Water quality conditions as disease related stressors in Clarias pond culture
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Quality characteristics of water in two Clarias culture ponds were monitored through a grow-out cycle then assessed as stressors relating to disease and mortality. The two ponds were part of a small-scale commercial fish farm growing Clarias and Ophicephalus. The culture system was representative of the Clarias practice in Thailand where distressed fish and relatively high mortalities are typical. The two Clarias ponds, about 1,000 m2 each, were stocked with 0.6 g. fingerlings at the rate of 7 7 and 78/m2 on March 1, 1980. The fish were fed a mixture of ground trash fish and rice bran (9:1) then harvested after 5 months giving yields of 3,931 (3.73 kg/m2) and 5,088 kg (5.3 kg/m2) from pond No. 1 and pond No, 2, respectively. Correspondingly, average weights of harvested fish were 122 and 144 g, food conversion ratios were 6.4 and 4.9, and survival rates were 40.3 and 47.4%. Two incidences of relatively high mortalities were observed at the same time in each pond; the first occurred 49 to 53 days after stocking and the second was during the period 120 to 150 days after stocking. Pathological examination of the distressed and dead fish revealed that they were highly infested with Aeromonas sp. bacteria. Water conditions in the ponds were characterized by low Secchi disc transparency (av. 9 and 10 cm. through the grow-out period), very little DO (av. late afternoon values over the last 3 months of 0.8 and 1.3 mg/1), generally high CO2 values (in excess of 20 mg/1 through abou t 60 days of the cycle), high BOD (av2. 8.5 and 8.8 g O2/m3/day), and high un-ionized ammonia (2.2 and 3.0 mg/1, av. through the first 60 days).