In fishes, the identification of disease may require laboratory examination by a specialist, but sick fish can easily be recognized by an observant fish farmer. Four common symptoms are changes in behaviour, reduced vitality, failure to feed and the presence of lesions.
1. Behaviour change – Fish in good health cannot be seen in ponds except during feeding periods. Should the fish gather in the vegetation near the incoming water supply, or in any particular area of the pond where they can be readily seen, the disease should be suspected.
2. Signs of reduced vitality – Healthy fish swims quickly away from the disturbances along the bank. If fish do not rush away when the fish farmer approaches, some type of disorder should be suspected. Other symptoms include drooping fins, loss of balance or general sluggishness.
3. Failure to feed – Under good water conditions, healthy fish feed vigorously, often taking food immediately after it is provided. Low oxygen concentrations or high water temperatures, as well as diseases, may cause fish to stop feeding, but the failure of the fish to accept feed is a positive sign that pond conditions are not good.
The farmer should take immediate steps to find out why his fish is not feeding.
4. Lesions – Lesions or sores are common in diseases that attack warm water fishes. The obvious ones are open ulcers or large discoloured areas on the body. Others include hemorrhagic areas in the head, body or fins; cysts in the skin, muscles or internal organs; and inflamed areas surrounding a parasite. The presence of any lesion is a positive indication of injury or disease and calls for careful examination.
Correct diagnosis of a disease is the first step toward control. Before any treatment is initiated one should know the water, the fish and the chemical to be used.
Signs and Symptoms of Vibriosis in Fish and Prawns
Vibrio anguillarum, an organism responsible for vibriosis in fish, has received an increasing amount of attention over the years. Originally associated with the red disease of eels, vibrio is now recognized as a pathogen (cause of disease) in other species of fish.
SEAFDEC fish disease researchers have found it to be the cause of mortality in prawns. While it is not clear how vibrio causes diseases in prawns, it is known to cause human gastroenteritis in Japan and the United States.
Vibriosis can occur any time of the year, reportedly even in water temperatures as low as 4°C. It is most prevalent in the summer months. Outbreaks can be expected when the water temperatures reach 14-20°C. Signs of vibriosis are not usually evident until the fish have been in salt water for two weeks.
Diminished feeding activity; erratic, spinning swimming patterns and gathering around the edges of the pens could serve as indications to the fish farmer.
Diseased fish show haemorrhage around the base of the pectoral and anal fins or a bloody discharge from the vent. Vibrio disease tends to approach epidemic proportions when the water temperature rises.
Practical Ways of Destroying Fishpond Pests
Atty. Ceferino Delos Santos, Jr., President of the Western Visayas Federation of Fish Producers, Inc., suggests the following materials and ways of eliminating fishpond pests:
1 – Don’t use chemical formulations containing hydrocarbons in ponds because hydrocarbons tend to sterilize or render infertile the pond soil after some time. Instead, use tobacco dust or other organic materials rich in Retenone or Saponin.
2 – Rice straw may make good compost but takes a long time to decay. It is better to make compost heaps on top of the dikes by mixing rice straw with pond mud and some ammonium sulfate. Cover the compost pile with mud to keep air out of the compost. If available, mix clostridium bacteria with the compost.
These bacteria thrive under anaerobic conditions and hasten the decay of the rice straw.
Finally, use hydrated lime rather than agricultural lime because hydrated lime is faster acting and cheaper. Furthermore, too much agricultural lime will harden the pond bottom.
Prevent Predators From Entering Your Pond
Here are some ways of preventing the entry of predators into the pond. Doing these will also prevent the escape of the cultured fish.
Seal the water gates by placing closure slabs in a parallel line starting from the bottom to a height which is at level with the water line desired to culture the fish.
Pack mud tightly between the slabs. When using pipes or wooden culverts to take in water, place a pair of slabs cut to fit the pipe or culvert. Then add two more slabs on top.
Attach to the pipe a bag of fine mesh nylon net or wire screen by tying them with strings or rubber strips.
The net or screen is placed on the pond side of the pipe or culvert. The pond water should be a few inches below the pipe or culvert to enable the fish farmer to see the net or screen for cleaning when it gets clogged.
Another way is to make a sleeve-like device, one end tied to the pipe and the other end tied to a bamboo pole inside the pond.
The net or screen will prevent the entry of unwanted species, and the escape of the cultured fry or fish.
Another method used is to stuff the pipe or culvert with steel wool, a cloth bag filled with coarse sand, or such other materials which are not soluble in water to act as a filter.
Using Organic Pesticides Recommended By Successful Fish Farmers
Fish farmers should get rid of pests like Chironomid larvae, polychaete worms, and snails in the pond soil because these animals also feed on lablabs thereby competing with fish for food.
A serious question in implementing a pest control program is what pesticides to use. Shall we use organic or chemical pesticides?
Mr Ceferino de Los Santos, Jr.. a successful fishpond operator in Iloilo, advises fish farmers to be careful in choosing and applying chemical pesticides. Most chemicals, he said, leave residual effects which are dangerous to both fish and man.
For instance, chlorinated hydrocarbons like DDT, Endrin, Chlordane, and Gamma BHC, have long-term residual effects which are suspected to cause soil sterility and poor growth of lablabs and other natural foods. Chemicals which belong to the organic-phosphate group like Gusathion leave a residual effect for two weeks.
If the pond is flushed and drained of treated water properly, the chemical – or most of it — is removed.
Its use should be done with extreme caution because animals killed by this poison should not be eaten by people or other animals.
Triphenyltin compounds sold as Duter, Brestan, Baylucide, and Aquatin, which are known poisons for snails and worms, should not be used for ponds because they also cause soil fertility and may
cause stunting in fish.
Instead of chemicals, organic pesticides such as tobacco dust, Derris roots, and other materials containing Saponin and Rotenone, like tea seed cake or Gogo bark should be used, de Los Santos said. Tobacco dust should be applied at the rate of 200-400 kilos per hectare depending on the density of the pests.
Spread this material over the pond after it has been drained, dried, and fertilized. You must, however, soak the sacks containing the tobacco dust in water overnight. Then spread them evenly over the pond soil.
This will prevent the dust or shavings from being blown away by the wind. Tobacco dust or shavings could also be applied by soaking them in drums of water placed above the water gate.
When it is time to let in water into the pond to start the growth of the lablab, the water is allowed to flow very slowly while you pour the solution of water and tobacco dust into the incoming water.
This will save the time and effort of transporting and spreading the material over the whole pond. Water is kept at a maximum depth of 5-10 centimetres in the pond to allow the nicotine to act on the pests for a period of 10 days to 2 weeks. Lablab will start to grow during the same period.
The same technique is followed in spreading the derris root solution. Apply 40 kilos of roots to one hectare with 10 centimetres depth of water. Reduce the volume of water to reduce the quantity needed.
If available, tea seed cake is a good selective poison for pests. It will also kill fish, but not shrimp and prawns. The main problem is that this material is not easily available in large quantities in the country. Its active ingredient is Saponin.
One part per million of Saponin will kill fish. Treatment of up to 11 parts per million will not kill shrimp. It works better in high temperatures.
In fish farming, identifying disease may require laboratory testing by a specialist, but sick fish can also be identified by observing their behaviour, vitality, feeding habits, and the presence of lesions.
Four common symptoms of illness in fish are changes in behaviour, such as gathering in particular areas of the pond or failing to swim away from disturbances; reduced vitality, such as drooping fins, loss of balance, or sluggishness; failure to feed, which can be caused by diseases or poor water conditions; and the presence of lesions, such as ulcers or inflamed areas, which indicate injury or disease.
Vibriosis, caused by the organism Vibrio anguillarum, is a disease that can affect various species of fish and prawns. It is most prevalent in the summer months, and signs of the disease may not be apparent until the fish have been in salt water for two weeks.
Symptoms include diminished feeding activity, erratic swimming patterns, gathering around the edges of pens, and haemorrhage or discharge from the fins or vent.
Fishpond pests can be eliminated using organic materials such as tobacco dust or rice straw compost mixed with pond mud and ammonium sulfate, and treated with clostridium bacteria and hydrated lime.