Herpes is suspected of taking a huge toll on cultured carp in 10 prefectures, according to findings reported Thursday to a farm ministry panel.

The first signs of carp dying were observed in fish farms in Lake Kasumigaura, Ibaraki Prefecture, in early October. The prefecture, which accounts for some 50 percent of farmed carp in Japan, reported that some 1,125 tons of carp had been lost as of Thursday. It said it had shipped farmed carp to 21 prefectures.

The fish were being bred at farms in Kasumigaura and Lake Kitaura. The dead fish were infected with the carp herpes virus.

Apart from Ibaraki, mass carp kills were reported in Aomori, Saitama, Yamanashi, Nagano and Miyazaki prefectures. Carp kills on a smaller scale have also occurred in Mie, Okayama, Kochi and Fukuoka prefectures, according to the Agriculture, Forestry and Fisheries Ministry.

Of these nine prefectures, seven had received fish from Lake Kasumigaura, and the herpes virus was found in dead fish samples in Aomori, Yamanashi, Mie and Miyazaki. In Okayama and Kochi, however, the dead fish had been farmed locally, indicating the possibility of another infection route.

Ministry officials said they will try to track the route of infection, beginning by conducting hearings with fish farmers.

Officials in Mie Prefecture are reporting a loss of between one and nine fish per day. In Okayama the losses on some days ranged between 30 and 50 fish. In Fukuoka, about two carp were dying each day, officials said. All prefectures said they saw the phenomenon worsen around mid-October and have issued instructions to incinerate the dead fish and refrain from shipping carp.

The carp herpes virus, first detected in Israel in 1997, has spread to other parts of Asia, including Indonesia and Taiwan, as well as Europe and the United States. Infected fish stop eating and start swimming erratically, according to experts.

It is highly contagious, but the virus does not multiply at temperatures above 30 degrees and is not harmful to humans.

Although it has never before been detected in Japan, the agriculture ministry listed it in June among the designated diseases under the law on sustainable fish farming. All imported fish must be inspected for the diseases cited under the law, but government officials said that no carp had been imported to Japan since the change took effect.

According to the National Research Institute of Aquaculture of the Fisheries Research Agency in Mie Prefecture, an independent administrative institution, the disease most likely spread to other prefectures through fish bred in Lake Kasumigaura. However, it is unclear how the virus first entered Japan.

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