22 SEPTEMBER 2015 • WORLD AQUACULTURE • WWW.WAS.ORG protein levels used for the culture of carnivorous fish such as salmonids almost never provide the correct N:P ratio for plants. For example, in terms of balancing nitrogen production by fish with nitrogen uptake by plants, high-protein (43 percent) fish feeds cannot meet the phosphorus requirement of plants unless solid fish wastes are also used as a nutrient source in the system (Lennard 2005). Even when fish waste solids are fully mineralized to release the phosphorus they contain, often there is still not enough phosphorus present to meet the optimum N:P ratio required by most plants. In addition, many countries, including the USA and especially countries in Europe, have strict limits on phosphorus content in fish feeds to address the environmental effects of concentrated phosphorus releases from open production systems (flow-through systems, net pens) and will therefore, never produce an acceptable N:P ratio for plants. Nutrient mass balance analysis of standard pelletized fish feeds also demonstrates that, in a scenario where fish waste nitrogen production is balanced with plant nitrogen uptake, phosphorus, calcium, potassium, magnesium and sulfur are all limited and optimum plant availability and supply is rarely possible. Nutrient Availability in the UVI Aquaponic System The University of the Virgin Islands (UVI) aquaponics system approach is arguably the progenitor of all recirculating aquaponic system designs and methodologies. To overcome the limitation in fish feeds of nutrients required for plant production, the system employs a nitrogen over-supply strategy to compensate for the limited phosphorus, magnesium and sulfur derived from the standard feeds used, 32 percent protein in this case, for tilapia culture. The UVI method advocates relatively high fish feed inputs and therefore relatively high fish numbers and biomass that oversupply nitrogen to the system so that other nutrients required are supplied in amounts needed by the plants. Like all RAS and aquaponic methods, the UVI approach uses a base addition strategy to control pH in the system. Because calcium and potassium are at very low concentrations in fish feeds, the UVI approach uses bases containing calcium (Ca(OH)2) and potassium (KOH) that also adds these nutrients to the system to meet plant requirements. Although the UVI approach is highly successful at growing fish and plants, it is approximate in terms of plant nutrient availability and complete plant nutrient supply cannot be always guaranteed. The Mass Balance Approach Nutrients are limited and unbalanced in aquaponic systems when standard pelletized fish feeds are used. Some researchers have suggested that designer fish feeds that address the plant nutrient limitations of standard fish feeds are appropriate for aquaponics (McMurtry et al. 1993, Seawright et al. 1998). In reality, other aquaculture sectors are currently far larger and more broadly applied than aquaponics. The aquaponics industry is so small and disparate that the manufacture of designer aquaponic fish feeds is most likely impractical and cost prohibitive. In addition, there are current questions associated with the concentrations of some of these nutrients that can be tolerated by fish. There is anecdotal evidence that some euryhaline fish species, including salmonids, are potentially intolerant of the elevated potassium concentrations required for optimized hydroponic plant production. Therefore, it is more sensible to adapt aquaponics production methods to currently available fish feeds, rather than the opposite. An alternative to the nitrogen over-supply approach is a mass balance approach to balance nutrient availability with plant nutrient requirements. The approach is to chemically analyze fish feed to determine the relative amounts of each nutrient. The aquaponic system designer can use this information to determine the amounts of each particular nutrient present with reasonable accuracy and then develop a strategy to make up any differences. The beauty and simplicity of this mass balance approach is that it allows the designer to account and compensate for any type of fish feed that is used, either low-protein feeds for omnivores or high-protein feeds for carnivores, and allows the provision of exact nutrient mixtures and concentrations that can be customized to meet the nutrient requirements of any particular plant variety. This approach, therefore, allows the designer to formulate aquaponic system designs and methods that are tailored to the optimized and efficient production of any combination of fish and plants, thus enabling the market-driven choices that lead to economic success. The mass balance approach also potentially ensures that all the assigned advantages of aquaponics are actually present: • The efficient and optimized use of nutrients with no nutrients wasted, meaning all added nutrients are utilized. • The efficient and optimized use of all of dissolved and solid-bound fish wastes, meaning, again, all fish generated nutrients are utilized. FIGURE 3. The hydroponic component of the commercial aquaponic analogue built in New Zealand (containing a commercial salad mix crop) to compare the SymbioponicTM aquaponic method with standard hydroponics.
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