A Bacteriological Investigation on the Mass Mortalities of Red Seabream Pagrus major Larvae with Intestinal Swelling

Abstract

At Momoshima Field Station of Japan Sea-Farming Association, plenty of red sea-bream Pagrus major juveniles have been raised from newly hatched larvae every year in earthen ponds. Since 1982, a peculiar mass mortality has been noticed in the ponds and mortality rate has become higher year by year. The characteristic symptoms of the diseased fish include remarkably swollen intestine and spleen, the former being full with undigested zoo-planktons and the latter with erythrocytes.
Bacterial examinations on the mass mortalities of the larvae with intestinal swelling were made at the station in 1986. As the result, bacterial number in the intestine of diseased fish proved higher than that of normal fish by one hundred or one thousand times. Vibrio, the most predominant group, was found 20% greater in diseased fish as compared with normal fish, however, different Vibrio species were isolated as predominant species at each case of mortality.
Seven strains of Vibrio, which was predominantly isolated from diseased fish, were tested of their pathogenicity against red seabream larvae by oral administrations. However, neither the disease symptom nor apparent mortality were observed among the tested fish. Although the cause of this disease could not be clarified by this study, an importance of the primary cause (s) leading to digestive disorder was discussed.