TOKYO - Researchers are studying reasons behind the mass extinction of tuna fish in Tokyo aquarium's most popular attractions, whic has left the workers scratching their heads over the unexplained scale of the deaths.

The doughnut shaped tuna tank in Tokyo Sea Life Park was home to nearly 160 fish, but almost all fish have died in an inexplicable manner, a spokesman said.

The second to last fish was found floating on Tuesday with a broken spine, the spokesman added, according to Japan Times.

"We have had the tuna tank since the aquarium opened in 1989 but never experienced this kind of mass-dying," the spokesman told reporters 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 said.

The 2,200-ton, 30-meter-diameter tank once housed 69 bluefin tuna, 52 eastern little tuna and 38 oriental bonito.

The tank's population suddenly started plunging in December, and just 30 fish remained alive by mid-January, the park said.

"The 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 a type that is usually fatal in fish farms."