Saprolegnia diclina isolated from pond cultured red drum (Sciaenops ocellatus) in Hong Kong

Mass mortality among pond cultured red drum (Sciaenops ocellatus) was obser ved in Hong Kong. Affected fish were lethargic and lost their appetite but no lesions on the body surface were apparent. Patches of white to brownish cottony growth on the gills of affected fish were observed and microscopic examination revealed mats of hyaline mycelia with mature zoosporangia and o ogonia which were identified as Saprolegnia diclina. During induced sporula tion, production of primary and secondary zoospores, oogonia, and antheridi a were observed. A physiological study of the growth and sporulation of a r epresentative isolate determined its optimum growth requirements. The isola te can grow from pH 4 to 10, in distilled water, at salinities of 5-30%,, a nd temperatures of 4-30 degrees. Maximum growth was observed at pH 5 and 8- 10, at salinities of 5-10%,, and 25-30 degrees. Production of zoosporangia only occurred in distilled water, 5 and 10%, salinities, with zoospores rel eased in distilled water and 5%, salinity. Zoospore release was also observ ed from 4 to 30 degrees with greater abundance at 25 and 30 degrees, while oogonia and antheridia were produced in distilled water and from 5 to 30%, salinities and at 20-30 degrees.