A mystery disease has decimated Tokyo Sea Life Park’s tuna population, leaving one sole fish surviving in the aquarium’s signature tank.

The aquarium's large tank – previously home to around 160 fish – has been reduced to one following the mystery outbreak.

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

"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."

Investigators are looking into the case, with a range of hypothesis already on the table, including the tank’s lighting, stress among the fish, or possibly a poisonous substance in the water.

The signature tuna tank, which measures 98ft (29.8m) in diameter, previously held 69 blue-fin tuna, 52 eastern little tuna and 38 oriental bonito. The remaining fish is a blue-fin tuna, with the rest of the tank’s population starting to deplete in December, while only 30 fish were still alive by the middle of January.

The spokesperson added that an earlier examination had found a virus among some of the dead fish, but it wasn't one that is commonly fatal in fish farms.

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