WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2014 15 co-products (e.g. brine shrimp) grown in the system. During the 1990s and early 2000s, we interacted with fish farmers throughout the world who were interested in the PAS approach. Interest usually focused on the fish-management advantages of high-density raceway culture compared to traditional pond culture. Of course the potential for eliminating water discharge while maintaining high yields was also attractive. A few individuals in Georgia and California installed commercial-scale PAS units, although most catfish farmers in the southern United States were reluctant to re-capitalize their existing fish farming operation using an unfamiliar technology. Split-Ponds Craig S. Tucker Tom Schwedler and I worked together in the Mississippi State University catfish research program at Stoneville before he moved to Clemson University in 1985. Through Tom, I met David Brune and John Collier, and I was aware of their work on the PAS from its earliest stages in the late 1980s. From the outset, I was impressed by the sound ecological and engineering theory of the PAS but, based on my experiences with commercial catfish farming in Mississippi, I had concerns about large-scale implementation of the technology on existing farms. Mississippi catfish farmers already had considerable investment in infrastructure (earthen ponds, wells, aerators, and so on) and I doubted that many farmers would consider making wholesale changes to their production system. David drafted a design based on a simplified PAS configuration he had originally proposed in 1999 (Brune et al. 1999). The new design could be built inside existing ponds, using large, slow-turning paddlewheels to circulate water throughout the pond and a smaller paddlewheel to pump a sidestream flow of water through concrete raceways. This configuration functionally resembled in-pond raceways constructed on some catfish farms in Alabama in the late 2000s (Brown et al. 2011). Although the new PAS design used existing ponds as the starting point for construction, I was also concerned about the willingness of Mississippi farmers to accept two other features of the PAS: the very high fish biomass loadings in raceways and the apparent requirement for continuous cropping of algal biomass to sustain high algal productivity. Fish farms in Mississippi experience frequent electrical power outages, and dissolved oxygen concentrations could decline to lethal levels within minutes after loss of power for aeration and water flow at the high fish biomass loadings used in PAS raceways. Although the solution is simple— installation of emergency electrical generators—I doubted whether many farmers would make that additional investment. Also, the extra effort involved in algal management with tilapia would not be acceptable to most Mississippi farmers. Tilapia must be overwintered in heated facilities and, although a few farmers might be able to hand-process and locally market some tilapia, infrastructure was lacking to process large quantities of tilapia if catfish-tilapia co-culture was widely adopted. These concerns led me to build a simplified version of the PAS in 2001 that would make use of existing catfish ponds, confine fish at lower densities than in PAS raceways but greater than those in traditional ponds, and not incorporate tilapia or other algal-cropping methods. The new system physically resembled the Polyculture Production System (PPS) described by Les Torrans, but it functioned more like a PAS. The initial, pilot-scale system was constructed at Stoneville, Mississippi, by building an earthen levee across an existing 0.4ha pond to divide the pond into two unequal sections (hence the name ‘split-pond’). The pond had a proportionately smaller algal basin (about 80-85 percent of the total pond area) and a larger fishholding basin than the PAS. The levee dividing the two basins was breached with two open channels and water was circulated between the two sections using a large, slow-rotating paddlewheel (Fig. 10, Brown and Tucker 2013). Barriers or screens in each channel prevented fish escape. Two larger split-ponds based on this design were built at Stoneville: a 2-ha system in 2008 (Fig. 11) and a 3.2-ha system in 2012. Three additional 3.2-ha split-ponds are currently under construction at Stoneville. TOP, FIGURE 10. A slow-rotating paddlewheel used to pump water between basins in a 2-ha split-pond. The paddlewheel is 3.7-m long and 1.8-m in diameter and turns at 2-3 rpm with paddles immersed to a depth of 1.2 m. The paddlewheel pumps approximately 50 m3/min and is powered by a 1.1-kW electric gearmotor. Screens in the open channel to the right prevent fish from escaping the fish-holding area. BOTTOM, FIGURE 11. The 2-ha split-pond at Stoneville, Mississippi. The 0.4-ha fish-holding basin is in the foreground and the 1.6-ha algal basin is in the distance. The two basins are separated by an earthen levee that is breached in two places. The slow-rotating paddlewheel on the far right pumps Water is pumped out of the fish-holding basin by the slow-rotating paddlewheel on the far right and returns through the channel on the far left. Two, 7.5-kW paddlewheel aerators provide supplemental dissolved oxygen at night. (CONTINUED ON PAGE 16)
RkJQdWJsaXNoZXIy MjExNDY=