Occurrence 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:
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 (Ruangpan & Tubkaew, 1993 cited by Yaowanit Danayadol, 1999).
Treatment – At early stage, treat with 0.10-0.15 ppm malachite green mixed with 25 ppm formalin (Danayadol & Direckbusarakom, 1987).
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 ( Danayadol et al. 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> 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.
F. maritimus shows yellowish colonies on cytophaga agar containing 50-100%seawater. It is long and gram negative. This bacteria grows well at 10-370C, at pH 4 -7, exhibits positive reactions to oxidase and catalase and shows a negative reaction on gelatin and arginine.
Treatment : Potassium permanganate and oxytetracycline are actively used against the disease in ill fish, especially at the early stages.
Viral Nervous necrosis (VNN) : The VNN or whirling disease was formerly known as the encephalomyelitis in several species, such as in barramundi (Lates calcarifer, Glazebrook et al. 1990) , seabass (Dicentrachus labrax, Breuil et al. 1991) and in some Japanese fishes (Muroga 1997). VNN has been detected in culture grouper since 1983. An important clinical sign is whirl-swimming of infected fish whose swimbladder 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 (Danayadol et al. 1995).
VNN is caused by the Nodavirus which infects the optic nerves and brain of victims (Danayadol et al. 1995). Infected organs usually vacoulated due to constant lesion. Homogenated filtrate of their brain or eye can bring about pathogenecity after intr-muscularly injection.
Occurrence of the VNN was indicated in grouper fry collection from natural waters along the southern coast of Thailand. High mortality of >80% occurred through the VNN during the nursing stages from juvenile to fingerling.
In 1993 a serious outbreak of the disease caused by iridovirus occured in grow-out net cages holding 20g-5kg groupers overall the cultured area of southern part of Thailand. About 90% of total production (one million individual fish) were lossed from this disease in that year. Disease fish showed no lesions but evidenced a pale body color before they suddenly died (Danayadol et al. 1997). This disease turned up again just only in the year 1994.
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. This virus was thereafter shown to have the same DNA sequences as a virus known to have infected several species in Japan (Miyata et al. 1996).
In 1997, a new species of iridovirus was reported in infected small grouper larvae (1.0-1.5cm) (Kasornchandra and Kongpradit, 1997). Infected fish were observed to have darkened pectoral fins and caused lower mortality (20-30%) than the earlier one. Isolation in EPC (epithelioma papulosum cyprini) and GF (grouper fin) cells was successful, this virus had a bigger diameter (220-240nm). Transmission of the disease can be accomplished by intra-peritoneal injection and cohabitation of viral cells.
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.
Treatment : Kongpradit et al. (1997) reported that this disease is caused by an icosahedral shaped iridovirus, 140-160nm (in diameter). Isolation of the virus in GF (grouper fin) and EPC cells was possible. Pathogenecity was successful through waterborne transmission of cultured virus. Infected fish showed signs within 5 days after infection and the onset of mortality occurred. Mortality rates of experimental transmission as high as 100% were reached within 10 days.
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:
- 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.
- Weak fish, i.e. individuals refusing to “school ” with other fish and ythose 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.
- Maintain a distance of 1m between compartments to ensure easy and continuous water flow and maintain ideal water quality for the fish.
Starve the fish 24 hours before harvesting. Harvest depends on the demand of the local and export market.
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 180° C 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
source: DA – BFAR RF8, Tacloban City