Why are Tuna Warm-Blooded?

When you’re deep sea fishing near Hawaii for a living, you learn a lot of interesting facts about fish. Among other things, these facts help up figure out their behavior and habits. Here’s one of the weirdest fish facts you’ll ever know.

Some fish are warm-blooded. We learn in school that mammals are warm-blooded while reptiles and fish are cold-blooded. This is overwhelmingly true, but some fish developed a version of warm-bloodedness to help control their body temperature. The only fish that are warm-blooded like this are tuna and mackerel sharks (including everyone’s favorite, the Great White Shark).

This warm-bloodedness isn’t as complete as that of mammals. Tuna have blood vessels that help them control the temperature of organs and swimming muscles. The key place tuna lack this trait is in their gills. This is key because water passes through these, exchanging a lot of heat as it does so.

In fact, there’s only one fish that has these kind of blood vessels in their gills. The opah is the only fish that has full-body warm bloodedness, but you won’t find it unless you’re in the waters off Antarctica. Believe us, it’s a lot more comfortable deep sea fishing near Hawaii.

Tuna have what’s called regional warm-blooded traits. This means that they’re really somewhere between warm- and cold-blooded. Bet you never thought of those two things as a sliding scale before, did you?

Tuna in Hawaii don’t have to raise their body temperature much because the waters here are always warm and relaxing. Yet their warm-blooded traits do influence tuna’s behavior. Tuna in Hawaii can get an extra burst of speed from the muscles being warmed up. In a 10 second sprint, a bluefin tuna can accelerate up to 30 mph. That’s pretty good for an animal that weighs 130 pounds on average, and can get up to a weight in excess of 900 pounds. That burst changes the way we fish tuna in certain ways, and science tells us why they have that burst in the first place: their muscles are already warmed up.