Anyone who has experienced burnout (I know it's all of you, lets not forget what site we are using) may find this fact a little reassuring.
The vast majority of fish are poikilothermic, meaning that their body temperature is determined by the surrounding environment. The Bluefin tuna (Thunnus thynnus) however, is considered to be regionally warm-blooded, or somewhere in between the two extremes. This predator is able to use muscle movement to generate heat and transport this heat in their blood to some regions of the body, in a sort of basal ectothermy.
This is a very helpful adaptation for the roughly 1.5m terror, the ability to stay close to optimal temperature for energy production combined with a smooth, teardrop shaped body built to reduce drag makes this fish fast, agile and formidable.
Unfortunately for our friend here, the extreme power of their swimming muscles comes at a cost. Tuna can accelerate around 30 mph in a ten second burst, built for ambush and not prolonged speed. When stressed, the fish can push their body to the limit and reach such quick acceleration that the heat produced by the sudden muscle use can cook the tuna inside-out in an event fishers call 'burning'.