Fish
Tuna
Thunnini · also called tunas, tunny
A tuna is a fish engineered for one thing: moving fast through open water, for its whole life, without a break. It is warm where other fish are cold, rigid where other fish flex, and it breathes only because it keeps swimming. Stop a tuna and it suffocates and sinks.
Overview
Tuna are the tribe Thunnini, around 15 species in warm and temperate seas worldwide, from the 2 kg bullet tuna to the Atlantic bluefin at over 3 m and 600 kg. All are fast-swimming open-ocean predators that hunt schooling fish and squid, and several are partly warm-blooded — almost unique among fish.
The warm-blooded fish
Bluefin, yellowfin, and their relatives keep a heat exchanger, the rete mirabile, that traps the warmth their muscles make. The core of a bluefin can run 10 to 20 degrees C above the sea, keeping its muscles powerful and its gut fast even in cold water. This lets bluefin follow prey from the tropics into near-polar seas that would leave an ordinary fish sluggish.
Shaped by drag
A tuna’s body is a stiff spindle. The fins fold into grooves, the tail stalk carries stabilising keels, and thrust comes almost entirely from a tall crescent tail whipping while the body stays still. It breathes by ram ventilation — mouth held open, water forced over the gills by forward motion — so a tuna that stops moving stops breathing, and with almost no swim bladder it also sinks.
Value and fishing pressure
Tuna are among the most valuable wild fish on Earth; a single Pacific bluefin has fetched over three million dollars at auction. That value drives intense fishing. Fast-breeding skipjack copes well, but the slow-maturing bluefins, which gather predictably to spawn, were fished down hard.
Status
Tuna as a group are Not Evaluated. Skipjack and much yellowfin fishing is reasonably sustainable; Atlantic bluefin has recovered strongly since stricter quotas around 2010; Pacific bluefin remains depleted but is slowly improving.
Tuna: common questions
How are tuna warm-blooded if they are fish?
They are not fully warm-blooded like a mammal, but several tuna keep parts of their body far above sea temperature, which is very rare in fish. The trick is a heat exchanger called a rete mirabile, a dense mesh where warm blood leaving the swimming muscles runs alongside cold blood coming in from the gills and hands over its heat before it can be lost. This keeps the red muscle, and in bluefin also the eyes, brain, and gut, warm and working efficiently. Warm muscle contracts faster and delivers more power, and a warm gut digests large meals quickly, which lets bluefin hunt in cold water that would slow an ordinary fish to a crawl.
Why can't a tuna stop swimming?
Two reasons, both to do with how the body is built for speed. First, tuna breathe by ram ventilation, meaning they hold their mouths slightly open and let their forward motion push water over the gills rather than pumping it with the cheeks, so if a tuna stops it stops breathing. Second, tuna have reduced or lost the gas-filled swim bladder that most fish use to hover, so a motionless tuna slowly sinks. The streamlined shape that makes a tuna fast also makes slow, hovering swimming inefficient. The result is a fish that swims continuously from the day it hatches until it dies.
How fast can a tuna swim?
Fast, though the highest figures are often exaggerated. In a hard burst after prey or away from a predator, a large yellowfin or bluefin can probably reach around 70 kilometres an hour for a few seconds, making tuna among the fastest fish alongside marlin and wahoo. Cruising speed is far lower, roughly 3 to 7 kilometres an hour, sustained around the clock, which is how they cover ocean basins. The crescent tail, the finlets that smooth the water flow, the retracting fins, and the rigid body all serve to make both the sprint and the endless cruise as efficient as possible.
Are tuna endangered?
It depends entirely on the species and the population. Skipjack, the small tuna in most canned products, is abundant and generally fished within sustainable limits. Yellowfin is heavily fished and overfished in some oceans but not globally threatened. The bluefins are the concern, since they grow slowly, mature late, and gather predictably to spawn, which made them easy to overfish. Atlantic bluefin was in serious trouble by the 2000s but has rebounded well under stricter international quotas, and Pacific bluefin, though still depleted, is now slowly recovering. Careful management, not a blanket ban, is what has turned the bluefins around.