Prevention better than cure
Fish are as susceptible to disease as any other farmed crop, whether animal or vegetable. Commercial operators are well trained to spot early signs of trouble and take the necessary steps, but for owners of small ponds, keeping their fish healthy may be the first, and sometimes a terminating, challenge.
Often the first sign of ill health is that the fish show less
enthusiasm for their food. Other obvious signs include ulcers or spots
on the skin, or there may be an overall change in colour or behaviour.
Expert
advice is obviously essential because it can be very difficult to
decide what the problem is and how it should be treated. For example,
catfish sometimes develop a liver problem caused by poor quality food.
Improving the food solves the problem. There are also some infectious
diseases where the life-cycle of the disease-causing agent passes
through several different animals or organisms, the host fish being
just one part of the life-cycle. Control may be relatively easy if the
life cycle can be interrupted before the fish are affected. In some
cases, this may require no more than netting the pond to keep birds
away.
Not all diseases are so easily controlled. Epizootic ulcerative syndrome, a disease which affects freshwater fish in Asia, is caused by a fungal pathogen that has so far proved impossible to treat once fish are infected. The disease was first recorded in 1971 in Japan, from whence it has spread rapidly westwards, reaching Pakistan in 1996. All kinds of fisheries are affected; rice paddy fish and small scale fisheries as well as intensive culture systems. Over one hundred freshwater species have been recorded as being susceptible to the disease, including the African catfish. Marine species do not, however, appear to be affected.
The fungus can only attack fish that are already suffering some degree of skin damage. Acid water run-off from soils can cause mild skin lesions on fish that are then at risk of the disease. Parasites, or even netting can also cause sufficient skin damage to allow the fungus to penetrate and invade the muscle tissue beneath. Scientists at the Institute of Aquaculture at the University of Stirling in the UK are working on ways to stimulate the non-specific immune system of the fish with techniques that are cheap and appropriate at all levels of fish culture. Non Asian fish are also being tested for susceptibility but, as with other fish diseases, environmental conditions play a major role in the level of resistance.
There is a very real risk that this disease will continue to spread. Within the region currently affected, some 250 million farming families depend on rice and much of the incidental fish harvested from the paddy fields is an important element of their diet. The main months for harvesting rice paddy fish are between September and February when the disease is most prevalent. Although the disease is not dangerous to human health provided the fish are well cooked, ulcerated fish are, of course, difficult to sell and confidence in freshwater fish farming, especially among potential investors, has been badly affected.
