Something fishy: Tokyo aquarium baffled by mysterious tuna deaths

A lone tuna is all that remains in the vast doughnut-shaped enclosure at Tokyo’s Sea Life Park. The 2,200-tonne tank, which 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. 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.

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.

A spokesman for Tokyo Sea Life Park