Responses of some bio-geochemical cycling bacteria and their activities to management protocols under polyculture with Indian major carps and freshwater giant prawn

Estimations of the population size of some biogeochemical cycling bacteria and their activities were made in outdoor tanks (4500 l) in triplicate used for four treatments: monoculture of freshwater giant prawn, mixed culture of Indian major carps (catla and rohu) and polyculture of Indian major carps and freshwater giant prawn and a reference control with application of feed alone during the grow out period of 125 days, while maintaining the same initial manural dose and supplementary diet for all the treatments. Enumeration of different groups of bacteria at regular intervals from water and sediment samples of these tanks revealed that mean counts of heterotrophic bacteria — HB (water — 11.81 × 103 ml− 1, sediment — 33.8 × 105 g− 1), ammonifying bacteria — AB (water — 9.65 × 103 ml− 1, sediment — 28.8 × 105 g− 1), protein mineralizing bacteria — PMB (water — 1.86 × 103 ml− 1, sediment — 18.45 × 105 g− 1) and nitrifying bacteria — NB (water — 33.66 × 103 ml− 1, sediment — 13.62 × 104 g− 1) were distinctly reduced (ANOVA, P < 0.001) in the carp–prawn polyculture compared to prawn monoculture or mixed carp culture. On the contrary, there was significant rise (ANOVA; P < 0.001) in the counts of aerobic cellulose decomposing bacteria — CDBA (water — 3.93 × 103 ml− 1, sediment — 17.06 × 105 g− 1), anaerobic cellulose decomposing bacteria — CDBAn (water — 1.50 × 103 ml− 1, sediment — 44.26 × 105 g− 1) and Vibrio sp. (water — 11.21 × 103 ml− 1, sediment — 39.21 × 105 g− 1) in the mixed carp culture than the rest. Reduction in the counts of bacteria in the carp–prawn polyculture implied a better utilization of feedlot resulting in less accumulation of feed remnants by the compatible and symbiotic association of fish and prawn. As a consequence, a clearly more balanced ecosystem and benign environment with reduced level of feedlot-dependent pollutants has been developed. It becomes apparent that main differences among the culture systems were the function of organic matter utilization which was zero in the control, and maximal in the polyculture and the resulting organic substrates accumulation in the water. In the carp–prawn polyculture, there was significant reduction in the counts of those groups of biogeochemical cycling bacteria (AB, PMB, NB, CDBAn, Vibrio sp.) which mostly belong to the Gram-negative group. This indicates the possibility of exploiting carp–prawn polyculture as an associative system for reducing the Gram-negative water borne enteropathogenic bacteria that causes epidemiological problems.