Tuna run warm-blooded engines through open ocean
Fifteen Thunnini species in five genera—from fifty-centimetre bullet tuna to giants—share a rare fish trick: with opah and mackerel sharks, they keep body heat above seawater. Streamlined hunters of warmer seas, they rank among the fastest pelagic fish and among the most heavily fished.
The tribe Thunnini holds 15 species in five genera, spanning the bullet tuna, which tops out near 50 cm and 1.8 kg, to the Atlantic bluefin, which can reach 4.6 m and 684 kg and may live around 50 years. The English word runs back through Spanish atún and Andalusian Arabic to Greek thýnnos, itself tied to a verb for darting along. Until 1999 Atlantic and Pacific bluefin were lumped as one species; B.B. Collette then showed they are distinct.
Every line of the body is built for speed. The first dorsal fin, stiffened with spines, can fold flat into a groove, seven to ten yellow finlets lead to a crescent tail, and a slender tail stalk carries three keels on each side. Yellowfin can hit about 75 km/h. The limit is physical rather than muscular: cavitation bubbles wrap the fins in a vapour film, and because the bony fins lack nerves, tuna do not feel the damage that dolphins find painful.
Tuna share with opah and mackerel sharks the rare fishy ability to run warmer than the sea. A rete mirabile, a tangle of veins and arteries, passes heat from outgoing blood to incoming blood, warming muscle, eyes and brain; bluefin hold a core of 25–33 °C in water as cold as 6 °C. Loads of myoglobin explain why the flesh is pink to dark red rather than white.
Appetite has followed. Annual catches of the five main market species climbed from about 300,000 tons in 1940 to over 4 million tons, mostly by purse seine, with skipjack about 60 percent of the haul. In 2006 Australia accused Japan of taking 12,000 to 20,000 tonnes of southern bluefin a year against an agreed 6,000, and that species is now threatened. A 278 kg bluefin fetched a record 333.6 million yen at a Tokyo auction. Dolphin-safe netting has had its own cost, shifting fleets to floating aggregation devices that raise bycatch of sharks and turtles.
Source: Tuna
