Mackerel spawns tuna — it is a curious phenomenon that cannot happen in the natural environment, but scientific research is underway to overcome this issue. This research is in no way intended to challenge the Providence of Nature. Explaining the significance of the research, Yutaka Takeuchi, Associate Professor of the Tokyo University of Marine Science and Technology, said: “Man needs to produce food in order to live in nature. It is science — and man — that can realize it without causing burden on the natural environment. At the same time, Takeuchi poses a question over the recent situation where too much attention is given to tuna on the ground that it should be a high-grade fish species.OPRT interviewed Takeuchi regarding the state-of-the-art tuna science at the university’s research center in Tateyama, west of Tokyo, where the project is being promoted.
QUESTION: Why is there a need to have mackerel spawn tuna?TAKEUCHI: Success has been made in the technology to have tuna spawn eggs and raise those eggs. But enormous costs are needed to keep the tunas that become parents. I hear the costs required in a year amounts to several hundreds of million yen. Also, it is not easy to release and propagate the seeds of the tuna species for which stock status is deteriorating. But if the fish is of the size of mackerel, it is possible to keep them in large quantities and increase the seeds. The cost for doing it is in the order of million yen a year. It is realized by way of the transplanting technology –i.e. the “surrogate parent” technique that causes mackerel to spawn tunas.
QUESTION: Has that technology been successful in the area other than tunas?TAKEUCHI: Success was made in the technology to have land-locked salmon spawn rainbow trout. At present it has become possible to have land-locked salmon spawn rainbow trout’s eggs at a full rate and also to have the former produce the latter’s sperms. I have been engaged in the development of such technology, and have been trying to apply it to marine fish since 2005.
QUESTION: What kind of technology is it?TAKEUCHI: Inside the gonads that produce eggs and sperms, there are stem cells which make up the eggs and sperms. When a part of the gonad is transplanted in the body (i.e. abdominal cavity) of a juvenile whose immunity functions have not yet developed, that part enters into the juvenile’s gonad. In the case of mackerel, the cells that make up tuna’s eggs and sperms are incorporated into mackerel’s gonad, and it duplicates and increases tuna’s eggs and sperms in large volume. Since normal mackerel also produces mackerel eggs and sperms, tuna’s sperms and eggs are produced only in a small quantity, say, one out of 1,000 eggs. But it is theoretically possible to have mackerel produce only tuna eggs and sperms by applying the technique of sterility processing, called triploidy. As this was realized in the case of land-locked salmon, it was made possible to produce rainbow eggs and sperms on a 100% basis. However, in the case of marine fish, this is more difficult than freshwater fish because its eggs are very small. It is no easy for marine fish because the difference in size between freshwater fish egg and marine fish egg is like that for salmon roe versus a pollock roe, in other words, a volleyball versus a marble.
QUESTION: How far has the research advanced at present?TAKEUCHI: The research has been initiated on the question whether or not a mackerel can bear a tuna because both species belong to Scombridae. But the study had not advanced smoothly. The cells to become tuna eggs and sperms, transplanted in mackerel, had disappeared in a span of a few days. While it is possible to transplant the cells of a mouse in chicken, it is not possible to do so from tuna to mackerel. After numbers of trials, it was found that the fish seemed to tend to be affected by the spawning period, habitat environment, and notably water temperatures.
QUESTION: Are the fish subject to impact of the environment in the stage of cells as well?TAKEUCHI: I think so. We started experiments using mackerel from southern area, and succeeded in the transplantation in the summer of 2009. We made a success again in the summer of 2010 as well. We are hoping that the southern mackerel would produce tuna eggs and sperms.
QUESTION: It is a remarkable technology.TAKEUCHI: We felt a heavy pressure because there was a mounting attention and expectation even before the technology was established because the project was related to tunas. I think it better to focus on efforts to complete the technology first and then give attention to its application. We felt pressed as we had to establish the technology while there is high-level attention. At the same time, we have been coping with the project as a real challenge because of strong expectations from many quarters.
QUESTION: What will happen when this technology is completed?TAKEUCHI: Juvenile tunas are born in aquariums in which mackerels are kept. Massive release of seeds will become possible to increase the tuna population. The advantage of this technology is that, when it is completed, it would become possible to have mackerel produce juvenile tunas on a 100% basis. Tunas born from mackerel spawn only tunas. Thus, in terms of technology, the genetic diversity can be fully secured. Apart from the argument whether extinction is really possible for any fish, it would become possible to increase artificially the population of fish species threatened with extinction, while securing genetic diversity.
QUESTION: This is really a technology that surpasses the power of nature, isn’t it?TAKEUCHI: I don’t agree with you on this point. We cannot beat nature when we compete with it. But man faces the need to produce food in order to live in nature. What science — and man — can do is to realize that goal without causing unnecessary load on the natural environment.
QUESTION: Do you think that the days will come when we can eat tunas without worrying about their population?TAKEUCHI: When the development of surrogate parent technology advances, we may be able to increase tuna seeds for farming or release the seed to recover the deteriorated stocks without much difficulty in the future. But I believe the basics should be whether we humans can secure food we need for our survival with minimum burden on the natural environment. With this concept in mind, I will continue with the development of surrogate parent technology.