Abstract
Since 1995 in Yamaguch Prefecture, Japan, mass mortalities of farm-raised kuruma prawn Penaeus japonicus due to unknown etiology have occurred from July to August when the pond water temperature reached around 30°C. A blue-green alga Chroococcus turgidus bloomed in the ponds, and the pH values of the pond water were found to be extremely high; frequently higher than 9.5. Many of the dead prawns displayed soft shells and whitish-opaque necrotic gills which were frequently covered with scar-like and/or granule-like foreign bodies. The intramuscular injection of the filtered homogenate of C. turgidus did not cause any significant damage to the prawn. The alga raised the pH of the medium to 9.73, much higher than any of the other phytoplanktons tested. Sea water with a pH of 9.48 or above prepared by adding sodium hydroxide solution killed the prawn with similar disease signs in the gills to those of the naturally diseased prawns. These results indicate that high pH of pond water by growth of C. turgidus is the main cause of the death.
