STIMULATING POND PRODUCTION THROUGH ENHANCEMENT AND LINKAGE OF THE BIOTIC FOOD WEBS IN THE PRAWN FARMING SYSTEM

Abstract

A study on stimulating pond production through enhancement and linkage of the biotic food webs in the prawn farming system was carried out at the Fisheries Field Laboratory, Bangladesh Agricultural University, Mymensingh (1° and 3 experiment) and at Boilor Union under Trishal Upazilia of Mymensingh district, Bangladesh (2°° experiment) during 2009 and 2010. In the first experiment, the production performance of giant freshwater prawn Macrobrachium rosenbergii, with monosex male tilapia Oreochromis niloticus, was evaluated in C/N controlled periphyton-based polyculture (C/N-CP) system. The experiment had three treatments: Ctrl, CN (carbon: nitrogen 20) and CN+P (CN plus substrate for periphyton growth) with three replications for each. Prawn (individual weight 0.23g) and tilapia (individual weight 14.97g) were stocked at the same density of 3 m™ and 1 m”, respectively in all treatments. Bamboo branch (mean diameter 2.8 cm and 15 branch m”) were posted vertically into the pond bottoms as periphyton substrate. Prawns were fed twice daily with locally formulated and prepared feed at 10% of body weight at the beginning of study (up to 30 days), and assuming 80% survival, feed application was gradually reduced to 3% in the last month. Additionally maize flour was applied to the water column separately as carbohydrate source for increasing the C/N ratio 20 in CN and CN+P treatment ponds. Water quality parameters, except total alkalinity did not differ significantly (P>0.05) among, the treatments. Both the organic matter and total heterotrophic bacterial (THB) loads in the sediment were significantly (P<0.05) higher in treatment CN+P followed by treatment CN and Ctrl with a significant (P<0.05) increase over time in different treatments. Periphytic biomass in terms of dry matter and chlorophyll a values constantly decreased during the culture period. The feed conversion ratio (FCR) was significantly (P<0.05) lower (2.02) in CN+P treatment than Ctrl (2.96) and CN (2.63) treatments. The addition of substrates did not influence prawn and tilapia size at harvest as well as specific growth rate (SGR), while CN ratio had influence on harvesting size and SGR of prawn. Again, the addition of substrates improved survival of prawn by 20% from CN ratio treatment. Substrates contributed 66% significantly (P<0.05) higher net yield of prawn resulting in 24% significantly (P<0.05) higher combined net yield (both prawn and tilapia).In the second experiment, the comparison of performance between hybrid Red tilapia strain (Mutant, O. niloticus x O. mossambicus ) and GIFT strain of Nile tilapia was evaluated in C/N-CP -based prawn farming system. The experiment had two treatments namely, TR and TG with three replications for each. Hybrid Red tilapia (individual weight 12.10 g) and GIFT tilapia stain (individual weight 11.98 g) were stocked with prawn (individual weight 2.47 g) in TR and TG treatments, respectively at the same density of prawn and tilapia followed in the previous experiment. Management practices were same for two treatments. Water quality parameters did not differ significantly (P>0.05) between the treatments. The abundance of total benthos and periphyton as well as total periphytic biomass differed significantly (P<0.05) between the treatments, and among different months with a decreasing trends (exception to some extent) over the experimental period. The individual harvesting weight, individual weight gain, specific growth rate ((SGR, % bw d°), survival, gross and net yield, combined gross and net yield, economic return (BCR 0,82) were significantly higher (P<0.05) in TG than TR treatment. In the third experiment, the effects of stocking density of tilapia with the inclusion of silver carp (Hypohthalmicthys molitrix) was evaluated in C/N-CP prawn farming system The experiment had three treatments namely, Tyo000, T 15000 and T2900 (named according to the stocking density of tilapia with an individual weight of 0.42g) with three replications for each. Prawn (individual weight 5.27g) and silyer carp (individual weight 9.21g) were stocked at 30,000 and 1,250 ha”, respectively. Management practices were same for all treatments. Water quality parameters, except transparency and chlorophyll a did not differ significantly (P>0.05) among the treatments. The periphytic abundance and biomass differed significantly (P<0.05) among the treatments, and even among the different months. Although the individual ‘harvesting weight, individual weight gain, SGR “were significantly higher (P<0.05) in Tjo000 treatment compared to T1500 and T9000 treatments, respectively, but the gross and net yields of tilapia were significantly higher (P<0.05) in treatment T20000 followed by T5000 and T1oo09 treatments resulting in higher combined gross and net yield of both prawn and tilapia (16.05% and 16.92%, 32% and 33.59% from the later two treatments, respectively) with higher economic return (BCR 0.53) during a 122 days culture period. As a whole, the study revealed that prawn, tilapia and silver carp with a stocking density at 30,000, 20,000 and 1,250 ha", respectively was found to provide an optimum and sustainable production as well as economic benefit in the C/N- CP based culture system.