The present project was planned to study gradual replacement of Cirrhinus mrigala with Cyprinus carpio and its impact on pond ecosystem. 900 fish individuals belonging to six species viz. Catla catla, Labeo rohita, Cirrhinus mrigala, Ctenopharyngodon idella, Hypophythylmichthys molitrix and Cyprinus carpio were stocked in four earthen ponds. The dimensions of each pond were 220 X 198 X 7 feet length, breadth and depth. The stocking density in pond 1 (T1) was C. catla 150, L. rohita 200, C. mrigala 200, C. idella 150, H. molitrix 150 and C. carpio 50. The stocking density of C. mrigala and C. carpio in pond 2 (T2) was 150 and 100, in pond 3 (T3), 100 and 150 and in pond 4 (T4) it was 50 and 200, respectively while the stocking density of all the other four fish species remained constant in all the four ponds. All the fish were fed with a diet of 25.16% crude protein @ 2% body weight. C. idella and C. mrigala showed maximum growth in T1, C. catla and H. molitrix in T2, L. rohita and C. carpio in T3. Maximum growth was observed in T3 followed by T4, T1 and T2. Among fish species C. idella and C. carpio showed higher growth rates than the rest of fish species. Our results reveal that in polyculture system stocking density of C. mrigala and C. carpio in a ratio of 1: 1.5 gives better results.
In treatment 1 pond higher SGR 0.552% was observed in H. molitrix and lower 0.238 in C. carpio. Similarly, in T2 pond maximum SGR 0.703% was observed in H. molitrix and minimum 0.260% in C. idella. Maximum SGR 0.409 % was observed in H. molitrix in on T3 pond while it was minimum 0.153 in C. mrigala. In T4 pond maximum SGR 0.322% was observed in L. rohita while it was minimum 0.139 in C. idella.
During present study major water quality parameters remained in the favorable range for fish culture, average value of temperature remained in the range 27.08 to 28.66 oC, salinity 0.86 to 0.99 ppt, DO 5.15 to 5.91 mg/L, EC ranges from 2.23 to 2.32 Ms/cm and pH ranges from 8.04 to 8.23 were observed within the optimum range throughout the experiment. Statistically significant variations in nitrate content of water were observed in T1 and T4 ponds while non-significant differences for nitrates were recorded in T2 and T3 ponds. Similarly, phosphates in water showed significant differences in T1 as compared to T3 and T4 ponds while T2, T3 and T4 showed non-significant differences for phosphates. Light penetration varied significantly between T1, T2 and T4 ponds and its values varied from minimum 18.17±0.946 cm in T4 to maximum 25.50±1.057 cm in T1 ponds
The phytoplankton density was similar in ponds during the given period except the start month of study the low plankton values in April due to the ponds being filled with fresh water and immediately stocked with fish. The density of zooplankton components was higher under T3 where silver carp gained more weight. A total of 11 species of Bacillariophyceae, 10 species of Cyanophyceae, 30 species of Chlorophyceae, 2 species of Euglenophyceae, 3 species of Cladocera, 4 species of Copepoda, 9 species of Myxophyceae, 9 species of Crustacea, 11 species of Rotatoria and 1 species of Olygochaeta were identified. The number of identified species was relatively small, consisting of eurytopic species only, possibly as a result of the heavy activity these ponds suffered. Microcrustaceans, being valuable food organism for many fish species, were represented by genera Daphnia, Cyclops, Bosmina, Moina and Scapholeberis.