Bluefin tuna can keep their hearts pumping during temperature changes that would stop a human heart: Adrenalin is the triggering key factor. Scientists at the University of Manchester and at Stanford University in the USA discovered that adrenalin keeps the heart of Bluefin tuna pumping: “When tunas dive down to cold depths their body temperature stays warm but their heart temperature can fall by 15°C within minutes. The heart is chilled because it receives blood directly from the gills which mirrors water temperature. This clearly imposes stress upon the heart but it keeps beating, despite the temperature change. In most other animals the heart would stop”, explained Dr Holly Shiels. Shiels elaborated on their findings, “We discovered that changes in the heartbeat due to the temperature coupled with the stimulation of adrenalin by diving adjusts the electrical activity of the heart cells to maintain the constant calcium cycling needed to keep pumping. If we went through this temperature change, our calcium cycling would be disrupted, our hearts would stop beating and we would die. ” The scientists are now exploring the bluefin's heart at the warmest temperatures possible as this may be where their limitations are. To date, the studies have pushed the bluefin heart to 28 C. This is a temperature that sea surface can reach in the spawning grounds in the Gulf of Mexico. It may rise as the earth warms. My biggest bluefin to date died despite the fact, unfortunately. I hooked the fish in 1997 off the Azores island of Faial. He had taken a marlin lure and went down more than 750 m. I brought him up to 400 m when he died. After 45 minutes of winding and planning it to the surface, we found out why: The fish had hit the sea floor and its gills had been covered with mud. So, it seems that the reason for the heart attact was a lack of oxygen, not the chilly temperature down there. You can find more about bluefin and other tuna in my book. It is now available as an App for IOS and Android systems. Take a look at www.ultimatefishingbooks.com  for some free reading.