Bacterial Diseasesand Controlof Tongue Fish (Cynoglossus Semilaevis) In Recirculating Aquaculture System

Tongue fish （Cynoglossus semilaevis） was usually cultured in recirculatingaquaculturesystem （RAS）.In recent years, mass mortalities frequentlyoccurredand causedeconomiclosses.In order to understand and control the diseases, the epidemiologicalcharacteristics of bacterial diseaseshas been performed in RASfarms located in Qingdaoand Yantai city. In which,ascites, skin-ulcer disease andabscess diseasewere found to bethe major bacterial diseases.The fish associated with ascites had a swelling belly. In severe cases, a portion ofdigestive tract even bulged out from anusbecause of the fluid squeezing in abdominalcavity and digestive tract. Numerous bacteria could be found in ascites through lightmicroscope.Using TSB and TCBS solid medium,one strain （CS11061001B） wasisolated from ascites and dominate90%of bacterial colony in TSB plate.The strain wasidentified as Listonella anguillarum through physiological&biochemical tests and16SrDNA sequencing analysis. For histopathological studies, most tissues and organs suchas liver, spleed and intestine were collected and processed. Severe lesion was clearlyshown, and could cause functional damage.The other two dominance strains （CS10111602A&CS10111602B） were isolatedfrom diseased tongue fish associated with skin ulcer disease.They were identified asVibrio splendidus and Vibrio harveyithrough physiological and biochemical tests and16S rDNA sequencing analysis.In challenge experiment, the infection fishes wereintramuscularly injected with the two strainsrespectivelyand showed skin ulcer clinicalsigns.Dominance strains also could be isolated again from the focal lesion of infectedfish. Based on histopathological analysis, the affected fish showed the skin epithelialcells necrosis and shed off, as well as muscle fiber shriveled, fractured, erosion, andblood infiltration. In addition, preliminary studies were completedin abscess disease. A dominancestrain （CS10111501A）was isolated from diseased fish. The strain is gram negativeappearing circular and transparent colony on TSB medium plate and slow-growth.Based on16S rDNA gene homology analysis, this strain is similarity as Vibriopomeroyiand the BLAST result is99%in National Center for BiotechnologyInformation （NCBI）.Therefore, the strain was identified as Vibrio pomeroyi.The purification process of water includes filtration of arcscreen, strongaeration,biological filtration and UV sterilization in the recirculating aquaculture system（RAS）.The pathogens of skin ulceration,Vibrio splendidus andVibrio harveyiwereselected to be the indicator bacteria. Bacterial distribution and extermination in RASofflatfish Cynoglossus semilaevis were conducted. The results showed that the pathogenswere distributed on fish body, bait residues, organic dirt on tank wall and eachinstrumental process of RAS. Unhealthy fish stock （juveniles） and tools used in thecultivation could also bring pathogens into the culture system. Extermination trailsillustrated that the filtration of arc screen, strong aeration and UV sterilizing were theimportant parts of the elimination for the bacteria. The disinfection rate of pathogens inthe organic dirt on tank wall was higher than90%by using KMnO₄at the concentration25×10^-6for1.5min. While the disinfection rate of pathogens on the body surface of theflatfish reached94.49%by using H₂O₂at the concentration100×10^-6for10min.The epidemiology, etiology, and histopathology analysis were studied for majorbacterial diseases in RAS of Tongue fish.And it is the first time to report the distributionand extermination of pathogenic bacteria in RASof flatfish. It will be helpful to providethe theoretical data and reference for the healthy management and disease control inRAS.