Are fish endotherms

No, catfish are not endothermic. Endothermy uses methods such as shivering, sweating, panting, storing and burning fat to keep the body temperature at a constant rate.

Catfish, and indeed all fish are actually poikilothermic. This means that there body temperature rises and falls with their environment. When a fish is in cold water their body temperature falls to match it, and when it moves to warm water it rises again. Fish can move deeper in the water too cool down and swim higher when they need more warmth. By doing this, the fish is able to regulate its body temperature.