Clarias are voracious feeders and will consume most types of food. However, as fry, even in the presence of adequate food, there may be difficulty if they do not have an optimal bacterial flora in their intestine. This is probably a function of the provision of high levels of vitamins by bacteria rather that any digestive function of these bacteria, but certainly clarias fry reared in particularly clean conditions rarely survive even on an ideal diet.
Little is known of the nutritional requirements of clarias but as omnivore/carnivores, it is reasonable to assume that they have a high demand for quality animal protein. Trash fish diets provide this readily, but where boiled broken rice and rice bran are used as major components of the diet, this provides a much poorer source of protein. It is also of vegetable origin and it may be deficient in specific amino acids, process unsuitable fatty acids, or have inadequate levels of vitamins.
Trash fish which has been allowed to decay before use, apart from fouling up the pond, may also be markedly deficient in specific amino acids and vitamins, which are destroyed by the enzymic digestion associated with putrefaction. Use of animal faeces as a component of the diet, as occurs in certain integrated farming systems, is of value in terms of the bacterial protein contained in the faeces as well as the undigested waste food present. There are however serious considerations, from a public health point of view, in terms of transmission of animal diseases via the fish, to the consumer. To help obviate these problems, clarias should always be starved for 3 days before marketing and preferably held in clean running water for that period. The fish should always be cooked well, at high temperature, in order to destroy any contaminants. Pelleted foods are beginning to be used on a larger scale in Thailand and so the range of problems associated with their use is likely to increase unless the quality of pellet production is very high.
There are two well recognised nutritional diseases of farmed clarias in Thailand. These are the Fatty Liver Syndrome and the Vitamin C Deficiency Syndrome.
1. Fatty Liver Syndrome. This condition is associated with the feeding of high levels of rice and poor quality trash fish. In such circumstances the liver is unable to metabolise all of the fat taken in through the diet. The fat localises in the liver, where it leads to swelling and fatty infiltration of the liver cells, resulting in loss of liver function. Affected fish have large livers, often white or bronze in colour, and usually show pale gills due to concomitant anaemia. Starvation followed by good quality feeding will usually resolve the problem but in the worst stages of the disease any handling leads to damage to the liver, and possibly fatal haemorrhage.
2. Vitamin C Deficiency Syndrome. This condition manifests itself in a number of ways depending on the severity of the deficiency, the age of the fish and the type of food used. Clarias in the wild generally obtain Vitamin C from macrophytes and algae consumed with their prey. In the pond situation there is neither prey nor available plant vitamin C because the turbidity of the water prevents photosynthesis and thus any algal or macrophyte growth. Thus levels of Vitamin C in the diet must be high to compensate. The three types of Vitamin C deficiency found are:
i. Crackhead. This is a condition of fast growing fish, where there is failure of ossification and underrunning of the osseous plates of the occipital bone, where it meets the body and, occasionally, the symphyses of the other skull bones, leaving a very ragged edge to the symphysis. The skin surface above this often becomes infected leading to serious underrunning necrosis of the skull. Affected fish appear greyish, and there ie a dark roughened line where skin lesions occur. Knocking on the skull produces a very hollow sound, and when sections are cut off the skulls of affected fish, they appear less brittle than normal, and there is evidence of delayed calcification or rarefaction, of the smaller skull bones, with necrotic inflammatory zones at the sites of secondary infection. The condition appears to be particularly associated with the feeding of boiled skinless rice as a high proportion of the diet, and with poor water quality. Although Vitamin C defiency has not been proved to be the cause, the signs, and the curing by improved diet and water quality, make such deficiency the most likely postulate (Fig. 14).
ii. Bone rarefaction spinal deformity. This condition results in severe spinal deformity with decalcification of the spinal vertebrate which, on X-ray or microscopic examination, often appear to be deformed or factured and haemorrhaged. Affected fish grow very slowly, and once deformed, even if the diet is improved, will remain thus. The condition has been associated with pelleted foods of known Vitamin C deficiency and with poor trash fish, and can be a source of severe loss (Fig. 15).
iii. Failure of fibrous wound healing. Vitamin C deficient clarias show marked loss of efficiency at healing skin ulcers. This is due to failure of the scar tissue collagen to form. It can lead to severe secondary infection which can be fatal.
Toxic Components of the Diet
No major effects associated with toxic components of the diets used in Thailand have been established but seed dressings, plant toxins or aflatoxins and trash-fish-mix binders or other contaminants may all contribute to losses of farmed fish. They certainly do play a role in mortalities in African or American catfish culture and thus must be suspected.
