The park yesterday 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 today.
"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."
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.
Also Read
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.