As mentioned earlier, the environmental conditions in high-density culture ponds of Clarias are conducive to a heavy incidence of disease and mortality, which often decimates almost half the fish stocked. The three most common diseases of cultured catfish are Trichodinainfection of the gills, bacterial infection of the kidney and Gyrodactylus infection. Infections by Aeromonas spp., Flavobacterium spp., Flexibacter columnaris, Pseudamonas spp. And Edwardsiella tarda, have been identified in C. batrachus and C. macrocephalus. Most of theseinfections are believed to be brought in with the fry or fingerlings. Treatment with 25–50ppm formalin in the pond or a one-hour bath of 250ppm formalin in tanks is recommended, before the fry or fingerlings are stocked.
Accumulation of H2S is another cause of mortality in Clarias ponds (Colman et al., 1982). Dissolved oxygen levels do not appear to be so critical for the survival and growth of the species.
Studies of the economics of Clarias farming in Thailand have shown that, in spite of risks involved, most farms make substantial profits (Kloke and Potaros, 1975). For a single crop, the net income to gross returns averaged 37.7 per cent, the net income to the total cost ratio averaged 71.4 per cent and on an annual basis the return of the total capital averaged 108.1 per cent (Kloke and Potaros, 1975).