For centuries, humans have mythologised the bluefin tuna, an elite, warmblooded fish that can traverse the Atlantic basin in less than a month and a half and grow to weigh three-quarters of a ton. Romans put bluefin on their coins; Salvador Dali painted them.
Now, researchers are using hard science to prevent the fish from going extinct.
Analysing facets including chemical markers in the tuna's ear bones and satellite readings generated by tags attached to migrating fish, marine biologists are beginning to decipher how separate bluefin populations travel and spawn. And those distinctions, they say, may help determine whether fishery managers can preserve the Atlantic's remaining giant tuna.
“We know phenomenally more about bluefin now than we did 15 years ago. We know enough to save this species,” said Michael Sutton, vice president and director of the Centre for the Future of the Oceans at Monterey Bay Aquarium in California. “We don't have the will.”
As demand for the tasty fish has soared among sushi connoisseurs — and as Americans and Europeans face off over who bears the most responsibility for plummeting Atlantic bluefin stocks — scientists have helped answer one of the key questions now confronting fishery managers: Two separate populations of bluefin swim in the Atlantic basin. In other words, humans cannot afford to let either stock disappear completely, because they are genetically distinct.
For years, Europeans have been overfishing bluefin tuna that breed in the Mediterranean. In fact, for the past four years European Union officials have set catch quotas at nearly double the levels their scientists recommended, and fishermen have exceeded those already-elevated quotas by 50 per cent each year. In the United States, the federal government has imposed greater restrictions, but fishermen can still bring home bluefin tuna they incidentally catch as the fish are spawning each spring in the Gulf of Mexico.
“We have some excellent science,” said John Graves, a professor at the Virginia Institute of Marine Science who studies bluefin by conducting DNA analysis. “The real challenge here is to get the science translated into management, and that's where we're hitting a roadblock.”
The fact that bluefin from the Mediterranean and those from the Gulf of Mexico mix most of the year has led some fishermen to think the stock off America's East Coast has been faring better than it actually is, Graves said. Instead, he added, fishery managers need to realise that western Atlantic bluefin off the East Coast are in dire shape. There are two other species of bluefin, Pacific and southern, which face their own fishing pressures.
“What you have now are essentially two stocks, but the only time the stocks are separated is during spawning,” Graves said, adding that the two Atlantic populations do not interbreed. “We need to protect the western stock, because that's a unique genetic entity.”
For years, researchers knew little about how pelagic fish such as bluefin travel, feed and mate. But new developments have allowed scientists to better understand a species that can fetch more than $170,000 for a single fish.
“It's the Porsche of the ocean,” said Sutton, whose aquarium keeps bluefins in its Outer Bay exhibit. “It's as fast as a sports car, it weighs as much as a sports car and it's as valuable as a sports car.”
Satellite tags — which can record the temperature of the ocean as well as the depth at which the fish swim and the level of light to which they are exposed — have provided scientists with a precise record of how bluefin tuna travel. Stanford University marine biologist Barbara A Block and her colleagues tagged two fish within a matter of minutes off western Ireland; within eight months, the fish were more than 3,000 miles apart. One had travelled to waters just northeast of Cuba; the other swam off the coast of Portugal.
Block's team has used a different kind of electronic tag, which they surgically implant in bluefin tuna, to record at regular intervals not only where the tuna go but how their bodies react to the surrounding environment. This year, a student of Block's, Steve Teo, published a paper chronicling that the tuna warm as they engage in courtship, and tend to stay in warmer waters such as the Gulf of Mexico during this period.
Jay R Rooker, a marine biologist at Texas A&M University, has used a different technique to reach the same conclusions as Graves and Block. Rooker and his colleagues examine otoliths, or ear bones, of bluefin to determine where the fish originate and journey, because the chemical composition of these ear bones reflects the water in which the fish have been swimming. By reading the isotopes of carbon and oxygen contained in these ear bones, Rooker can isolate bluefin from the Mediterranean and the western Atlantic.
“If you look at them over many years, they do look different,” Rooker said.
The discovery that the two stocks are genetically different has prompted the realisation that the number of western-stock bluefin that US fishermen are catching is inflated by migrants from the Mediterranean. These findings have prompted several scientists and conservationists to call on the National Marine Fisheries Service (NMFS) to shut down longline fishing in the Gulf of Mexico between April and June while bluefin tuna are spawning there. Federal officials have resisted these calls and are fighting a lawsuit on the issue filed in federal district court in Washington by Earthjustice on behalf of the advocacy group Blue Ocean Institute and its president, Carl Safina.
NMFS Director William T Hogarth agrees that bluefin tuna are in trouble off the East Coast. “We're catching nothing,” he said, noting that US fishermen have collected less than 15 per cent of their official catch quotas for the past five years.
But he has devoted most of his energy to pressing the Europeans to cut back on bluefin catches, something they have steadfastly refused to do. “It's totally out of control,” Hogarth said, adding that when it comes to halting fishing in the Gulf of Mexico, “If the science tells us that is what it's going to take to recover bluefin tuna, then the federal government will consider it and probably do it.”
Safina, speaking by telephone as he conducted research in Belize, said the science was already settled.
“To say there's not enough science to tell us whether we need to protect the last few fish that are trying to breed on our side of the ocean, that is just nonsensical,” he said. “I believe that is illegal. The law requires better stewardship than (government officials) sitting on their hands and doing nothing.”
LA Times
