WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2015 29 (CONTINUED ON PAGE 30) of seed stock. Such mortality typically occurs soon after stocking and ponds will be restocked. The loss of animals from a few days after stocking until harvest is more likely related to disease or a direct effect of poor water quality such as oxygen depletion. Moreover, poor water quality often predisposes animals to disease and survival provides an indicator of the adequacy of water quality management. There usually is a lag between the actual occurrence of mortality and the awareness of this mortality by producers. Considerable resources, especially feed, often are wasted by managers not being aware of mortality promptly. The resulting uneaten feed can lead to poor water quality and greater pollution loads. Energy Use Energy is used to construct aquaculture facilities and it is used directly at the farm level for many purposes during production and harvesting. Energy also is embodied in fertilizers and feeds. For the purpose of a survey of the performance of aquaculture, energy use can be estimated from fuel and electricity use and energy embodied in fertilizers and feeds. Carbon emissions for aquaculture will differ with the type of energy used but energy use should be an adequate surrogate for carbon emissions. Of course, a more exact estimate can be made by adjusting carbon emissions for the combinations of fuel used at different farms. Waste Discharge The main concern about aquaculture effluents is an increase in nitrogen and phosphorus concentration leading to increased phytoplankton abundance (eutrophication) in receiving water bodies. There are several ways of assessing the degree of eutrophication in a water body: Secchi disk visibility, chlorophyll a concentration, nitrogen and phosphorus concentration, algal species composition, primary productivity and diurnal fluctuations in dissolved oxygen. The latter of these techniques seems to be the most appropriate because dissolved oxygen concentration can be easily and quickly measured in situ with a portable dissolved oxygen meter. The other methods, excluding Secchi disk visibility, are more tedious to conduct. Secchi disk visibility in receiving water bodies often is affected by sources of turbidity other than phytoplankton, limiting its usefulness as a single indicator of eutrophication. Feed Conversion Ratio Feed conversion ratio or FCR (feed applied ¸ net production) is a critical variable because it is an indicator of several important aspects of aquaculture production. The lower the FCR, the less feed needed per unit production. Thus, the lower the FCR, the lower the production cost, the greater the efficiency of feed use and the smaller the waste load. The large impact of reducing FCR on resource use is illustrated in Table 2. Huge resource savings could result by industry-wide improvement in FCR. For example, world shrimp production by aquaculture in 2011 was about 3.93 t (faostat.fao.org). A reduction in FCR of 0.1 is equivalent to a global savings of 29,868 t nitrogen, 5,895 t phosphorus, 74,670 t fish meal, 94,320 ha agricultural land, 126 million m3 water and 3.2 million GJ energy. Of greater importance to the producer, it would reduce feed use by 393,000 t, a considerable reduction in expenditure for feed. There are few instances where sufficient data are available at the farm level to reveal the range and normal efficiencies of resource use for different species, culture methods and regions. Without such information it is impossible to set target efficiency levels that are both economically feasible and ecologically meaningful. The WWF initiative will be invaluable in efforts to advance responsible aquaculture. TABLE 2. Effect of the feed conversion ratio (FCR) on resource use in shrimp aquaculture. The estimates are based on production of 1 t of live shrimp. Nitrogen Phosphorus Embodied in feed FCR Feed Actual Embodied Actual Embodied Fish meal Land for feed Water Energy (kg) ingredients (m3) (GJ) (ha) 1.0 1,000 56.0 20.0 12.0 2.98 190 0.238 318 8.08 1.2 1,200 67.2 24.0 14.4 3.58 228 0.286 382 9.70 1.4 1,400 78.4 28.0 16.8 4.17 266 0.333 445 11.31 1.6 1,600 89.6 32.0 19.2 4.77 304 0.381 509 12.93 1.8 1,800 100.8 36.0 21.6 5.36 342 0.428 572 14.54 2.0 2,000 112.0 40.0 24.0 5.96 380 0.476 636 16.16 2.2 2,200 123.2 44.0 26.4 5.56 418 0.524 700 17.78 2.4 2,400 134.4 48.0 28.8 7.15 456 0.571 763 19.39 2.6 2,600 145.6 52.0 31.2 7.74 494 0.619 827 20.01 *Feed was assumed to contain 5.6 percent nitrogen and 1.2 percent phosphorus.
RkJQdWJsaXNoZXIy MjExNDY=