WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2017 57 Farm Trial Results The results and technical details provided here have value as field results, obtained under real farm conditions and not using the classic scientific method. Such work is needed and will hopefully follow. Two identical 6600-m2 ponds were used for the trial. Stocking density in both ponds was 25 PL/m2 (Penaeus monodon), as is common in semi-extensive ponds. Both ponds were aerated as needed to maintain dissolved oxygen concentration above 4.5 mg/L. Both ponds were amended with molasses as needed to keep total ammonia concentration at a low level. Vertical substrates were installed in one pond. The other pond had no vertical substrate. Shrimp were harvested from both ponds after 190 days. Survival was high in both ponds, although it was a bit higher in the pond with substrate (90 vs 82 percent). Average shrimp weight in the pond with substrate was 50 g versus 42 g in the pond with no substrate, the larger size resulting in considerable economic benefit. Production (normalized to kg/ha) was 11,250 kg/ha in the pond with substrate versus 8,610 kg/ha in the pond with no substrate. Feed conversion ratio in both ponds was around 1.2. Farm workers reported that the introduction of vertical substrates led to a very obvious decrease in sludge accumulation on the pond bottom. In the pond without substrate, four sludge drainings/week were necessary to avoid excessive accumulation. In contrast, in the pond with vertical substrate, only ten sludge draining events were necessary during the entire crop. Gross production costs (INR/kg harvested shrimp) were about the same with or without substrate (Table 1). Comparing expenses (normalized to INR/ha, about INR 50 ~ US$ 1), purchase and installation of substrates added roughly 3 percent to total costs. (The cost of substrate installation is considered only in the first year, rather than taking annual depreciation, because of a lack of good data on life expectancy of the substrate.) Feed, carbohydrate and aeration costs were also greater in the pond with substrate. The main factor affecting profitability was the higher farm-gate price of the larger shrimp. The value of 50-g shrimp harvested from the pond with substrate was about 50 INR/kg greater than that of the 42-g shrimp from the pond with no substrate, adding a net income of about US$ 11,000/ha for the pond with vertical substrate. Discussion The results obtained pertain to the effect of vertical substrates where stocking density was much lower than that necessary to develop a suspended biofloc system. Relatively high feeding rates are needed to develop the dense and active microbial population making up the suspended biofloc system. The introduction of vertical substrates provides an environment with high local concentrations of organic matter that enables development of microbial biomass. This community can be manipulated in the same way as that used in BFT and the biofilm provides similar services (ammonia control, natural food items) to those of a suspended biofloc community. Introduction of vertical substrates promotes development of a system based on a fixed biofilm, in contrast to the suspended biofloc community used in conventional BFT systems. In densely stocked ponds with high feeding and aeration rates and low water exchange rates, suspended biofloc communities develop naturally. The concentration of feed residues in such ponds is sufficient to support bioflocs. In addition, in densely stocked ponds, the adsorption capacity of substrates will not be insufficient to trap the large amounts of suspended organic particles and to significantly affect sedimentation of organic residues to the pond bottom. More work is needed to define quantitatively under which conditions the fixed-film biofloc system or the suspended biofloc system has an advantage. Suspended bioflocs and biofilm on vertical substrates have several points in common and some different features. • Both consist of an array of bacteria, algae, protozoa, planktonic species, insects, nematodes and others. • Both can control ammonia concentration by encouraging heterotrophic assimilation of inorganic nitrogen, by supporting (CONTINUED ON PAGE 58) TABLE 1. Estimated production costs (1000 INR/ha) of shrimp produced in ponds with or without vertical substrates. Shrimp were stocked at 25 PL/m2 and cultured for 90 days. Input parameter Vertical substrates No substrate Pond preparation 20 20 Vertical substrates, nets & accessories 120 0 Feed 1418 1193 Carbohydrates (molasses) 79 16 Water pumping 45 65 Aeration 911 542 Fertilizers, lime 10 10 Labor, technicians 175 175 Maintenance 50 50 Harvesting expenses 23 17 Overhead 150 150 Total 3000 2239 Cost/kg shrimp 267 260
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