10 March 2012 Figure 3 illustrates the impacted area from the sea bream farm, assuming an annual production of 600 t and a harvest size of 450 g. The surface occupied by net pens is concentrated in a central circular area of the represented spot. Emitted particles fall to the seabed at a certain distance from this central area, depending on particle settling velocity, depth, and direction of currents, which is mainly determined by tide in Melanara Bay. Thus, solids accumulate on the seabed in an ellipsoidal pattern (Fig. 3). To find the best management strategy that integrates economic and environmental perspectives, 18 production scenarios of sea bream farming were analyzed: three harvest sizes (350, 800 and 1200 g) and six annual production levels (from 500 to 1500 t), divided into four lots annually. Figure 4 shows changes in the impacted area resulting from emissions from net pens for different annual production levels and harvest sizes. As expected, the area of environmental impact increases with scale of production and harvest size, although the impact area increases substantially when production exceeds 700 t/year. Figure 5 shows the economic performance (net present value in 20 years) resulting from different management strategies. Economic returns increase with annual production. However, the influence of harvest size is not straightforward. The most profitable harvest size is 800 g and the final size of 1200 g is suboptimal. Managers have economic and environmental information (Figs. 4 and 5) on the effects of different management strategies. Fig. 4. Degraded area as a function of annual production of sea bream harvested at three sizes. Fig. 5. Net present value as a function of annual production of sea bream harvested at three sizes. The Value of a Broad Modeling Approach Recently, marine aquaculture in net pens has been facing an image problem because of real or perceived negative environmental effects, which calls into question the sustainability of fish farming in net pens. With the purpose of centering this debate on reasonable terms, it is useful to apply methodologies that rigorously assess the net social benefits of the activity, including monetary returns and ecological costs. In addition, these methodologies should be dynamic, that is, alternative management strategies and their effect on obtained social benefits should be included in their hypotheses. The bioeconomic model described here points the way because it provides producers and managers a simple quantitative tool for decision making on farm management, taking into account financial returns and environmental impacts derived from marine aquaculture. Notes 1University of Las Palmas de Gran Canaria, Spain. References Díaz-Almela, E., N. Marbà, E. Álvarez, R. Santiago, M. Holmer, A. Grau, S. Mirto, R. Danovaro, A. Petrou, M. Argyrou, I. Karakassis and C. M. Duarte. 2008. Benthic input rates predict seagrass (Posidonia oceanica) fish farm-induced decline. Marine Pollution Bulletin 56:1332-1342. FAO (Food and Agriculture Organization of the United Nations). 2010. The State of Fisheries and Aquaculture 2010. Food and Agriculture Organization of the United Nations, Rome, Italy.
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