In Bangladesh, like other tropical countries, the overall conditions are in favour of pond fish culture. Long summer and monsoon in association with lengthy and bright sunshine accelerate good growth of fish under semi-intensive culture system. However, there are also some common problems and constraints that often hinder fish production. Some common hazards of fish culture are described here with possible means of prevention and remedy. Some common hazards of fish culture and their possible solutions are considered below.
Dissolved oxygen level is one of the most important production factor in fish culture. Loss of fish due to reduction in dissolved oxygen content of water is not uncommon in fish ponds of bangladesh. Low level of dissolved oxygen decrease appetite and growth rate of fish. Repeated exposure to low dissolved oxygen make the fish susceptible to diseases. Oxygen deficiency appears as frequent hazard factor when the intensity of fish production increased.
Influence of dissolved oxygen levels at different concentrations on warm water fishes are given in Table-4.1
Table 4.1. Influence of dissolved oxygen at different concentration
Dissolved Oxygen | Effect on fishes |
<1 ppm | Lethal when exposed longer than few hours |
1–5 ppm | Fish survive but poor reproduction and |
slow growth under continuous exposure | |
> 5 ppm | Normal growth and reproduction |
Lethal concentration of dissolved oxygen for some common culturable species of pond fish in Bangladesh is given in Table 4.2.
Table 4.2 Lethal concentration of dissolved oxygen for some common culturable fishes (Doudoroff and Shumway, 1970)
Fish species | mg/l |
Catla catla | 0.7 |
Labeo rohita | 0.7 |
Cirrhinus mrigala | 0.7 |
Hypophthalmichthys molitrix | 0.3–1.1 |
Ctenopharyngodon idella | 0.2–0.6 |
Cyprinus carpio | 0.2–0.8 |
a. Accumulation of organic matters
In some ponds high concentration in detritus can accumulate because of reasons such as over manuring, direct inflow of community waste water etc. In all cases, the increase in organic detritus is followed by an increase in heterotrophic bacteria which also is an oxygen consumer. Usually during day time enough oxygen is produced by photosynthesis to balance the demand but during the night the oxygen reserves in the water are depleted and fish die before sunrise.
Prevention
by using proper combination of fish species under polyculture system.
continuous use of bottom rakers (horra) throughout production cycle in ponds with high detritus level.
b. Algal bloom
Because of intensive manuring and excessive use of chemical fertilizers planktonic algae are often over produced. In this “deep green” water the light penetration is limited to a thin surface level. When the day is calm, dissolved oxygen level in the surface water is very high (above 100% saturation), while the oxygen balance is negative in deeper layers. In windy days, the oxygen from the upper level is mixed with the oxygen poor water of deeper levels. Thus making enough provision of oxygen for night time respiration. But if the night is warm and calm oxygen deficiency will often occur before sunrise.
Prevention
By using higher number of silver carp in the fish stock the danger of phytoplankton bloom can be considerably decreased.
By daily monitoring of water colour and use of fertilizer. When the symptom of algal bloom appears, fertilization should be stopped for some days.
c. Cloudiness:
Cloudiness over a period of 2–3 days reduces light irradiation considerably. As a result the production of dissolved oxygen through photosynthesis is also considerably reduced. This factor can cause problems during monsoon months.
Steps to prevent fish mortality by anoxia
With the appearance of the signs of potential causative factors (algal bloom, calm evening etc.) the pond should be checked during the second half of the night. When the symptoms of oxygen deficiency start (gulping of fish). The following actions should be taken immediately.
Splashing the water surface with bamboo poles.
Add fresh water by pump from neighboring water body or circulate the pond water. Let the water splash as it enters the pond.
Stop or minimize manuring on cloudy days.
Traditionally the slopes of fish ponds are prepared too steep. The slope should be at least 1:2. Old construction should be strengthened at the weak points from time to time. The best solution is to make a fence by bamboo poles on the original embankment and fill the collapsed part with compressed dry soil. As soon as possible plant grass on this part to avoid erosion. Leakages and possible connections with other water bodies must be stopped. The above mentioned method can be used with some soil filled bag also.
Cultivation (horticulture) on embankments should be limited to the top. The sides and 1–2 feet on the brink of embankment should be kept for grass to stop soil erosion.
Decomposing roots of dead or cut trees also create holes in the dike which can cause erosion. These holes should be closed with soil.
In some places this problem causes more damage than other, hazards. Some suggestions which may give some protection beside proper guarding:
Putting bamboo twigs or branches of trees in the water along the dike sides, closely blocking the use of nets.
Fixing barbed wire under the pond surface in criss-cross manner is fairly effective.
Fencing the farm/fish pond properly.
Snakes and birds can cause problems in fish ponds by feeding on fingerlings. Chicken/ducks are often attacked by wolves and jackals.
Protection
Aquatic vegetations in ponds whether floating, sub-merged or emergent inhibit fish production when they are in excess. They absorb nutrients from pond soil and water and hinder production of fish food organisms in ponds, hinder easily movement of fish in ponds, absorb oxygen at night causing anoxia, prevent penetration of sunlight to pond water, shelter fish pathogens and make harvesting difficult.
All aquatic vegetations are to be removed manually from the pond at the time of pre-stocking management. These vegetations, however, can be used favourably in fish culture as valuable manure. Some grass carp can be stocked to keep control of excessive growth of aquatic vegetation.
One of the major hazard to pond fish culture in Bangladesh is the recurrent flood. Every ear, a considerable number of ponds get inundated with flood water. During inundation, most of the stocked fish get away from the pond and at the same time predator and weed fishes enter into the pond and thus the farmers are discouraged to take up pond fish culture in many areas of the country.
To protect the pond from flood, pond dykes if possible, should be raised high above the flood level or some changes have to be made in stocking and harvesting schedule viz. ponds in flood-prone areas are to be stocked with large fingerlings immediately after possible flood period (Sept.-Oct.) and harvested before the next possible flood (June-July).
Many ponds completely dry up in summer season (seasonal pond). If there is any such possibility, the water level should be maintained above four feet. Pumping water from shallow tube well or nearby water source could be a solution. But most of the rural ponds do not have this facility. In these ponds, the production can be maintained with proper management especially by stocking the ponds with fish as soon as water level increases and harvest them before the water level goes down too low to become risky for fish production. Repairing and strengthening of dikes are necessary before the monsoon starts.
Outbreak of fish diseases in most cases originate from management mistakes such as --
Diseased fish can be detected by their unusual behavior and appearance of some clinical and sub-clinical symptoms.
Warning symptoms in pond are:
Some common diseases of fish and their symptoms and treatment measures
DISEASE | CAUSATIVE AGENT | COMMON SYMPTOMS | TREATMENT MEASURES |
Bacterial disease: | |||
Bacteriemia (Haemorrhagic septicaemia) | Aeromonas hydrophila, Pseudomonas fluorescens and possibly others | Shallow ulcerations, haemorrhages and in severe cases the abdomen is swollen and the scales protrude. Internally the body cavity is filled with opaque fluid, and sometimes haemorrhages over swim bladder. | Over crowding, and oxygen depletion are some of the contributing conditions that must be avoided. Terramycin (oxytetracycline) with feed @7.5 g/100 kg body weight/day for 10–12 days. Furazolidone @5–7.5 g/100 kg body weight/ day for 2–3 weeks. Pond treatment @3–5 ppm of potassium permanganate is also a practical approach. |
Columnaris disease | Flexibacter columnaris | Lesions on the head, back and gills. The lesions start as small raised whitish plagues with a reddish peripheral zone and develop into large haemorrhagic ulcers. | Dipping in 500 ppm copper sulphate for 1–2 minutes; Adding chloromycetin @5–10 ppm to pond water; Terramycin with feed @75 mg/kg fish/day. Pond treatment @2–4 ppm potassium permanganate. |
Fungal diseases: | |||
Saprolegniosis | Saprolegnia spp. | ||
Ulceration of the skin, fin erosion, exposure of muscles and jaw bones and in some cases tufts of minute white hair like outgrowths may occur in the affected regions. | Dip treatment in 3% common salt solution or in 500 ppm copper sulphate solution or in 500–1000 ppm of potassium permanganate solution. Swabbing with 10,000 ppm of potassium dichromate is also recommended. | ||
Branchiomycosis | Branchiomyces spp. | Characterized by necrosis in the gill due to intravascular growth of this fungus. Histologically hyperplasia, fusion of gill lamellae and areas of acute necrosis are seen. | Improvement in water Quality, avoidance of over feeding and manuring; and addition of freshwater are quite effective. Draining and liming the pond or treatment with bleaching powder is essential before initiating the next culture operation. |
Myxosporodiasis | Myxobolus sp. | Presence of white cysts of varying diameters on the body, fins, gills, opercula etc. In some cases, emaciation, dark colouration together with presence of cysts and spores in kidney tissues without showing external cysts. | Infected fish should be immediately removed from the pond. Before initiating the next culture operation the pond should be dried if possible and/or thoroughly disinfected with bleaching powder @50 ppm. Provision of settling tank before the water intake in the pond also reduces the risk of infection. |
Metazoan diseases: | |||
Monogenetic Trematode infection | Gyrodactylus sp. and Dactylogyrus sp. | Heavily infected fish show increased production of mucous, frayed fins, skin ulcers and damaged gills, Microscopic observation of the skin temporary mount of a portion of gill show the presence of the parasites. | Bath in 100–250 ppm of formalin ranging from 1 to 3 hours, is very effective, Dip in 2–5% salt solution till the first sign of distress is equally beneficial. Bath or pond treatment with some soft organophosphorus insecticide (Malathion and @ 0.25 ppm in pond) lesion/smear is also equally effective. |
Black spot disease | Diplostomum sp. | Development of small black or brown spots on several parts of the body and fins. Microscopic examination and dissection helps in locating rolled up and slowly moving worms embedded in the connective tissues. | Fish exhibiting black spots may be given an hour bath in 10 ppm picric acid solution. Removal of aquatic snails and preventing the entry of birds are some of the preventive measures. Infection does not spread from fish to fish and hence it is not worth treating infected stock. |
Argulosis | Argulus sp. | Development of haemorrhagic patches over the body and presence of the parasite in large number in and around the lesion. | Benzene hexachloride application in pond @0.02 ppm with a second subsequent treatment after a week. Affected fish should also be given dip in 500–1000 ppm potassium permanganate solution which helps in avoiding secondary infection as well as accelerate the healing process. Malathion @ 0.25 ppm in pond also effectively controls the infection. Malathion also requires a second treatment after a week interval. |
Lernaeasis | Lernaea sp. | Anaemia, severe ulcerations and presence of attached cylindrical parasite of 1 to 2 cm length hanging outside. Sometimes causes mass mortality in carp nursery and rearing ponds. | Baths in concentrated solution of salt and potassium permanganate is reported to be effective. However, the author has found very little improvement by potassium permanganate treatment. Juveniles are embedded in the skin and hence remain unaffected. Bromex completely cures the infection when applied @0.15 ppm. |
Leech infection | Piscicola sp. | They are not very dangerous. They affect the fish by their attachment and feeding. Area of attachment normally exhibit excessive mucus production, and some signs of haemorrhages. Open wounds are often infected by bacteria and fungi, Attacked fish show attached parasite, irritation and restlessness. | Removal of aquatic vegetation and maintenance of pond hygiene is the most important preventive measure. Hard objects such as stones, logs etc. should also be removed. Disinfection of pond with unslaked lime @ 250–300 kg/ha should be done prior to next rearing operation, Short bath in 3–5% salt solution is very effective treatment. |