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Fisheries > Culture Fisheries > Seafishes >Grouper

Fish disease management

Occurrences of diseases are common in intensive culture systems, which generally change various parameters in the environment. Nutritional deficiency and environmental stress indirectly lead to diseases. The causative agents of diseases are parasitic organisms, bacteria and viruses. The diseases that they cause are as follows:

Parasitic Diseases

White Spot

It is a skin parasite commonly occurring at every stage of the grouper's life. The fish is especially prone to this disease when regularly exposed to highly turbid water, which usually occurs during the monsoon months or during heavy rains. In the initial stages, affected fish scrape their bodies on the sides on the pond or tanks. Later stages show fish having opaque eyes and white spots on the body surface and gills. Mortality up to 100% can occur in all stages within three days in every size of fish. Cryptocaryon irritan is the parasite known to infest white spot in grouper.

Treatment - At early stage, treat with 0.10-0.15 ppm malachite green mixed with 25 ppm formalin.

Bacterial Diseases

Flexibacteriosis

This is a common disease found in both freshwater and marine fishes, is caused by a gliding bacteria, Flexibacter sp.; specifically F. maritimus. A serious outbreak of this disease in groupers, known as the red boil disease, was reported in 1996. It was named after the clinical signs of reduced scales and severe hemorrhage on the body surface, casing it to resemble boiled skin. A high mortality rate of greater than 80% can be seen within a week. It was felt that stress from grading was the most significant cause of the disease, making the fish susceptible to invasion from bacteria.

Treatment: Potassium permanganate and oxytetracycline are actively used against the disease in ill fish, especially at the early stages.

Viral Diseases

Viral Nervous necrosis (VNN)

The VNN or whirling disease was formerly known as the encephalomyelitis in several species, such as in barramundi (Lates calcarifer), seabass (Dicentrachus labrax) and in some Japanese fishes. VNN has been detected in culture grouper since 1983.

An important clinical sign is whirl-swimming of infected fish whose swim bladder is generally hyperflattened. There are no lesions on the body surface - the only indication of the disease is darkened skin. The infected fish always swim near the water surface with their body in a curved position. Larvae and juveniles are generally more susceptible to the disease than fingerlings and adult fish. Mortality of up to 90% was reported in larvae and juveniles within a week of infection but was much lower (2%) in fingerlings and adult fish. High mortality of >80% occurred through the VNN during the nursing stages from juvenile to fingerling.

Iridovirus disease

In 1993 a serious outbreak of the disease caused by iridovirus occurred in grow-out net cages holding 20g-5kg groupers overall the cultured area of southern part of Thailand. About 90% of total production was lost from this disease in that year. Disease fish showed no lesions but evidenced a pale body color before they suddenly died.

Virus particles were found in enlarged macrophage cells in the spleen and head kidney of infected fish. It caused necrosis in hematopoitic tissues, including the appearance of enlarged macrophage cells. Hexagonal shape viruses, 120-135 nm (in diameter), were detected. Infected fish were observed to have darkened pectoral fins and caused lower mortality (20-30%) than the earlier one. Transmission of the disease can be accomplished by intra-peritoneal injection and cohabitation of viral cells.

Blister Disease

The outbreak of Blister disease has been observed in fingerling groupers since 1988. Infected fish exhibit an initial loss of appetite, followed by blisters appearing on the body surface and a complete refusal to feed towards the end. Even the blister disease cause a small daily mortality. Natural infection may cause mortality of 60-80% within a month. Infected fish showed signs within 5 days after infection and the onset of mortality occurred.

Health management

It is generally recognized that many diseases in fish culture are often associated with stress. Stressed fish can easily be infected with disease causing agents and this affects growth. The following tips may minimize stress on fish and prevent disease outbreaks:

1. Observe any unusual swimming behavior of the fish, especially during dawn and late afternoon. Fish gasping for air usually indicates low levels of dissolved oxygen. Should this happen, thin out stocks by transferring some of them into another compartment.

2. Weak fish, i.e. individuals refusing to "school " with other fish and those observed as lowing balance while swimming, should be separated from healthy stocks immediately. Stocks found to have sudden loss of appetite and with red "spot-like" wounds on the skin and fish are likely to have a bacterial infection. Use Povidone-iodine, commercially known as "Betadine solution" at 15 parts per million for 5-10 minutes for 3 alternative days, as an affective treatment for bacterial infection. Methylene blue can be used for swabbing. Transfer treated fish to a new compartment.

3. Maintain a distance of 1m between compartments to ensure easy and continuous water flow and maintain ideal water quality for the fish.

Harvesting

Starve the fish 24 hours before harvesting. Harvest depends on the demand of the local and export market. For harvesting, the net cages are hauled up and the fishes are caught by using large scoop nets. Scoop live marketable size grouper (400 g and up) from the cages. Hold grouper temporarily inside the conditioning tank and provide aeration for about 1-2 hours. Adjust water temperature gradually to 18oC by adding packed ice.

Place 3-5 fish an oxygenated double sheet plastic bag, with water at 3-5 cm or at least covering the nostrils of the fish. Place crushed ice on top of the plastic bags to maintain the water coolness during the transport. Place 3-5 fish in oxygenated double sheet plastic bag, with water at 3-5 cm or at least covering the nostrils of the fish. Place crushed ice on top of the plastic bag to maintain the water coolness during the transport. Place plastic bags inside the square Styrofoam box (30 cm x 30 cm x 20 cm) with a cartoon cover having a tag "live fish" and then ready for transport.

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