AFP

TOKYO

Workers at a Tokyo aquarium are scratching their heads after the deaths of dozens of fish that have left just one lonely tuna roaming a once-flourishing tank.

The park on Tuesday found the second last fish floating dead in its vast doughnut-shaped enclosure that was once home to nearly 160 fish and among the venue’s most popular attractions, said a spokesman for Tokyo Sea Life Park. ‘We have had the tuna tank since the aquarium opened in 1989 but never experienced this kind of mass-dying,’ he told AFP on Wednesday. Researchers are studying a range of possibilities including the tank’s lighting and other factors that could have caused stress among the fish, or even the presence of a poisonous substance in the water, he added.

The 2,200-ton tank that is 30 metres (98-feet) in diameter once housed 69 bluefin tuna, 52 eastern little tuna and 38 oriental bonito. In December the tank’s population suddenly started plunging with just 30 fish alive by mid-January, the park said. ‘The second last one that just died apparently crashed into the acrylic wall twice. It suffered a broken backbone, which was unfortunate but not very unusual for tuna kept in a tank,’ the spokesman said. ‘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.’