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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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