Study On The Major Bacterial Diseases Of Cultured Turbot (Scophthalmus Maximus) During Larval Stages

Turbot (Scophthalmus maximus) was firstly introduced into China by Yellow Sea Fishery Resesrch Institute in 1992. After achieving technological breakthrough on the fry domestication, broodstock rearing, artificial seed breeding and compond feeds, the fish was extensively cultured along the coast in north China. However, due to the high stocking density, poor management and the susceptivity to disease during larval stages, diseases occurred frequently. They caused high mortalities and reduced the seed outputs of hatcheries.Epidemiologic investigation from 2002 to 2003 showed that white fin and dark-thin diseases commonly occurred, they were caused by bacterial agents resulting in acute death. The gross signs of white fin disease displayed the whitish coloration of dorsal and ventral fins, ulceration with no hemorrhage, as well as anorexia and darken body for moribund animals. The larvae of 25 ~ 40 days was sensitive to the disease and the mortality in 5 ~ 6 days could reach 90%.The fish larvae of 7 ~18 days was susceptible to dark-thin diseases The body of the infected fish turned to black, and the head region looked much larger than tails. The infected fishes were smaller because of their metamorphism was lowered.The dominant bacterium Y13 was isolated from moribund fish associated with white fin disease, it was considered as the causative pathogen to the disease upon succeeding artificial infection by immersion and wounding methods. Histopathological analysis showed that Y13 can cause systemic infection, the target organs including the liver, spleen, kidney, brain, gill and so on. Morphological, physiological and biochemical character of Y13 was achieved by API 20E and other methods. Y13 was gram-negative, curved rods, oxidase and catalase positive, no swimming, pigment producing negative, glucose ferment and H2S production negative. In addition, the optimum salinity for Y13 was 3% and the optimum temperature was 20 ~ 30癈. The pH growth scope was 5.0 ~ 9.0. In general, thecharacteristics of Y13 were similar to Vibrio anguillarum, phylogenic analysis also gave the same result, i.e., Yl 3 was classified as V. anguillarum.Two dominant bacterial isolates Bl and B2 were obtained from the dark-thin fish. They were gram-negative, curved rods, oxidase and catalase positive, O/129 sensitive, glucose ferment. Challenge experiment showed that they were both pathogenic to the fish larvae. Indentification by general methods and phylogenic analysis indicated that Bl was Vibrio splendidus and B2 was Vibrio sp.Vibriosis was considered as the main diseases of turbot larvae. Antibiotic sensitive tests have been conducted on the purpose of controlling the above diseases. Based on the testing, the chosen drugs were applied to the affected fish, and achieved excellent efficiency for the prevention and treatment of the diseases.