THE fish of the Scombridae family are all beautifully coloured for upper, sunlit layer of the ocean: the back a dark, intense blue, the belly a silvery white to merge with the mirrored ceiling of the sea.
Mackerel add dorsal ripple marks to confuse their predators still further. Admiring the colours of some fresh-caught fish as I split them for the grill the other evening, I pondered on the camouflage value of this dark-blue-and-silver for the mackerel's huge ocean-going relative, the Atlantic bluefin tuna. What could possibly menace such a muscular, seven-foot torpedo? But even Thunnus thynnus has to start off young and small. A fish hatched off south-west Spain in June weighs barely half a kilo today. And as an adult it does, indeed, have a predator to reckon with: the killer whale, three times its size. For a tuna, 18 years is a good age. Then, at a weight of 500 kilos or more, it may bite incautiously at a squid dangling in mid-water and itself end up in raw, incredibly costly, slices in a sashimi restaurant in Tokyo, The presence of a Japanese tuna fleet somewhere out there off Galway has been a revelation, and not just of Tokyo restaurant prices. The sheer application, the scale of an essentially passive fishing effort, the patient weighing of odds, have something unwestern about them: the world of Zen-per cent.
Yet all we're seeing is the standard Japanese technology for fishing tuna - long, long lines floating on a big, big ocean. A 70-mile line is not at all exceptional, and it makes more sense when you learn that there are only perhaps, 25, hooks to the mile, each baited with a squid. The aim is not to catch a lot of small fish, but to intercept some very big ones, swimming far apart.
The Scombridae species generally - mackerel, bonito, albacore, skipjack - are schooling fish, and the bluefin tuna, too, often schools near the surface when young. But on its long, fast-swimming migrations northwards and westwards, the size of a bluefin school can thin out to match the size of the fish, and the largest tuna of all may swim alone.
There's almost an inbuilt need to migrate, given the tuna's unusual physiology. It's a warm-blooded fish with a great thirst for oxygen, so it has to swim fast and constantly, to drive water over its gills. Its powerful muscles make it capable of extraordinary bursts of speed - well over 40 mph - but they also create a powerful predator's appetite, a driven quest for food.
Its home waters are the Mediterranean, and the Atlantic to the west of Spain and Portugal, but many bluefin migrate northwards after spawning - up the west of Ireland and on to Norway, Iceland or even Munansk. While shoals of herring survived in the North Sea, tuna would swing round the north of Scotland, sometimes in schools of 20 or more, to circle the nets of the drifters off the Yorkshire coast. There was even. for a while, an annual gathering of big-game fishermen at Scatorough, who landed tuna up to 850 lb or whatever that is in kilos. Jane Grigson, the cookery writer, remembered "dark perfect shapes hanging against the usual grey summer sky. Their tails brushed the ground almost, but tall men had to put their heads back to look up at them".
Design for a purpose is so often beautiful, and the streamlining of the bluefin is perfect. Its fins are all crescent-shaped, aerodynamic. The first dorsal fin flicks back into a groove and disappears; the pelvic and long pectoral fins fold, back into hollows, so that the spindle-shaped body, almost perfectly circular in section, can reach, speeds as smoothly as a dolphin.
THE fish's warmth, several degrees above the surrounding water, demands constant regulation. Bluefin can actually overheat, so they like to stay close to the "fronts", the meandering, eddying boundaries between masses of water of different temperature. And these fronts, for other reasons, are also rich in plankton and thus in food such as squid.
But temperatures can also fix the limits to the seasonal migration of some tuna species. Albacore, for example, the bluefin's smaller, metre-long, relation, follows water which has a surface temperature of 18C. In the average year, the edge of this isotherm nudges close to the southern coast of Ireland; in some summers, it reaches the latitude of Dingle Bay.
It's almost 30 years since an expert sent by President Kennedy pointed out the potential for Ireland of this tuna fishery, so long commanded by the French.
Today, after much developmental work by BIM and the Fisheries Research Institute, about 10 small trawlers from Castletownbere take a gamble on finding the schools of albacore, perhaps 160 miles to the south-west.
Locating them means days of steaming and searching, matching blips on the sonar to the movements of seabirds, the blowing of whales, the ruffles on calm water that betray the presence of a shoal. "Tuna," as one French skipper has said, "is a trade of patience and a hunting full of maybes." Once discovered, the albacore must be tackled with a drift-net of a mere 5.2 km - not much longer than those used for salmon. The restriction comes from ICCAT, the International Commission for the Conservation of Atlantic Tuna (of which, it should be said, both Japan and Korea are members, but not Ireland, which merely "observes").
America has been especially vocal on tuna conservation. Yet the fortunes of the Atlantic bluefin produce an unexpected irony. One migration route takes the young tuna across the Atlantic to the Caribbean and up the north American coast as far as Newfoundland. They are harassed for much of the way by legions of American sports fishermen, wannabe Hemingways out in launches who account for a remarkable 65 per cent of the Atlantic bluefin catch - far more than the commercial fishermen. Most of the fish they take are between one and five years old - and the bluefin does not spawn until it is eight. If its population starts a significant decline, it will not all be the fault of the far-flung Japanese, dangling squid in the deeps west of Gal way.
