The Status of Koi Herpesvirus Disease in Lake Kasumigaura and Kitaura

Youji Takashima, Naoki Watanabe and Takanori Yanai

Ibaraki Prefectural Freshwater Fisheries Experimental Station, Ibaraki 311-3512, Japan

ABSTRACT

[Object] Lake Kasumigaura and Kitaura in Ibaraki Prefecture is the second largest lake in Japan whose total lake and catchment areas are 220 km2 and 2,160 km2, respectively. In the lake, common fisheries such as trawling and fyke net as well as net cage culture are practiced. Net cage culture produces about 5,000 tons of food carp per year, which is one half of the total production in this country. At the beginning of October 2003, deaths of unknown origin of food carp in net cages occurred in Lake Kasumigaura. After inquiry of the cause, PCR testing detected DNA of koi herpesvirus (KHV) in affected fish and wild carp captured in Lake Kitaura, confirming the occurrence of KHV disease.

[Method] We conducted an interview survey on aquaculturists regarding the date, quantity, and circumstances of death of cultured carp in Lake Kasumigaura and Kitaura, and investigated the circumstances of death in net cage farms. Another interview survey was also conducted on other fishermen regarding the occurrence of abnormality in natural carp, and cultured and natural carp were sampled for PCR testing.

[Results] At the beginning of October when the deaths began, the water temperature was about 18⁰C, which decreased to 14-15⁰C between the end of the month and the beginning of November. It reached 12-14⁰C at the end of November when mass death seemed to come to end. The interview surveys revealed that the death scale was larger in net cages with larger feeding and the tendency for the death of larger fish was greater than that of the yearlings. Symptoms of affected fish included hallowing around the eye, whitening of the body surface, and branchial anemia, congestion, and decomposition.

The death of cultured carp in Lake Kasumigaura was generated in two areas dense with net cage farms, and then was likely expanded. The cumulative amount of death according to the interview surveys increased rapidly: 200-300 tons in mid-October, 660 tons at the end of the month, and 1,200 tons at the beginning of November. The damage amount reached one quarter that of the annual food carp products. Abnormal natural and PCR-positive fish were found after the occurrence of death in cultured carp.

Downstream of Lake Kitaura, abnormal wild fish presenting with emaciation and surface rubor began to be caught from the end of September. It was thus inferred that, after that, the occurrence of this abnormality in wild fish spread upstream. In mid-October, deaths of cultured carp in net cages began.

Since KHV disease is one of the designated diseases under the Law to Ensure Sustainable Aquaculture Protection, to prevent the spread of this disease according to this law, Ibaraki Prefecture requested aquaculturists of Lake Kasumigaura and Kitaura to exercise restraint on the transfer of cultured carp in net cages on November 2, and issued orders prohibiting transfer on November 12 and implementing incineration and landfill disposal on December 21. The incineration disposal commenced from January 20.