Click for detail! Interestingly, Leatherback Turtles have a warmed body core and Swordfish, a rudimentary heat exchanger allowing them to reduce heat loss.
Warm-blooded Fishes like Mackerel Sharks and Tuna perform better = swim faster and migrate further. Despite of the high cost of keeping one's temperature elevated, these species appear to derive a comparable advantage over ectothermic Fishes, and I cite.
In conclusion, our comparative analyses indicate that a potential ecological advantage of RM endothermy in fishes is the ability to cruise faster, which not only increases prey encounter rates, but also enables larger-scale annual migrations and greater access to seasonally available resources. We suggest that this advantage, coupled with the previously recognized benefit of thermal niche expansion, could outweigh high energetic costs incurred by RM endothermy and, thus, has facilitated the radiation and diversification of tunas and endothermic sharks. Our analyses also indicate that fishes with RM endothermy are similar to birds and mammals in many respects, including not only high metabolic rates and temperature dependence of muscle function, but also fast cruising speeds and the capabilities of large-scale migrations.
Now you know! :)