A single tuna roams a once-flourishing tank at an aquarium in Japan after more than 150 of its fishy friends died unexpectedly . Workers at Tokyo Sea Life Park are scratching their heads after the most recent death on Tuesday , leaving just one lonely tuna in the vast doughnut-shaped tank . Researchers are trying to find out the cause of their deaths , and one option being explored is poisoning . Unexplained : The cause of the deaths of more than 150 fish is being explored , but is not currently known . Lone fish : Just one lonely tuna remains in the vast doughnut-shaped tank at Tokyo Sea Life Park in Japan . A spokesman for the tourist attraction said : 'We have had the tuna tank since the aquarium opened in 1989 but never experienced this kind of mass-dying . 'We are studying what caused the fish deaths , but we have n't figured it out yet . We suspect that it could be due to new factors that were not present before . ' The tank 's lighting and other factors that could have caused stress among the fish , or the presence of a poisonous substance in the water are being considered . The 2,200-tonne tank 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 . Researchers are trying to find out the cause of their deaths , and one option being explored is poisoning . 'The second last one that just died apparently crashed into the acrylic wall twice , ' the spokesman added . 'It suffered a broken backbone , which was unfortunate but not very unusual for tuna kept in a tank . 'An earlier examination has found some sort of virus among some of the dead fish , but it was n't the kind that is usually fatal in fish farms . '
More than 150 fish died at an Japanese aquarium and only one survived . Workers at Tokyo Sea aquarium Park do not yet know what caused their deaths . Poisoning and factors that could have caused fish stress being explored . Tank contained 69 fish and 52 eastern little tuna , plus 58 oriental fish .