WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2017 55 (CONTINUED ON PAGE 56) pond liners is often beyond the financial capacity of small farmers. Incorporating vertical substrate presents an opportunity for moderate intensity heterotrophic (BFT) systems. Pond mixing, sediment suspension and lining To keep the biofloc in suspension and minimize sedimentation, a continuous and fairly vigorous mixing of water is required. Vigorous mixing induces bank erosion (Boyd 1995) and sediment suspension. These processes lead to the mixing of organic matter with eroded soil that are deposited in areas of the pond bottom where water current is low. These mixed deposits can be 10-20 cm thick (Avnimelech et al. 1986). Draining or aeration of this layer is very difficult as a practical matter. The need for vigorous aeration to avoid sedimentation is one of the reasons pond liners are required. (There are other reasons for this, such as the ease of cleaning, shortening of the period between harvest and stocking to a few days and the practically unlimited possibility to construct ponds on practically any soil type.) Including substrate can reduce the aeration and mixing requirement compared to intensive BFT systems. Suspended bioflocs and surface-attached biofilms are similar biocontrol systems Similar to industrial biotechnology systems, BFT ponds are operated as a completely mixed, suspended growth system. Such a system maximizes the capacity of microbial biofloc development. Developed biofloc systems contain 106-109 bacteria/mL and can adapt to a wide range of feed loading. The technology requires ample aeration, lined ponds and proper drainage systems. Excessively high microbial biomass is not needed for good system function and can be deleterious (Ray et al. 2012). The approach presented in this article is based on a different spatial distribution of the biota. The controlling microbial consortium is not uniformly distributed through the water column but is concentrated on the surfaces of vertical substrates that may be placed only in certain parts of the pond. The microbial community growing on vertical substrates can be controlled in the same way as that in a suspended biofloc system, by controlling the C/N ratio. In addition, both systems consist of a mixture of organisms: a variety of bacteria, algae, protozoa, rotifers, crustaceans and others (Asaduzzaman et al. 2010, De Schryver et al. 2008). Thus, the two systems – uniformly dispersed and locally attached communities of aquatic biota – are similar. Because of the localization of the substrates, it can function at any shrimp or fish biomass and there is no need for a high stocking rate. This is in contrast with suspended BFT where the average organic substrate concentration, not a local one, has to be high for the system to function properly. Although the processes in suspended or attached (fixed-film) BFT systems are similar, controlling the two systems may require different approaches. Evolution of Shrimp Production Practices at Hitide Seafarms The trial with vertical substrate was conducted at Hitide Seafarms, located in Kattur, Tamil Nadu, India (11.360 N, 79.809 E), a 25-ha shrimp farm near the outlet of the Kollidam River to the Indian Ocean. Hitide Seafarms was established in 1990 and followed traditional practice of pumping from an estuary to exchange pond water. Water Theoretical Considerations Accumulation of organic matter on the pond bottom Sedimentation, the downward movement of particles, is a process common to all water bodies, from oceans to shrimp ponds. In all water bodies, from natural systems to man-made intensive ponds, organic matter accumulates on and within the bottom soil. The greater the biomass and feeding rate, the faster and greater is that accumulation. An intrinsic property of bottom soils is the inefficient downward supply of oxygen and, in most cases, the prevalence of anaerobic or anoxic conditions in the pond bottom. This leads to a few severe results: 1) development of anaerobic conditions in the pond bottom and the resultant production of toxic materials, stressing fish or shrimp and limiting production (Avnimelech and Ritvo 2003), 2) inefficient microbial degradation of organic residues, compared to equivalent rates under aerobic conditions (Reddy et al. 1986), and c) slow and inefficient recycling of accumulated nutrients from the pond bottom into the pond food web. Sedimentation of organic particles, with a density just slightly greater than that of water, is slow. Moreover, lateral water movements lead to horizontal components super-imposed on vertical settling. The particles have an oscillating horizontal movement, even though the vertical vector dominates, and particles settle to the bottom (Fig. 1A). Vertical substrates in the water intercept and trap organic particles as they move, especially so if the substrates are hydrophobic (Fig. 1 B). A plastic surface placed in a pond is soon covered by a gelatinous biofilm (made of organic debris and microbiota), usually within hours of contact, leading later to the coverage of the surface with trapped detritus and an active biofilm. In contrast to organic matter accumulating on the pond bottom where the supply of oxygen is limited, organic matter attached to vertical substrates in the aerobic water strata has a good chance to be effectively metabolized by microorganisms, contribute to the recycling of feed materials and not induce production of anaerobic toxic substances. Periphyton attached to vertical substrates has been studied and used in aquaculture for years (Assaduzzaman et al. 2010, Azim et al. 2005, Bratvold and Browdy 2001, Milstein et al. 2009 Scott and McNeil 2001). The role of vertical substrates in minimizing organic sedimentation and being an important tool in managing biofloc systems, however, has not been demonstrated. Option of intermediate stocking density Intensive BFT systems are based on domination of heterotrophic microbial activity. With respect to ammonia control, extensive systems are based on algal uptake. In intensive systems, the capacity of algae to control ammonia accumulation through photosynthesis is limited by the availability of light caused by self-shading at high algal density. No such limitation exists when a heterotrophic bacteria population dominates. Relatively high feeding rates are needed to promote dominance by heterotrophic bacteria. In BFT shrimp ponds, sufficient heterotrophic bacteria development occurs only when stocking rates are above 100 PL/m2 (McIntosh 2000, Taw 2010). High feeding rates require intensive pond aeration, which necessitates pond lining. Such biofloc systems require a very reliable aeration system. Investment in aeration systems and
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