The effect of organic fertiliser and formula feed in pond culture of the freshwater prawn, Macrobrachium rosenbergii (de Man)

Abstract

The supplementation of pelleted prawn feed with chicken manure significantly increased the mean growth rate, weight at harvest and marketable percent of the freshwater prawn Macrobrachium rosenbergii (de Man) under the conditions of complete pond trials described. Unproved prawn production, when both feed and manure were applied to ponds, may be attributed to manure. The application of manure only resulted in significantly reduced yield, but did not produce significantly different mean growth rate, weight at harvest, marketable percent and marketable yield as compared to when feed only was applied. There was a definite requirement for pelleted feed but the application of both feed and manure was beneficial. At the levels employed, simultaneously decreased pellet input and increased manure load actually increased prawn mean growth rate, %Might at harvest, yield, marketable percent and marketable yield. There were no significant differences between treatments for the measured parameters of water and sediment chemistry, or benthic macroinvertebrates. Mineralisation of organic matter and assimilation of inorganic nutrients appeared efficient and the water was eutrophic in all treatments.

A second pond experiment evaluated the frequency of supplemental manure application, as every 3.5. 7, or 14 days. Prawn mean growth rate, weight at harvest, yield and marketable yield were superior when manure was applied every 14 days. Decreasing application frequency resulted in increased autotrophic biomass and Improved efficiency of algal production but higher oxygen requirement. The manure may have induced short term shifts between autotrophy and heterotrophy. Phytoplankton appeared to be generally nitrogen dependent. There was an overall decrease in sediment nutrient concentrations with time, and rapid organic decomposition and mineralisation were indicated.

In the final experiment, which employed enclosures, water and sediment total bacterial biomass were determined. After two months of the trial, water total bacterial biomass in the treatment which partially replaced feed with manure was 3.7 times that in the feed only treatment. 2.0 times that in the manure only treatment, and 1.7 times that of the commercial feed plus manure treatment. In all treatments, water bacterial biomass showed a general increase over time in all treatments, whereas the sediment bacterial biomass was more erratic. The occurrence of high sediment bacterial levels in the feed dominated treatments at an early stage in the experiment indicated overfeeding. High total nutrient concentrations in the water indicated a large capacity to support algae and the low inorganic nutrient concentrations indicated their rapid utilization. Prawns suffered a space effect when enclosed, and although survival was high in all treatments, mean weight at harvest was small.

Both the feed conversion and financial ratios favoured the treatment wherein feed was partially replaced by applications of manure every 14 days. Correlations between prawn production and feed, for data combined from the three experiments, indicated that each of growth rate, mean weight at harvest and yield were strong but less than those when both feed and manure were added. Manure was strongly correlated with marketable yield, the index of economic concern.