Polyculture of red seaweed Gracilaria tenuistipitata with postlarval giant freshwater prawn Macrobrachium rosenbergii at different densities and feeding rates in the nursery phase
摘要
A polyculture system of postlarval freshwater prawns (PL) Macrobrachium rosenbergii and red seaweed Gracilaria tenuistipitata was tested at different stocking densities and feeding rates to evaluate the effects on water quality, growth, and feed conversion ratio. The first experiment used a 3 × 3 factorial design with three prawn densities (1000, 1500, and 2000 PL m−3) and three red seaweed densities (0, 1, and 2 kg m−3) that were randomly established in triplicate tanks with a salinity of 5 g L−1 for 30 days. The results showed that the prawn and seaweed density had no interaction effect (p > 0.05) on prawn growth, survival, and production, nor on feed conversion ratio (FCR). Prawn growth in terms of weight, survival, and FCR at densities of 1000 and 1500 PL m−3 was not significantly different (p > 0.05) and was superior to that achieved at a density of 2000 PL m−3. Furthermore, a seaweed density of 1 kg m−3 resulted in the highest growth rate and production. These results suggest that a combination of 1500 PL m−3 and 1 kg m−3 seaweed would be optimal for nursery-rearing PL; this was applied to a feeding trial. In the second experiment, the PL and red seaweed polyculture system was assessed at various feeding rates during the nursery phase. The seven feeding treatments comprised a control group (monoculture) that was fed commercial feed (CF) with a 100% feed ration (15% of biomass per day), while the prawns in the six polyculture groups received CF at feeding rations of 100%, 80%, 60%, 40%, 20%, and 0% of the control ration. After 30 days, the growth rate, survival, and production of the prawns in polyculture treatments fed at rates from 60 to 100% of the control were equal to or better than those of the control treatment, with a feeding ration of 80% yielding the largest prawn size and lowering FCR by 37.84%; this could indicate that a 20% reduction in feed ration is appropriate for nursing PL in polycultures. In addition, in both experiments, the polyculture groups exhibited considerably lower levels of nitrogen and phosphorus compounds than the monoculture groups, and the concentrations of these compounds decreased with reduced feeding rates, which helped improve the water quality in the rearing tanks.