Abstract
Production of carps in semi-intensive polyculture systems in Bangladesh is not up to
the satisfactory level yet, which needs to be increased sustainably to ensure food
security of teeming population. An extensive literature based investigation namely
Study-I revealed that there are huge potentials of increasing existing stocking density
and productivity of carp polyculture ponds incorporating catfish, cichilid, perch, SIS
etc. However, very high stocking densities being practiced in semi-intensive and
intensive culture practices of Pangasius, koi and shing are facing various
environmental problems viz eutrophication, algal blooms, low dissolved oxygen,
disease outbreak and low productivity. On the foreground developed through
extensive literature review, two on-station experimental studies were designed namely
Study-II and Study-III based on integrated multi-trophic aquaculture (IMTА)
principles aiming at increasing productivity of carp polyculture ponds and preventing
environmental degradation by incorporating two extractive aquaculture species (snail
and kolmi). Study-II was conducted in nine earthen ponds of 40 m² each having 3
treatments (T) in triplicate. The treatments were: T₁ - carps and shing; T2 - carps, shing
and snail; and T3 - carps, shing, snail and kolmi, and stocking densities were: carps -
20,000 fingerlings/ha at a ratio of 3: 1: 2: 2 (catla: silver carp: rohu: mrigal), shing - 100
fingerlings/m³ cage and snail - 62 kg/ha. Carps in ponds were fed with traditional
supplementary feed consisting of rice bran and wheat bran, and shing in cage was fed
with snail mixed feed ensuring required amount of protein content. Kolmi (water
spinach) was cultivated in the floating trays in IMTA ponds. Although fish production
did not vary significantly (P>0.05) among the treatments, the overall total biomass
production differed significantly (P<0.05), and the highest production was found in T3.
Water quality parameters were within the suitable ranges of fish culture, even the best
in T3 where both total amount of stocked biomass and yield were the highest. Study-III
was executed to understand whether the inclusion of additional biomass and increase
of aquatic plant cultivation area increase total pond productivity and maintain water
quality parameters sustainably through the application of IMTA principles. Although
production trends of various species in both of the studies were similar, total
production was much higher in the Study-III, in spite of lower individual growth of
fish species in Study-III compared with Study-II. Fish production was significantly
(P<0.05) higher (123%) in IMTA ponds than that of CP ponds in Study-III, which was
also higher compared with the previous study by 123%. The overall total yield was
higher (489%) in IMTA system than that of CP system. Furthermore, the concentrations
of NH3-N and PO4-P were found lower by 38 and 21%, respectively in Study-III, possibly due to the higher yield of kolii by 135% compared with Study-II. The higher
level of biomass production with the presence of lower level of organic and inorganic
wastes confirmed that the waste extractives (snail and kolmi) worked well in IMTA
system by biomitigation process. Through an economic analysis namely Study-IV, IMTA was found efficient from the view point of financial investment and return. In
the Study-IV based on Study-III, net returns in CP and IMTA systems were Tk 223,126
and 642,330, respectively asas calculated for 1 ha of pond area. Here, net return and BCR
were higher by 188 and 35%, respectively in IMTA system compared with CP system.