Mystery deaths afflicting tuna at Tokyo aquarium

by Shusuke Murai

Staff Writer

A Tokyo aquarium known for its tuna breeding program is facing a crisis: the unexplained deaths of its main exhibits.

Seven weeks ago, the Tokyo Sea Life Park had 165 bluefin tuna, kawakawa and striped bonito on display. On Monday it had just 11.

Takashi Sugino, a manager with the center’s breeding program, first realized in early December that the fish in a 30-meter doughnut-shaped tank were ailing.

The aquarium in Edogawa Ward has been unable to determine the cause of death, despite using computers to monitor possible changes in the environment where the fish died, Sugino said.

One third-party observer said such rapid mortality in the entire population was “unprecedented.” Keiichi Mushiake, a director of the Research Center for Tuna Aquaculture at the Seikai National Fisheries Research Institute, said he had never heard of similar cases elsewhere. Although fish may die from disease or human error, those causes are unlikely at such a reputable aquarium.

The center is known for its breeding of bluefin tuna, a species listed as being under threat of extinction. In 1989, the aquarium became the first in the world to show bluefin tuna swimming in their natural conditions.

Breeding tuna is difficult. Even the slightest change of environment, such as unusual light levels or a loud noise, can prove fatal to the highly strung fish. Sugino and his team have worked carefully to re-create the natural habitat as much as possible.

There was construction work in another tank at the site in early December, but Mushiake said this was unlikely to be the cause of death because fatalities continued even after construction finished.

In early November, the aquarium added 31 kawakawa, a fish also known as the mackerel tuna, in a routine restocking procedure, one that has taken place regularly over the past 25 years. Nothing out of the ordinary was noticed at that time, Sugino said.

Meanwhile, total extinction is a possibility. The 11 surviving fish are showing signs of unusual behavior, such as nervousness and lack of appetite.

Sugino added that he is sad to see visitors disappointed, because the center’s main attraction is supposed to be a tank containing a large school of tuna.

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