Mackerel Give Birth to Tuna!? The 20-Year Hurdle in Development and the Shock of “Surrogate Parent Fish” That Produce an Endless Supply of High-Quality Salmon from Rainbow Trout

Can Mackerel Give Birth to Tuna? What Is the “20-Year Barrier”?…

In recent years, poor catches of mackerel and other species have persisted. According to a survey by the Ministry of Agriculture, Forestry and Fisheries, Japan’s domestic mackerel catch—which stood at approximately 1.62 million metric tons in 1978—hit a record low of about 261,000 metric tons in 2023.Catches of species such as Japanese common squid and salmon have also plummeted. With fish increasingly becoming a luxury ingredient these days, “surrogate parent fish” may be the savior.

Farming tuna requires large aquaculture ponds, and it takes three to four years for them to grow from eggs to maturity.Feeding costs are high, and the longer they’re raised, the greater the risk of disease. So, what if we used the small fish known as “suma”—which belongs to the same family as tuna—as surrogate parents? Smaller tanks would suffice, and suma reach maturity in about eight months. The research team led by Professor Goro Yoshizaki at Tokyo University of Marine Science and Technology is studying this surrogate parent technology.

It’s been over 20 years since they began their research. I thought tuna born from smelt might be hitting the market by now, but

“We’re not there yet. The males produce tuna sperm, but the females won’t lay eggs,”

said Professor Yoshizaki.

How do you turn a fish into a surrogate parent?

The method developed by Professor Yoshizaki involves transplanting stem cells—the source of eggs and sperm—into the surrogate parent fish. However, just as humans can sometimes experience rejection reactions after transplant surgery, this technique cannot be applied to just any fish.

“So far, we’ve tried various fish in the mackerel family, including Japanese mackerel and spotted mackerel, but it didn’t work out. I thought the Japanese amberjack might be the one… but we’re currently experimenting with a new species,”

Even if we’re going to transplant into the Japanese amberjack, we first have to build an aquaculture system capable of raising them.

“This is the difficult part. Mackerel are migratory fish; they are constantly migrating in search of water temperatures where they feel comfortable. They feel comfortable only within a very narrow range of temperatures and salinity levels, in areas with abundant oxygen. It takes three to four years to build an aquaculture system that meets those conditions. For the Japanese horse mackerel, it took seven years.”  

Twenty years—it’ll fly by in the blink of an eye.

Can rainbow trout produce an infinite number of king salmon?

Although no tuna have yet been produced from mackerel, researchers have succeeded in breeding king salmon from rainbow trout. While it takes 4 to 5 years for king salmon to mature from eggs to adults, rainbow trout reach maturity in about one year.They also require less space for rearing. Furthermore, while salmon species like king salmon spawn only once in their lifetime, rainbow trout can spawn as many times as needed. This means king salmon can be mass-produced. Is this already heading to the market?

“The next step is to breed the king salmon that are born and see how much we can improve their quality.”

Currently, the majority of the salmon Japan imports comes from Norway. Over the past 40 years, Norway has engaged in selective breeding, repeatedly refining the strain to ensure faster growth, greater disease resistance, and even better flavor. In contrast, the salmon born in Professor Yoshizaki’s lab are in their wild state. In terms of quality, they fall short of the Norwegian variety.While the potential is immeasurable,

“It won’t be easy to close that 40-year gap,”

Is that so…? With all due respect, what’s the point of this research?

“We’re researchers. Salmon lay eggs only once in their lifetime, while rainbow trout can spawn repeatedly. We’re interested in why that is. One hypothesis is that, although they have the ability to spawn multiple times, they do so only once because their lifespan ends before they can do it again.

Another hypothesis is that salmon are biologically designed so that their ovaries or testes can only produce eggs or sperm once. To solve this mystery, we transplanted cells from fish that can only spawn once into fish that can spawn multiple times.”  

As a result, we discovered that while rainbow trout always leave behind copies of their eggs and sperm after spawning, salmon do not.

“We realized that what matters isn’t the germ cells themselves, but the environment in which they develop. Unraveling mysteries like this is a major goal of our research.”

Preventing Fish Extinction and Paving the Way for Regenerative Medicine

Professor Yoshizaki is currently focusing his efforts on conservation research.

Throughout history, humans have driven various species to extinction. One such species is the Japanese wolf. As a result, the deer population has proliferated, devouring the shoots of plants and trees. This has left small mammals and birds without food, causing their numbers to decline steadily. The damage to farmers is also severe.

“By driving just one species—the wolf—to extinction, a future awaited that people at the time could not have foreseen. At this stage, we cannot predict what will happen in the future if this fish were to disappear. However, I believe that the species currently inhabiting the Earth are a precious legacy that should be passed on to future generations.”

Professor Yoshizaki is currently working on increasing the population of Musashi Tomiyo by using the freshwater form of Tomiyo as surrogate parents. He has also succeeded in increasing the population of Miyako Tanago by using a fish called Aburabote as surrogate parents.Furthermore, he says that the Mekong giant catfish can now be bred using a small catfish called the “Kaiyan” as a surrogate parent.

It has also become possible to freeze the germ cells of fish on the brink of extinction so that, even if they go extinct, they can be revived by transplanting those cells.

By applying transplantation techniques, researchers have used the small Kusafugu as surrogate parents to increase the population of male tiger pufferfish—from which milt can be harvested—and have even created “Yume-aji,” a hybrid of horse mackerel and Kaiwari.

“As we come to understand why rainbow trout leave behind copies of themselves—and how cells determine their fate—this could potentially lead to applications in regenerative medicine. Unraveling the mechanisms of fish reproduction gives rise to new technologies. Our goal is to elucidate those mechanisms.”

▼Goro Yoshizaki: Professor, Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology. Ph.D. in Fisheries Science. After serving as a postdoctoral researcher at the College of Agriculture, Texas A&M University, and as an assistant professor at the Faculty of Fisheries Sciences, Tokyo University of Fisheries, he has held his current position since 2012.He received the Japan Society of Fisheries Science Award in 2015, the Japan Society of Agricultural Science Award in 2017, and the Yomiuri Agricultural Science Award. His major publication includes *Can Tuna Be Born from Mackerel!?* (Iwanami Science Library).