The bluefin tuna is on the verge of extinction at a Tokyo aquarium known for a breeding program intended to save the species. Since late last year, the fish have been dying one by one, and researchers can’t seem to determine why.

Another bluefin died on Tuesday, leaving only one lonely tuna in its habitat at the Tokyo Sea Life Park in Edogawa Ward near the Tokyo Bay. The diseased fish was found with broken bones, but the cause of death is not yet known.

“We have had the tuna tank since the aquarium opened in 1989 but never experienced this kind of mass-dying,” a spokesman for the aquarium told Agence France-Presse early on Wednesday.

“The second last one that just died apparently crashed into the acrylic wall twice,” he added. “It suffered a broken backbone, which was unfortunate but not very unusual for tuna kept in a tank.”

In November, there were 69 bluefin tuna in the tank, a doughnut-shaped enclosure filled with 2,200 tons of water, Satoshi Tada, a spokesman for the government-run Tokyo Sea Life Park, told The Washington Post. The next month, the fish started dying at an alarming rate. In January, only 11 remained.

The aquarium has lost 68 bluefin tuna in four months, he said in an e-mail.

Keiichi Mushiake, director at the Research Center for Tuna Aquaculture at the Seikai National Fisheries Research Institute, called the case “unprecedented.” He told the Japan Times he had never heard of a mass death like this one.

Tokyo Sea Life Park opened more than two decades ago with a mission to reproduce “aquatic habitats in Tokyo and in the world,” according to its Web site. It started a breeding program for the bluefin tuna, a finicky fish put on the endangered list several years ago largely due to overfishing in the Northern Pacific Ocean. The aquarium’s exhibition, “Voyagers of the Pacific: Tuna,” once featured nearly 160 fish: bluefin tuna, kawakawa (or mackerel tuna) and striped bonito. But keeping the bluefin in captivity can be tricky since it is extremely sensitive to changes in light and noise levels. A loud boom can cause its demise, the Japan Times reported.

As for the deaths, the aquarium has considered several possibilities. In November, it put 31 mackerel tuna into the tank, though such restocking is common, it said. In December, it started construction on another tank nearby, but Mushiake told the newspaper the construction wasn’t likely the culprit. The fish continued to die even after the project was completed.

Though certain viruses were found in the tank, researchers doubt they would kill an entire clan.

“An earlier examination has found some sort of virus among some of the dead fish, but it wasn’t the kind that is usually fatal in fish farms,” a spokesman told AFP.

News reports earlier this year said the remaining fish seemed unusually nervous and wouldn’t eat.

In the past few months, the aquarium has attempted to control the situation. It started switching off the lights at night to put the fish at ease. It used computers to monitor the water and the fish, but no abnormalities were found.

Researchers are considering numerous factors such as nutrition or changes in the physical environment including light, sound and vibration. They are monitoring water quality, oxygen levels and searching for possible poisonous substances such as heavy metal, Satoshi Tada said.

“We are studying what caused the fish deaths, but we haven’t figured it out yet,” a spokesman told the AFP.” We suspect that it could be due to new factors that were not present before.”