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ABSTRACT
Growth performance of Nile tilapia, Oreochromis niloticus, and pearlspot, Etroplus suratensis, under polyculture was studied in inland saline groundwater ponds. Three treatments (1–3) each in replicate of two were maintained: In Treatment 1 (substrate ponds), bamboo poles as additional substrate were provided for the development of periphyton, while in Treatment 2 (fed ponds), no additional substrate was provided; however, the fish were fed on supplementary diet (40% CP). Treatment 3 served as control, where neither the fish were fed on any supplementary diet, nor any substrate provided in the ponds. Irrespective of the treatment (duration 90 days), ponds were fertilized with cow-dung at 7,500 kg/ha/year. Results revealed significantly (P < 0.05) higher fish growth in Treatment 1 due to the provision of additional substrate compared with diet (39%) and control (102%) ponds. The exponential value of “n” for both the fish species was higher (n = 3.2) in ponds provided with additional substrate than fish fed a diet or fish in control ponds. Further, irrespective of treatment, growth performance of O. niloticus was significantly (P < 0.05) higher than E. suratensis. Significant differences in most of the water quality parameters were not observed among the three treatments; however, bicarbonate, pH, electrical conductivity, NH4-N, NO2-N, and o-PO4 were significantly (P < 0.05) lower, while NO3-N remained significantly (P < 0.05) higher in ponds having substrate (Treatment 1). Further, a review of data indicates that some of the productivity indicating parameters (viz, TDS, turbidity, and alkalinity), and BOD5 were significantly (P < 0.05) higher in Treatment 2 compared with Treatment 1 and control ponds. Net primary productivity (NPP), chlorophyll a, and sulphate concentration remained significantly lower (P < 0.05) in ponds with substrate (Treatment 1). No significant variations in carbonate concentration, gross primary productivity (GPP), plankton density, and plankton species diversity were observed among the three treatments. Significantly (P < 0.05) high periphyton biomass in terms of dry matter (1.6 ± 0.03 mg/m2), chlorophyll a (10.3 ± 0.69 μg/cm2), pheophytin a (3.5 ± 0.36 μg/cm2), and mean periphyton productivity (0.9 mg C/m2/day) were observed at a depth of 50 cm in ponds with substrate (Treatment 1). On the other hand, autotrophic index (AI) remained low at 50 cm depth. Epilithic periphyton density and chlorophyll a concentration in substrate ponds were significantly (P < 0.05) lower compared with the other two treatments. No significant variation in sediment quality among different treatments were observed; however, NO3-N and benthos were significantly (P < 0.05) higher in diet and control ponds compared with substrate ponds. Significantly (P < 0.05) higher values of digestive enzyme activity (protease, amylase, and cellulase), accumulation of high muscle protein, muscle, and liver glycogen, and also higher values of viscero-somatic index (VSI) and hepato-somatic index (HSI) were observed in fishes grown in ponds with substrate compared with other treatments. Proximate composition of periphyton indicates high protein (35.0–41.1%) and energy contents (13.3–15.5). These investigations suggest that provision of additional substrate for the enhancement of periphyton production has a greater importance in growth enhancement of Nile tilapia and pearlspot when stocked at the densities used in this study (10,000 fish/ha).
