Abstract
Studies on the ecology of the pathogenic bacteria Aeromonas hydrophila in indigenous and exotic carps under polyculture conditions were conducted at BAU campus for a period of two years. Polyculture of three indigenous carps, viz., rui (Labeo rohita), catla (Catla catla), and mrigal (Cirrhina cirrhosus) and two exotic carps, viz., silver carp (Hypophthalmichthys molitrix) and common carp (Cyprinus carpio) in three different stocking densities (40, 60 or 80 fishes per decimal) revealed that the stocking density of 9880 fishes/ha with species combination of rui 8 : catla 6 : mrigal 6 : silver carp 14 : common carp 6 was the most suitable that gave a net production of 2089.62 kg/ha/10 month. Results of qualitative and quantitative study of bacterial flora in above fishes, water and mud showed that the total bacterial load in water was 1.2 × 10⁷ cfu/ml, in mud was 6.2 × 10⁷ cfu/g and in fishes was 5.0 × 10⁵ cfu/g in average. Amongst 50 bacterial isolates, the identified genera were Aeromonas, Bacillus, Flavobacterium, Pseudomonas, Micrococcus, Streptococcus, Staphylococcus, Corynebacterium, Acinetobacter, Moraxella, Nocardia, Enterobacter, Vibrio, Edwardsiella, and Salmonella, Aeromonas being the most dominant genus. Five species of the genus Aeromonas could be distinguished by morphological, biochemical and physiological characteristics as: Aeromonas hydrophila, A. sobria, A. caviae, A. veronii and A. jondea, A. hydrophila being the most dominant one. Pathogenicity of eight selected isolates of A. hydrophila isolated from kidney, liver and intestine of the carps through intramuscular and intraperitoneal injections at doses of 2 × 10⁶ and 2 × 10⁵ cfu/fish in each case and through immersion method at doses of 2.2 × 10⁸ /ml and 2.2 × 10⁷ /ml proved that all the tested isolates were pathogenic to the fishes. The mortality rates were 40% to 100% within 2 to 12 days. Effect of lime as a therapeutic in reducing the abundance of A. hydrophila and its survival in the carp culture environment was also conducted in four cemented tanks. The fishes were either immersed for 30 min in a bacterial suspension of the concentration of 2.2 × 10⁹ cfu/ml of A. hydrophila or released in 2.5 - 2.7 × 10⁴ cfu/ml of bacterial suspension for long time (throughout the experimental period of 15 days). Each of the above treatment was either subjected to lime treatment or not. Mortalities in the tank without lime treatment were higher than in tank with lime treatment in both the methods. A. hydrophila was recovered from the water of all tanks every day but recovery decreased gradually up to 15 days after which no A. hydrophila could be detected. So it was proved that A. hydrophila could not survive for a longer period in water without host. Lime directly reduced the abundance of the pathogen. It was revealed from the above studies that Aeromonas hydrophila bacteria were present in the culture habitat of the indigenous and exotic carps. By its opportunistic pathogenic nature it may cause threat to the culture of the above carps causing histopathological changes leading to their high mortality. However, it was proved that the pathogen may not survive long time in the water and lime treatment might be a suitable measure for the control of the pathogen. It needs further study to know the detailed ecology of the pathogen.