Atlantic bluefin tuna heat their own muscles to hunt in cold seas
Few fish run warm, but the Atlantic bluefin tuna controls its body temperature better than any other. A heat-trading web of blood vessels keeps its swimming muscles warm, letting a fish that can top 680 kilograms chase herring and squid through the frigid, food-rich North Atlantic and dive past a kilometre deep.
The trick is countercurrent exchange. In a network of vessels called a rete mirabile, warm blood leaving the muscles passes its heat to cold, freshly oxygenated blood coming in, so little warmth escapes into the sea. All tunas are warm-blooded to some degree, but the bluefin takes it furthest. Its blood carries one of the highest haemoglobin concentrations among fish, across an exceptionally thin barrier between blood and water, so oxygen loads fast. Power flows through a pair of tendons to a crescent-shaped tail that beats while the body stays rigid, a highly efficient stroke. A light-sensitive pineal window in the head is thought to help it navigate a range spanning thousands of miles.
Adults average 2 to 2.5 metres and about 225 to 250 kilograms. The largest caught under game-fishing rules, off Prince Edward Island, weighed 679 kilograms and measured 3.84 metres, and in 1934 six men off Nova Scotia took turns fighting one tuna for 62 hours. Very large fish may live 50 years.
Life starts perilously. A female can release up to 40 million eggs at once, which hatch in two days; within a week the larvae are quarter-inch cannibals, and only about 0.1 per cent survive their first year. Eastern fish spawn around the Balearic Islands in the Mediterranean, western fish in the Gulf of Mexico, where they dodge the warm surface by staying below 200 metres, and researchers proposed a third ground, the Slope Sea off the northeastern United States, in 2016. Most tagged fish roam the ocean but return to spawn where they were born.
That habit of massing to spawn made them easy targets, with spotter planes guiding purse seines in the Mediterranean, and the Japanese sushi market drove prices high. By 2009 stocks had fallen 72 per cent in the eastern Atlantic and 82 per cent in the western. A proposed global trade ban failed in 2010, but quotas brought partial recovery, and in 2021 the IUCN moved the species from Endangered to Least Concern, though western stocks remain severely depleted.
Source: Atlantic bluefin tuna
