ABSTRACT-Recently, Mycobacterium infection has been observed in cultured yellowtail Seriola quinqueradiata in Japan but not studied in detail. Diseased fish were lethargic, anorexic and emaciated, and showed hemorrhagic cutaneous ulceration and ascites. The necropsy and histopathological features showed that disseminated necrosis and numerous white nodules were found in the kidney, spleen, liver and heart. Numerous acid-fast bacteria were detected in the above tissues and granulomas. Myositis, hepatitis, splenitis and nephritis due to granulomas and gill inflammation were histologically observed. Almost all granulomas were classified into soft tubercletype. All bacteria isolated from the diseased fish were Gram-positive, acid-fast, rod and non-motile. As a result, they were classified into the genus Mycobacterium. The isolates were identified as Mycobacterium marinum on the basis of biological and biochemical characteristics and the analysis of a partial 16S rRNA gene sequence. An experimental infection test showed that a representative isolate had pathogenicity to yellowtail with disease signs similar to those of naturally affected fish. This is the first report on M. marinum infection in cultured yellowtail.
: Marine ranching of striped jack Pseudocaranx dentex was undertaken at the Goto Station of the Japan Sea‐Farming Association between 1988 and 1999. This stocking method aimed at protecting the released fish from predators and preventing dispersion by feeding for a certain period. The present study examined the reduction process in the number of ranched striped jack and the conditions required to keep the fish in the ranched area. We estimated the sighting rate and the dispersion coefficient from the ranched area per day from 1993 to 1999, on the assumption that natural mortality can be ignored, using the observed number of fish from the sighting observation. The number of fish that remained was represented by a decreasing exponential function of day. This decreasing process was divided into six phases at most. We found two phases when the number of fish that remained markedly decreased in 1993 and 1995. In the first phase, the dispersion was caused by lack of food due to an increased food demand caused by the growth of fish. In the second phase, the ranched fish moved out from the ranched area due to the cessation of feeding. This dispersion was also observed in other years when feeding was stopped.
The Goto station of the Japan Sea-Farming Association conducted marine ranching experiments with striped jack Pseudocaranx dentex from 1988 to 1999 in the Goto Islands, Nagasaki, Japan. Marine ranching is a stocking method where released fish are fed for some period after release in order to decrease initial mortality from starvation. Ranched fish were fed for 0, 90, and 15 days after stocking in 1997, 1998, and 1999, respectively. During the experiment, herons (grey heron Ardea cinerea, black-crowned night heron Nycticorax nycticorax, and great egret Egretta alba) were often observed preying on released fish in the ranching area. The present study estimated the number of fish preyed upon by herons during observation periods in 1997, 1998, and 1999 from the number of herons that flew into the ranching area and the predation rate on ranched fish by herons. The former was counted during sightings. The latter was estimated from the sighting observations and heron regurgitations. The number of herons that flew into the ranching area increased with the feeding period. The numbers of fish lost to predation by herons in 1997, 1998, and 1999 were estimated to be 0, 5,741, and 829, respectively.
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