WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2013 29 Several additional species — shrimp, tilapia, crucian carp and milkfish — whose total cultured production in 2010 was above 10 million t, obtain various fractions of their nutrition by filter feeding (Neori 2011). For instance, milkfish is a marine filter feeder that naturally feeds on a combination of benthic phytoplankton and macroalgae (lab-lab), but in deep, pelletfed ponds or in pens, milkfish filters greenwater.2 Tacon et al (2011), for instance, estimated that over half the total milkfish production in 2008 depended on such natural food. Therefore, 12.5 million t is at the low end for the estimated value of aquaculture production by planktivorous greenwater species in 2010. The true value, including supplementary greenwater consumption by fed species (such as tilapia, shrimp and milkfish), estimated here at about 25 percent of their total feeding world-wide, may be as high as 15 million t. The product of species production and their microalgae consumption provides a range of values, depending on assumptions. The minimum value is 123 million t of microalgae if only plankton consumption by silver carp and bighead carp are considered (Table 1). An intermediate value of 270 million t of microalgae is obtained if the Indian major carps (rohu and catla) are included. A maximum value of 310 million t of microalgae includes, in addition, as a rough order of magnitude only, a quarter of the biomass production by tilapia, shrimp, milkfish and the mrigal and crucian carps (25 percent of a total of 10.8 million t) with an average FCR of 15. These three estimates of microalgal production that feed fish are much larger than the entire 80 million t of aquaculture production reported by FAO for 2010. An additional unknown is the possibility that a very large quantity of greenwater algae is produced and then settles to the bottom of ponds or is discharged to nearby water bodies. The 14 million t of bivalve production, assuming an FCR of 20, would use 280 million t of microalgae from coastal seawater. Marine plankton, although a true natural food, often grows in water that is nutrient-enriched by humans through pollution, such as sewage and agriculture runoff discharges. Calculating the Production Cost of Greenwater Algae Although the exact species composition in greenwater aquaculture is not sufficiently defined and is certainly less controlled than typical mass cultures (Benemann 1992, Moriarty 1997), these microalgae support an unquestionably profitable aquaculture sector. Greenwater aquaculture produces hundreds of million tons of microalgae that sustain the production of tens of million tons of fish and shrimp. Greenwater algae must be produced at a low cost to allow farmers to make a profit from lowvalue fish such as silver carp (Neori and Nobre 2012). In general, extensive (greenwater) culture costs less than more intensive forms of culture. For instance, in China, production costs of greenwater-fed shrimp was US$1-3 /kg, compared with US$2-6 / kg in semi-intensive (greenwater supplemented by aquafeed) farms and US$4-8 /kg in intensive (heavily fed) farms (Tacon et al. 2002, Biao and Kaijin 2007). An estimate of the production cost of greenwater algae considers the market value of silver carp, the most common phytoplanktivorous fish in aquaculture. With a market value of US$1.2/kg in 2010 (FAO FIGIS), an estimated FCR of 11 (Neori 2011), and assuming food accounts for 50 percent of total cost, the production of the algae that feed silver carp should be US$1.2 × 0.5 / 11 = US$0.055/kg or US$0.275/kg dry weight, if water content of the algae is 80 percent. This value is an order of magnitude less than the lowest published values for the cost of mono-algal mass culture (Benemann 2009). Conclusions Estimates of the biomass of greenwater microalgae that feed fish, bivalves and shrimp in aquaculture range from 120 to 310 million t in 2010 for land-based aquaculture plus about 280 million t consumed by marine bivalves. The total amount of microalgae used in aquaculture (cultured and natural) is several times larger than the total FAO-reported world production of seafood in 2010 (168.5 million t). As the largest sector in aquaculture, greenwater plankton deserves a much larger R&D effort than it receives today. The complex factors controlling the composition and development of the different organisms that constitute green water can be studied and their management can be improved (Moriarty 1997). Multidisciplinary research — specific to site, management and organism — on greenwater aquaculture can develop indicative parameters and practical monitoring tools. Scientific attention and budgets allocated to green water may cost-effectively increase global seafood production. Studies could improve the understanding and control of greenwater food webs, elucidate the physiologicalenvironmental interactions between the food webs and the fish that grow on them, improve production of market-attractive marine planktivorous fish, and promote the use of polyculture and other multi-species culture approaches (like integrated multi-trophic aquaculture, or IMTA, partitioned aquaculture and aquaponics). The rising global demand for fish protein can be satisfied sustainably and cheaply without further taxing the shrinking supply of fresh water and arable land by improving traditional greenwater aquaculture and by furthering the domestication and production of marine planktivorous fish. Notes 1 National Centre for Mariculture, Eilat, Israel Oceanographic and Limnological Research Correspondence: neori@ocean.org.il or aneori@gmail.com, +97286375761 (fax), P.O. Box 1212, Eilat 88112, Israel 2 Peter Edwards, personal communication Acknowledgments The author is indebted to Peter Edwards for seminal suggestions and to A. Israel for useful comments on the manuscript. References Adámek, Z., O. Linhart, M. Kratochvíl, M.Flajšhans, T. Randák, T. Policar and P. Kozák. 2012. Aquaculture in the Czech Republic in 2012: A prosperous and modern European sector based on (CONTINUED ON PAGE 30)
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