As the global population exceeds 6.5 billion in recent years, aquaculture is destined to play a more important role in providing animal protein in terms of aquaculture potential and high feed conversion rates. Technical and non-technical aspects of aquaculture are discussed in this paper to help sustainable development of this knowledge-based industry, with emphasis on food safety and human welfare. Technical strategies include consolidated standards for each species, introduction of exotic species, improvement of feeds, selective breeding, disease control and others. Non-technical strategies include training of personnel, proper licensing system, implementation of appropriate regulations and laws, efficient monitoring system, and establishment of local and global aquaculture networks.
Salmonid fish are important species for hatchery reared and released fish. Flounders and shrimp are also important species for seed production and sea-farming in Japan. Viral disease is one of the limitations of successful propagation of these species. Methods currently used to control viral diseases are 1) hygiene and sanitation in facilities, 2) disinfection of rearing and waste water using U. V. irradiation, ozonization and electrolyzation, 3) selection of pathogen-free brood stock by cell culture isolation and detection of specific antibody against important pathogens with ELISA or viral gene with PCR, 4) health monitoring of hatched fry by cell culture isolation and detection of pathogens by immunological and molecular biological methods, 5) control of normal intestinal flora by feeding bacteria producing antiviral substances, and 6) temperature manipulation. Under these circumstances, hatched fish and shrimp are healthy and specific pathogen free, but there is still a possibility of infection by some pathogens in environmental waters after they are moved to ponds or net pens outside of facilities. For prospective studies, development of effective vaccines, vaccine injection machines and immunological tools for evaluation of vaccination effect are necessary.
As the mariculture technique had been developed, annual production of shallow water cultures exceeded that of adjacent water fisheries in recent years in Korea. For reducing production losses from infectious diseases in fish aquaculture, various strategies were employed including treatment of effective chemotherapeutics, development of herbal immunostimulants and vaccination, etc. For the treatment of bacterial diseases 27 antibacterial agents have been used under government license. Frequent use of these drugs was accompanied with occurrence of drug-resistant bacteria. Recently, new bacterial diseases have occurred in cultured fishes. As the studying on the control activities of medicinal herbs to bacterial growth, the wormwood Artemisia asiatica, the Galla rhois Rhus javanica and the veiled lady mushroom Dictyophora indusiata were revealed high antibacterial activities to the Gram-positive and negative bacteria. For the enhancement of nonspecific immune response in mariculture fish, medicinal herbs and other several materials were investigated as immunostimulants. Olive flounders Paralichthys olivaceus fed on the diet supplemented with 0.1% extract mixture of the wormwood and the barrenwort Epimedium koreanum showed higher relative percent survival (RPS), when they were challenged with bacterial pathogens. To prevent various infectious diseases, two vaccines were developed and commercially available including vaccines on Edwardsiella tarda and Streptococcus iniae.
Homologous DNAs (M DNA) to the infectious hematopoietic necrosis virus (IHNV) M gene were detected in river-homing chum salmon Oncorhynchus keta sperm, eyed-egg embryos, alevins and fry collected from nine rivers and four hatcheries in Hokkaido Prefecture, Japan, in 1998-2002, although no virus titer was observed in the samples. Real-time PCR assay revealed M DNA at the highest number of 1.7 × 104 copies/mg of tissue. Nucleotide sequence similarities between the M DNA and the viral M genes were ≥97%. When CHSE-214 and RTG-2 cells were inoculated with IHNV, the cells contained 3.8 × 101 to 1.8 × 103 copies of M DNA/105 cells and showed cytopathic effects (CPE). Salmonid fish fry and yearlings infected with the virus contained 1.3 × 101 to 2.6 × 103 copies of M DNA/mg of tissue. CHSE-214 cells transfected with the viral RNA produced M DNA without showing CPE. Reverse transcriptase (RTase) activities in the fish cells and the fish tissues were equivalent to 86-578 pg HIV-1 RTase/106 cells and 2.1-12 pg HIV-1 RTase/mg of tissue, respectively. These results suggest that M DNA is produced from IHNV RNA and retained in the fish.
Streptococcus parauberis, a Gram-positive bacterium, is known as the causative agent of mastitis in cows as well as streptococcicosis in turbot Scophthalmus maximus and Japanese flounder Paralichthys olivaceus. It has recently become one of the most important pathogens of cultured Japanese flounder in western Japan. In this study, we investigated the serological nature of S. parauberis strains isolated from Japanese flounder in Japan. As a result, the strains were distinctly divided into two groups, serotype I represented by strain KRS-02083 and serotype II represented by strain KRS-02109, by an agglutination test with rabbit antisera. Autoclave-extracts of bacterial cells derived from each serotype formed a type-specific precipitation line with each corresponding antiserum in an agar double diffusion test. Acidic polysaccharides were detected in the autoclave-extracts of the bacterial cells. Electron microscopy revealed capsular structure on the bacterial cells pretreated with each corresponding antiserum. This finding suggests that capsular polysaccharide antigens serve to differentiate two serotypes.
Antimicrobial susceptibility of Streptococcus parauberis against nine drugs were investigated in 64 strains isolated from Japanese flounder Paralichthys olivaceus at different culture areas of western Japan, and resistance genes were determined by PCR and Southern blot hybridization. Five of the tested strains belonging to S. parauberis serotype I (44 strains) were resistant to oxytetracycline and erythromycin and were found to possess both tet(S) and erm(B) genes. Minimum inhibitory concentrations (MICs) of oxytetracycline and erythromycin were 128 and 512 μg/mL, respectively. Southern blot hybridization analysis showed that the tet(S) gene was encoded on an 11-kb conjugative plasmid, while the erm(B) gene resided on the chromosomal DNA. All of the examined S. parauberis serotype II strains (20 strains) were tetracycline-resistant (MIC = 32 μg/mL) and were found to possess tet(M) gene. Both int (integrase) and xis (excisase) genes were also detected in all of the serotype II strains, suggesting the presence of Tn916-related element in these strains. These results present a threat of difficulty in treatment of S. parauberis infection by tetracyclines, especially those caused by serotype II strains.
After transporting Asian seabass Lates calcarifer from a fish farm to the Burapha University facilities, they began to die. The affected or moribund fish exhibited abdominal swelling, yellowish decoloration around the anus, darkening of the gills and exophthalmic eyes. The internal signs of the disease included abdominal cavities filled with a cloudy yellow and gelatinous fluid together with liver hemorrhage. Isolated bacteria from several organs were biochemically characterized as Photobacterium damselae subsp. damselae. It was further confirmed by PCR. An infectivity experiment by intraperitoneal injection with an isolate LCA 24907 showed its virulence to Asian seabass with LD50 of 8.1 × 105 CFU/g fish body weight. This is the first report of isolating P. damselae subsp. damselae from Asian seabass.