WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2013 27 (CONTINUED ON PAGE 28) commonly referred to as seaweeds, production in 2012 reached 20 million t, 90 percent as sea vegetables for human consumption in raw, cooked, dried and frozen forms. The leading cultured macroalga is Saccharina japonica (formerly Laminaria japonica, Japan kombu), of which over 5 million t were produced in 2010. Other macroalgae are cultivated for phycocolloids (alginates, carrageenans and agars), which are polysaccharides that form microbially- resistant gels that are used to thicken and stabilize different foods (ice cream) and drinks (beer). Phycocolloids are also used in the textile, pharmaceutics, microbiology, biochemistry, biotechnology, cosmetics and even drilling industries. However, most algal biomass in the world consists of microscopic, unicellular species known as phytoplankton (Ryther 1969). Polyculture fish farming requires the production of greenwater microalgae, which feed fish directly or via small herbivorous protozoans and zooplankton. The full scale of microalgae culture in polyculture has only recently been recognized and estimated. Muller-Feuga (2000) estimated production rates of ≤ 100,000 t/y, but a recent estimate (Neori 2011) has raised this number by orders of magnitude to 240 million t/y of greenwater algae. Bivalves in coastal marine farms ingest an additional quantity from natural populations of algae that is roughly similar. Principle of the Calculation The quantity of microalgae produced in ponds and consumed by cultured fish can be estimated by simply considering the biomass of microalgae necessary to feed and produce one ton of the main planktivorous cultured fish (and shrimp and bivalves). The quantity of production that depends on natural food was extracted from recent published reports (FAO 2012, Tacon et al. 2011). The quantity of plankton consumed by each of the main planktivorous species was estimated from aquaculture nutrition publications (Neori 2011). The leading filterfeeding fish species produced in 2010 (FAO 2012) were silver carp Hypophthalmichthys molitrix (4.1 million t), bighead carp Hypophthalmichthys nobilis (2.5 million t), Nile tilapia Oreochromis niloticus (2.5 million t), Gibelion catla Catla catla (2.4 million t), Crucian carp Carassius carassius (2.1 million t), rohu Labeo rohita (1.2 million t) and the leading marine filterfeeding fish, milkfish Chanos chanos (0.8 million t). Tacon and Nates (2007) considered the production of filterfeeding finfish species (silver carp, bighead carp, catla, rohu and mrigal) as largely based on the consumption of natural planktonic food organisms within the culture environment, the production of which is usually augmented and/or maintained through the external application of waste and fertilizers, but not aquafeed. Such production amounted to 12. 5 million t in 2010, or 32 percent of global total finfish production by aquaculture (FAO 2012). This estimate should be qualified and perhaps reduced by recent observations that Southeast Asian farmers have been increasing yields and growth rates of planktivorous carps and tilapias by supplementing natural food with pelleted or powdered feed.2 In India, an important food supplement is farm-made rice bran — oil cake mix (mash). However, in general, the species considered purely planktivorous — the silver, bighead, rohu, and catla carps — are used in significant proportions (20-25 percent of total production) and serve as filter feeders to remove excess plankton from green water (Table 1). TOP, FIGURE 1. A farm of macroalage and bivalves in coastal China (Photo: Max Troell). BOTTOM, FIGURE 2. Greenwater carp polyculture, Andhra Pradesh, India (Photo: Peter Edwards).
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