Are all fish cold-blooded?
Not all fish are cold-blooded. In 2015, researchers with the NOAA Southwest Fisheries Science Center revealed the opah, or moonfish, as the first fully warm-blooded fish. Although not as warm as mammals and birds, the opah circulates heated blood throughout its body, giving it a competitive advantage in the cold ocean depths from 150 to 1,300 feet below the surface.