooded or cold-blooded, nature in all of its exquisite beauty and complexity tends to adapt and not follow the rules that we attempt to impose on it. If you go to the provided resource, you will find an example of a fish that has been designated as being warm-blooded. This is the Opah (Lampris guttatus).
Always remember that nature has a way to adapt and evolve. Thus, we should be cautious about making global statements regarding natural phenomena.