TOKYO (AFP) - 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.

"We are studying what caused the fish deaths, but we haven't figured it out yet. We suspect that it could be due to new factors that were not present before." 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-tonne tank that is 30m 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."