Staff at a public aquarium in Tokyo are baffled by the deaths of over 150 fish in one of its display tanks.
The aquarium at Tokyo Sea Life Park has just one surviving fish — a bluefin tuna — left in the giant 30m/98ft Voyagers of the Pacific exhibit that up until December last year held almost 160 fish: bluefin tuna, mackerel tuna and striped bonito.
Staff are unable to find out why the population in the tank has suddenly crashed. Only 30 fish were left alive by the middle of January and earlier this week the body of one of the two remaining fish was removed.
Investigation is under way, looking at a range of possibilities including lighting, noise and vibration levels, nutrition, and toxins. Computers to monitor both the water and the fish found no abnormalities.
"An earlier examination has found some sort of virus among some of the dead fish, but it wasn’t the kind that is usually fatal in fish farms," a spokesman told AFP.
"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."
Tokyo Sea Life Park opened more than two decades ago and began a breeding program for bluefin tuna to help save the species.
Why not take out a subscription to Practical Fishkeeping magazine? See our latest subscription offer.
Don't forget that PFK is now available to download on the iPad/iPhone.