A large number of bluefin tuna and bonito have died at a well-known Tokyo aquarium since December, baffling officials over the cause.
Tokyo Sea Life Park in Edogawa Ward, known for hosting the world's first major exhibition on bluefin tuna migration, had 165 tuna and bonito on Dec. 1. Now, only seven are left.
If the trend continues, the aquarium, which has kept bluefin tuna for 25 years, could face its own tuna extinction.
"I've haven't seen so many deaths over such a short period since we opened in 1989. There might not be any fish left in the tanks by the end of the month," aquarium Vice Director Kazuomi Nishikiori said Thursday, standing in front of an empty 30-meter-diameter ring-shaped fish tank.
Three species of migratory fish have been dying at the aquarium - bluefin tuna, mackerel tuna and striped bonito.
Workers first noticed something unusual in early December. The bonito were no longer biting at their feed properly, and they had started floating up to the water's surface and then sinking to the bottom.
By the end of the month, the same phenomena began with the tuna.
On Dec. 1, there were 63 bluefin tuna, 67 mackerel tuna and 35 striped bonito at the aquarium.
But as of Tuesday morning, only three bluefin tuna and four striped bonito were left. The last mackerel tuna in the tank died Sunday.
Some of the surviving fish are exhibiting signs of abnormal swimming, such as moving up and down in the tanks.
"We wanted to see big schools of fish swimming around. It's too bad, isn't it?" said Takahiro Sekiguchi, a 26-year-old company employee from Kawaguchi, Saitama Prefecture, who visited the aquarium on Thursday with his family.
It is difficult to raise migratory bluefin tuna in captivity, though Sea Life Park has acquired a wealth of practical know-how about caring for the fish.
The aquarium uses a combination of LED, halogen and mercury lights to mimic the natural rising and setting of the sun. It is famous for being the only domestic aquarium to have a large school of bluefin tuna.
"Tuna and bonito are sensitive. If the way their lighting is switched off and on is changed even a little, they'll start swimming erratically," aquarium worker Takashi Sugino said.
Previously, the aquarium saw more than 100 fish smash violently into the acrylic panels of the tank in a year.
Deaths due to collisions from broken spines and other reasons still occur, but most of the tuna and bonito that have died recently showed no such signs.
Nothing abnormal has been found in their water, and autopsies have revealed no evidence of parasites or disease.
Last December, however, the aquarium began repairs on a tank that sits near the large fish tank. Vibration and noise from the construction may have stressed the fish.
A research organisation has been asked to help investigate the cause.
What do experts think is going on?
Keiichi Mushiake, head of the Nagasaki-based Seikai National Fisheries Research Institute's Research Center for Tuna Aquaculture, said: "Sea Life Park has plenty of experience caring for fish, so it's hard to imagine they made a rookie mistake, like not properly managing the water quality. We can't rule out a combination of causes, such as disease plus collisions."
Kinki University executed the world's first "complete aquaculture" of bluefin tuna in 2002 when it succeeded in artificial hatching, raising the fish to adulthood and having them spawn.
It offers the fish at restaurants in Tokyo and Osaka. "Kindai Maguro" has proved popular.
Takashi Kitagawa, a tuna specialist and associate professor at the University of Tokyo's Atmosphere and Ocean Research Institute, said: "The same problem could occur at other facilities that keep tuna. It's a fish that gets a lot of attention in connection with fishery resource issues, so I hope a solution can be found quickly."
The aquarium does not plan to introduce new fish until the reason for the deaths is determined.
"A lot of visitors come to see the tuna, so I feel bad about disappointing them. We intend to do everything we can to figure out the cause," Nishikiori said.