The tremendous growth in fish farming activity in Pakistan has highlighted various issues of fish husbandry including fish diseases and their control. The problems of fish diseases are related to stocking density, level of aquaculture technology applied and inputs going into the fish ponds. In fish farms, the fish are densely stocked and thus are more susceptible to different diseases. Uncontrolled and unregulated transport of fish and poor farm management is also considered as cause of spread of disease. Disease causes mortality, poor growth, loss of fecundity and minimize production.
Fish are affected by viral, bacterial, fungal and parasitic diseases/ infections. Moreover, poor water quality in fish ponds also put fish under stress which can leads to fish mortality. In ponds fish are also under stress of predation by some predators at the early age. The description of some commonly occurring fish diseases, predators and their control is given as under:
Viral Diseases in Fish
Below is a list of viral diseases that affect farmed fish in Pakistan:
Viral Hemorrhagic Septicemia
It is an infectious disease caused by cold water rhinovirus. Fish may be lethargic and congregate at tank/ponds sides or outlets, have pale gills, dark body color, and exophthalmos and in some cases intermittent periods of erratic spiraling swimming. Hemorrhage may be visible in the eyes and skin, within the muscle and internally in the viscera and intestine. In more chronic cases some of the above signs may be obvious with abdominal distension due to edema in visceral organs. Disinfection and quarantine is the most effective means of controlling epidemics. Iodophore treatment will readily eliminate the virus from eggs of carriers, making it reasonably certain that progeny will be free of this disease.
Spring Viraemia of Carp (SVC)
It is an acute hemorrhagic and contagious viral infection specifically of the common carp. The disease usually erupts in spring and causes mortality of the adults as well as the young stock. Viral outbreaks are often complicated with secondary bacterial infections. The signs of disease are; lethargy, distended abdomen, petechiation on gills, skin and around eyes, oedematous vent and trailing mucoid casts, exophthalmia and internally ascites with focal hemorrhages in swim bladder and other visceral organs. Still there is no known treatment.
Bacterial Diseases in Fish
Below is a list of bacterial diseases that affect farmed fish in Pakistan:
It is chronic progressive disease caused by certain Mycobacterium bacterial species. Many of these bacteria occur naturally in the aquatic environment. Contaminated feed (based on uncooked trash fish) is a common source. There is no fully effective treatment, therefore, the best course is to cull the diseased fish and disinfect premises with 10,000 ppm chlorine or 60 – 85% alcohol. Erythromycin, rifampicin, and streptomycin is effective against experimental infections. Kanamycin may be effective in some cases. Gloves should be worn when handling suspect or infected fish or when cleaning contaminated surfaces.
It is a disease of pond fishes. Distended belly and oozing out of white or yellow exudes, erected scales and abnormal swimming are the common symptoms of this disease. Use of antibiotics can obviate the problem. Oxytetracycline, chloramphenicol, and nifurpirinol have successfully controlled some outbreaks.
It is another bacterial disease which is very common in polluted pond environment. Bloody patches at the base of the fin and gradual erosion of fins are common symptoms. Regular baths in copper sulfate and potassium permanganate can cure the disease. Oral administration of antibiotics can be helpful in controlling the disease.
It damages the gills. The gills which normally secrete excessive mucous become firmly glued with each other and hardly separate and open during breathing. Fish feel difficulty in breathing and expire if the situation intensifies. Maintenance of pollution free pond environment is of extreme importance. Disinfection of pond can help. Baths in copper sulfate solution or potassium permanganate can relief the condition. Administration of oxytetracycline and chloramphenicol can successfully control if disease outbreaks.
Furunculosis is a fatal disease caused by the bacteria. High mortalities, without external signs of infection are often associated with acute furunculosis, although anorexia may be present. Other fish may appear dark in color, lethargic with reddening at the fin bases or head. Internally there may be widespread petechiae in the viscera and a swollen spleen. The hemorrhagic lesions may be present in the musculature with bloody discharge from the vent. Broad spectrum antibiotics are effective in controlling an outbreak, but increasing antibiotic resistance is observed and sensitivity should be tested. Nevertheless oxytetracycline, furazolidone, oxolinic acid and potentiated sulfonamides have been used successfully.
Bacterial Gill Disease
Bacterial gill disease is an important disease in gill tissue. Gill is the only target organ and clinical signs include lethargy, dyspnea, coughing and flared opercula. Early cases of this disease can be treated by prolonged baths in potassium permanganate and copper sulfate, however, advanced cases demand systemic antibiotics. Providing adequate oxygen is useful supportive therapy.
Fungal Diseases in Fish
Below is a list of fungal diseases that affect farmed fish in Pakistan:
It is infections in fish and fish eggs associated with the water molds and fungi. Lesions are focal, grey-white patches on the skin or gills of fish which, when examined underwater have a cotton-wool-like appearance. Gills, mouth or bronchial cavity can also be affected. Internal infections in the peritoneum or gastrointestinal tract in younger fry can also be seen and results in high mortalities. Freshwater fish eggs are also very prone to infection. Overcrowding and poor water quality may also give rise to infection. Affected fish are difficult to treat although formalin, formalin with bronopol (Pyceze) and salt as bath treatments show some benefit. Liquid paraffin in the feed is of benefit for fry which have fungal infections in the gastrointestinal tract. There is no protective vaccine and no effective treatment. To minimize fish losses in infected ponds water, exchange should be stopped and lime or hydrated lime and/or salt be applied. Malachite green is the most effective drug but is not approved due to human health concerns.
Parasites (Amoebic and protozoan infections) Diseases in Fish
Below is a list of parasitic diseases that affect farmed fish in Pakistan:
It is the infection of fish caused by costia, a protozoan parasite of skin and gills. Mortality in fry, flashing, thickened mucus on skin giving an opaque blue-grey appearance to body. Respiratory distress and mortality with gill infestation, epidermal or epithelial (gill) necrosis and sloughing are common symptoms of this disease. Formalin baths or flush treatments are effective. Salt may also help as a bath in some cases. Improving the tank or pen environment will usually help the situation with regard to any parasite infestation.
Amoebic Gill Disease
It is a significant disease which results in respiratory distress and mortality. The parasite can survive in the water and maintain its infectivity for at least 14 days. The most effective and frequently used treatment is freshwater baths. Briefly fish are bathed in freshwater close to zero salinity for 2 to 6 hours. Salt, formalin and Virkon flowing treatments all appear to help to control the parasite, although moving fish from severely affected tanks to clean tanks is also of benefit. Control by monitoring and early, accelerated harvest with rapid processing can be practiced if present.
Diseased fish are lethargic and are found in slower moving water. Affected fish can act as carriers of the parasite and the parasite has spread between rivers and farms mainly by restocking and transport of live fish.
Lernaeosis is the most prevalent disease among all the carps including all the stages of development. Fish feels restless, common red patches are visible on fish body and it dies if intensity of parasite exceeds certain limits. Organophosphate is usually effective, prolonged immersion treatment should be repeated every 7 days for 28 days. Diflubenzuron is less toxic to fish and is highly effective.
Anorexia in Fish
Depletion of oxygen in fish pond is called anorexia. This condition may be due to rise in water temperature in pond, over stocking of fish seed and other biological factors. In anorexia condition, fish come on the surface of water, stop feeding and show restlessness. The Anorexia condition in pond can be improved by addition of fresh water and agitating pond water.
There are some predators which prey on fish from fry to adult stage which include water insects, amphibians, reptiles, birds, carnivorous fish and even mammals. Harmful insects include water beetle, water bugs, water scorpion etc. which attack fish eggs and fry, but can be controlled by improvement in fish pond management.
Frogs and toads are also predators but the best way to control them is to eliminate the breeding grounds of these amphibians. Reptiles such as tortoises and snakes also prey on fish and can be controlled by netting them out of pond and destroying them.
Aquatic birds like king fisher, fishing eagle, Heron prey on small and big fish and can be controlled by shooting. The presence of carnivorous fish such as Mystus, Wallago and Channa species in pond also affects farm productivity. These prey fish may enter the farm through canal water or by stocking unidentified fish seed from natural waters. These unwanted fish can be eradicated by use of rotenone and even installing fine screens at water inlet.
Water rats and otter eat fish, eggs, fry and big fish. These animals even destroy fish feed. These predators can be controlled by fixing fences around the ponds and catching them in traps.