Holy mackerel: A breakthrough in tuna science?

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TOKYO — The next time you dine at a Japanese restaurant, try to steer clear of the tuna sashimi. If you're unable to resist the temptation  — and, let's face it how many of us can? — make sure you savor every last slice. In just a few years, it may have disappeared from the menu for good.

Our appetite for the undisputed “king of sushi,” whose succulent flesh is prized by diners at high-class restaurants from Tokyo to London and New York, is far from being sated.

Only last week the conservation group WWF warned that Mediterranean bluefin tuna stocks were on the verge of collapse, and the breeding population just three years from extinction, as a result of overfishing and a failure to curb our desire for melt-in-the mouth otoro.

Demand in Japan, and increasingly the U.S., Europe and China, is decimating stocks among the world’s four bluefin populations. The number of Mediterranean bluefin, for example, has more than halved since the 1950s.

Attempts at imposing ambitious quotas have had little impact. Although fisheries from several countries agreed on new bluefin quotas late last year, they were still some 47 percent higher than the levels recommended by their own scientists — a political fudge that environmental groups condemned as a “disgrace.”

While Japan is often cast as the villain of the piece for its voracious consumption of bluefin, it may also offer the key to the species’ survival, thanks to a team of researchers working out of a laboratory in Tokyo.

The team’s leader, Goro Yoshizaki, a professor at Tokyo University of Marine Science and Technology, has perfected a method of assisted reproduction in which sperm and ovaries from donor trout are implanted in salmon recipients. When the salmon reach maturity and mate, they produce a large number of hybrids, but also a smaller number of pure trout.

Last year his work reached a critical point when he identified the presence of sperm of a nibe croaker in the testes of a mackerel, saltwater fish that have physiological similarities to tuna.

Now Yoshizaki is in a race against time to save the imperiled bluefin.

He believes he is only a few years away from adapting the technology to enable him to transplant sperm and ovary stem cells from bluefin tuna to mackerel, and for the recipient mackerel, when mature, to produce a precious bounty of bluefin sperm and eggs.

The biggest obstacle is obtaining enough stem cells from bluefin testes to produce both eggs and sperm. Preliminary experiments have proved unsuccessful, but the professor is certain he is close to a breakthrough.

Success, he says, depends on his ability to exploit the sexual bipotency of enriched stem cells from bluefin to produce both sperm and ovaries in mackerel.

“The hypothesis is that the bluefin tuna has some stem cells in its testes, but that the concentration is very low,” he tells GlobalPost. “If that’s the case, and we can find a way to enrich them, then we should be able to repeat the success we had with the salmon and trout.”

Replicated on a big enough scale, the process could produce masses of tuna fry with enough genetic variation to survive and multiply in open sea after being raised in marine ranches, thereby helping replenish stocks of wild fish.

The approach has several advantages over the bluefin farming pioneered by Kinki University in western Japan, in which the sperm and eggs from farm-raised tuna are used to create test-tube fish, which in turn are reared for about four years in offshore pens until they are big enough to be sold.

The process is time-consuming and costly, and, aficionados insist, produces sashimi of an inferior quality.

The Japanese government, aware of mounting international criticism of its failure to rein in fishermen, have given Yoshizaki’s team a 300 million yen ($3 million) grant for the five-year project.

They have three years left to produce results. “In that time I want to produce at least one tuna bred using surrogate mackerel,” Yoshizaki says.

“And if I’m being optimistic, we should have all of the techniques we need to mass produce tuna through surrogate mackerel in less than 10 years. If we can make it work in conjunction with marine ranching, then we can ensure there will be a healthy population of bluefin tuna forever.”

The professor is almost evangelical in his enthusiasm for his work, but he also feels a heavy moral responsibility to succeed. “Japanese people consume a lot of tuna so it is up to us to do something to save this precious ocean resource.”

In the meantime, the bluefin tuna’s hopes of staving off extinction partly rest with consumers. Several sushi and supermarket chains have stopped selling it, while diners can refer to a selection of new ethical eating guides telling them exactly what and what not to order over the sushi counter. Needless to say, bluefin is a definite no-no.

More GlobalPost dispatches from Japan:

The endless lure of pachinko

Tokyo's cat cafes

Monks venture into bars, and rap

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