16 JUNE 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Unlike the Clemson PAS, water does not circulate continuously in the split-pond. During daylight and early evening, oxygenated water from the algal basin is pumped through the fish-holding basin and return flow to the algal basin removes fish metabolic wastes. At night, when dissolved oxygen concentrations decrease in the algal basin, circulation between the two basins ceases and oxygen is provided by mechanical aerators in the fish-holding area. Similar to the PAS, no attempt is made to manage dissolved oxygen in the algal basin and dissolved oxygen concentrations usually fall to 0 mg/L each night during summer months. Slow-rotating paddlewheel and aerator operation are controlled by oxygen sensors located in both sections of the splitpond. The two critical design parameters for split-ponds are 1) amount of aeration required in the fish-holding area and 2) water flow rate between the two basins (Brune et al. 2012). Split-pond aerator requirements are estimated by determining the aerator oxygen transfer needed to offset fish respiratory demands at the maximum projected fish biomass loading. Other sources and sinks of oxygen, such as air-water gas exchange and plankton and benthic metabolism are assumed to be insignificant at the high fish biomass loading rates in the small fish-holding area. Pumping rate estimates are based on the assumption that fish oxygen requirements are primarily met during daylight and early evening by oxygen in water pumped into the fish-holding area from the algal basin. An oxygen mass balance is used to calculate flow rate (volume/time) by dividing fish respiratory rate (oxygen mass/time) by the minimum desired dissolved oxygen concentration (oxygen mass/volume). Required water flow varies with time as fish grow and water temperature changes. The original goal of the split-pond was to take advantage of the fish-confinement benefits of the PAS, such as facilitation of feeding, inventory, harvest, health management and protection from predators. Because the system appeared to lack processes needed for enhanced algal productivity (shallow basin, turbulence throughout the system, and algal cropping), I did not expect maximum allowable feed input to be much greater than for catfish traditional ponds. However, within three years of use, it was evident that some set of ecosystem processes were operating that allowed higher feed inputs than expected. Net annual production (based on the combined water surface of algal basin and fishholding areas) in experimental split-ponds at Stoneville has ranged from 15 to more than 20 t/ha, which is marginally less than production from the PAS but two or three times greater than from most traditional catfish ponds. The potential for better fish production stimulated rapid commercial adoption, and more than 600 ha of split-ponds have been built by farmers in Mississippi, Arkansas, Alabama, and California. As farmers adopted the split-pond concept, pumps other than slow-rotating paddlewheels (such as high-speed screw pumps, axial-flow pumps, and others) have been used to reduce initial investment costs and facilitate system installation. Catfish production in commercial split-ponds is less than that achieved in experimental ponds, but is nevertheless impressive, with net annual production of 12 to more than 16 t/ha. Two regional studies are currently underway to examine production and economics of different split-pond designs. Partitioned Ponds and Drivers of Innovation Partitioned ponds are a significant development in pond aquaculture, with the potential to increase aquaculture production several-fold over that achieved in traditional ponds and little or no discharge of effluent or wastes into public waters. One variant, splitponds, has been rapidly adopted by farmers in the southeastern United States for catfish aquaculture. Time will tell whether partitioned ponds find a place in the aquaculture mainstream, although we believe the concept offers a number of advantages over large earthen ponds for commercial production of some species. Use of filter-feeding organisms to harvest algal biomass in the PAS can produce co-products that provide significant secondary sources of income for fish farmers. Current fishmeal, fossil fuel, and electricity costs seem to favor simpler partitioned pond designs—such as the split-pond—that do not involve controlled algal harvest and recovery. However, as fishmeal and fossil energy prices increase, recovery and utilization of otherwise wasted feed nitrogen is likely to grow in importance. Fish farmers may soon find themselves forced by economics to install and operate “designed ecosystems” in which photosynthetic algal co-production will become essential to profitable fish and shrimp production. The developmental history of partitioned ponds is an interesting example of innovation in science. Independent, contemporaneous development of partitioned ponds is an example of “multiple discovery,” where similar innovations are made more or less simultaneously by different people. The common perception is that science advances mainly through unique contributions made by individuals or groups with a singular combination of training, personality, and insight. In actuality, multiple independent discoveries are thought to be the common pattern of innovation (Merton 1961). Although innovation of partitioned ponds does not rise to the level of simultaneous development of calculus by Leibniz and Newton, it does represent a significant development in pond aquaculture, and has an interesting twist in that the two independent evolutionary lines of partitioned ponds were stimulated by different goals. The common occurrence of simultaneous discovery has been the subject of considerable study and speculation to explain why “multiples” are so prevalent in science (Simonton 1979, 1986, Garfield 1980). Frequent occurrence of multiple discoveries is attributed either to chance or to the concept of “zeitgeist.” Zeitgeist means “spirit of the times” and the zeitgeist theory of innovation is based on the idea that simultaneous discoveries arise from an existing set of cultural circumstances that make discovery inevitable. These circumstances (the prevailing zeitgeist) may be an acute need for the technology, the development of a body of knowledge to the point where the next discovery is unavoidable, or—usually—both. So what was the aquaculture “zeitgeist” in the 1980s that led to independent development of partitioned pond concepts? It was a decade of amazing growth of aquaculture throughout the world. In the United States, channel catfish production nearly doubled, increasing from 90,000 t in 1980 to almost 175,000 t in 1990. As the demand for aquaculture products increased, aquaculturists sought ways to intensify production and increase economic returns. Fish feeds were developed that satisfied all nutritional requirements. Supplemental aeration, which was relatively rare
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