Just as people in Japan had begun to resign themselves to the idea that eating coveted fatty tuna may soon no longer be possible, mackerel has appeared as an unexpected savior.
The background to this was a proposal to ban international trade in Atlantic bluefin tuna. The proposal was defeated at the recent meeting of parties to the Convention of International Trade on Endangered Species (CITES), but it could be reactivated at any time.
It turns out there may be a way to bolster the tuna population by using mackerel to breed the much sought-after fish.
Though female tuna release hundreds of thousands of eggs at each spawning, the number that reaches maturity in the wild is close to zero. However, if mackerel raised in captivity for about a year can be made to lay tuna eggs, tuna fry could be obtained inexpensively and in large amounts.
This would not only be useful for aquaculture, if the fry were released back into the ocean, but overfished wild tuna could be saved from extinction as well.
So, is it really possible for mackerel to produce the eggs of a tuna? And even if it is, wouldn’t the result be some kind of bizarre tuna-mackerel hybrid?
“It’s not a problem. We’re just using the mackerel as a surrogate, so to speak. The resulting fry are 100-percent genuine bluefin tuna,†explained a grinning Goro Yoshizaki, 44, an associate professor of aquatic bioscience at Tokyo University of Marine Science and Technology who has been researching the subject for close to 12 years.
Injecting tuna germ cells into a baby mackerel
Germ cells--the early stage stem cell of sperm and eggs--are present in male and female adult tuna. If the cells can be transplanted into the mackerel and they take hold, the female mackerel’s ovary will produce tuna eggs while the male mackerel’s testes will produce tuna sperm. Should these male and female mackerel meet and duly spawn, tuna fry will be the end result.
But just like transplants, there is a risk of rejection. Just as it is not easy to coax a human body into accepting transplanted organs, it is difficult to get a mackerel to accept cells from a tuna.
“However, we have found that if the transplant is performed on newly hatched fish, the rejection of foreign cells rarely occurs,†Yoshizaki said.
If the male and female tuna germ cells are transplanted when the mackerel are still very young, the resulting adults will grow to produce tuna sperm or eggs according to their gender. Transplanting the cells after sterility measures make certain the mackerel cannot produce their own eggs or sperm will ensure the fish produce only tuna eggs or sperm continuously thereafter.
But how does one locate the reproductive organs in baby mackerel that are less than 5 millimeters in length? For that matter, how easy is it to distinguish gender in a creature so small? What would happen if males were accidentally given female germ cells, or vice versa?
“That’s not a problem either,†said Yoshizaki, grinning once more.
The fact is that both male and female tuna germ cells have the ability to independently look for and implant themselves into the ovary or testes. If the cells are injected into the peritoneal cavity of the baby mackerel using the tiniest of needles, they will then start to move inside the fish’s body just like an amoeba. Furthermore, if the female germ cells reach the testes or vice versa, they will then change their sex accordingly, Yoshizaki said.
“This high degree of flexibility has been present in fish reproductive cells right from the beginning,†he added.
Surely this is one of life’s great mysteries.
The discoveries made by Yoshizaki and his colleagues were published in a U.S. scientific journal in 2006, creating quite a stir.
Using these principles, Yoshizaki was successful seven years ago in getting land-locked salmon to produce rainbow trout eggs and sperm. Since 2005, he has been trying to replicate the feat by using mackerel and tuna.
In the beginning, he didn’t have much success transplanting the tuna germ cells. Tuna are from southern waters, while Japanese mackerel are from northern waters. Yoshizaki wondered if the low water temperature used to raise the mackerel was affecting the tuna cells.
So, he decided to use a different type of southern mackerel instead.
Last September, he succeeded in getting implanted germ cells of tuna to take hold inside a mackerel. He is now at the stage of attempting to use the mackerel to breed tuna.
“In another seven or eight years, I believe we’ll be able to breed tuna from mackerel in a stable manner,†Yoshizaki said.
There is an additional benefit resulting from Yoshizaki’s research. The tuna fry he breeds don’t have to be artificially raised to maturity by way of fish farming only; they can also be released into the sea and reach adulthood naturally.
Though aquaculture is an important farming technique, when large tuna are raised in narrow fish pens, additional care must be taken to avoid having their excrement and leftover feed pollute the surrounding marine environment.
However, even if tuna numbers can be raised, there is concern that the fish they feed on, such as mackerel, could be depleted in the future.
“We aren’t employing any complicated techniques such as gene manipulation,†Yoshizaki said. “Just as with the stocking of salmon, we only need to increase tuna numbers to equal the amount that is harvested by man.â€
This constitutes the basic philosophy of Yoshizaki and his colleagues. It is one that is much admired by Toshiki Matsuyama, education officer for the Tokyo Sea Life Park which is involved in breeding and exhibiting tuna.
According to Matsuyama, “It’s said that as Atlantic bluefin tuna are targeted when they gather in the Mediterranean to spawn, their situation is more dire than that of the Pacific bluefin. If that’s the case, releasing tuna fry produced with Yoshizaki’s method offers the tremendous possibility that tuna numbers can be brought back up.â€
Hirotaka Akamatsu, the then-minister of agriculture, forestry and fisheries, and other officials were clearly delighted at the outcome of the CITES convention in March.
But, it is by no means the end of the problem. If anything, the crisis surrounding tuna is likely to deepen.
Yoshizaki’s research offers the tantalizing possibility of using mackerel to protect the fatty tuna that is so highly regarded in Japan.
This nation has a responsibility to learn more about this fish if its people want to continue consuming the delicacy.
