World Aquaculture 9 matter produced during fish culture. This is the input for the dispersion module, which calculates the impacted area depending on particle settling velocity, water depth, velocity and direction of current. Finally, interaction between discharges and the seabed are analyzed in the benthic effect module. Formal relationships among variables (i.e., model coefficients) in modules were extracted from published theoretical and empirical studies. If specific information on commercial growth of the species and physical conditions of the environment is available, functional expressions and parameters included in the model can be adapted to particular species and production strategies. Economic and management components of the model are represented on the left side of Figure 1. Specifically the economic results module calculates the financial results of the farm (profits, net present value, and internal rate of return) obtained from the commercial activity. Production is planned in the management strategy module, which includes some management choices in the farm, e.g. annual production, harvest size, or time span for the investment. Thus, economic and environmental consequences of different production alternatives can be estimated. Case Study: Sea Bream Net Pens in Melenara Bay The model was fitted to sea bream (Sparus aurata) culture conditions in Melenara Bay (Gran Canaria Island, Spain) (Fig. 2). Water temperature in this region ranges between 17.3 ºC in summer and 18-22 ºC in winter. The seagrass Cymodocea nodosa is the predominant plant in the soft sediment of the bay. It exerts an important ecological and physical role in the coastal ecosystem (stabilization of marine substrates, detritus generation, and nutrient provision). Accordingly this species is considered a bioindicator of human activities on the seabed. There are currently 12 net pens in Melenara Bay for sea bream and sea bass culture, each ranging from 13.2 to 16 m in diameter, 8 m high, and bound with a steel cable enclosing a surface area of 4848 m2. The annual production is around 1200 t, monthly production fluctuates between 25 and 225 t, and harvest size ranges between 450 and 1000 g. Extruded commercial feed is commonly provided Fig. 2. Location of the case study farm site on Gran Canaria, Spain. Fig. 3. Simulation of the impacted area in Melenara Bay derived from a facility with annual sea bream production of 600 t and harvest size of 450 g. to fish, although sometimes pelleted feed is used. To fit the bioeconomic model, a data set of sea bream growth in 12 separate cages was considered. The monthly number of individuals in a cage and the amount and composition of feed supplied were used to calculate the feed conversion ratio for the culture period. Other information needed to fit the model (metabolic parameters, settling velocity of particles, current direction and velocity, depth, degradation coefficients, and economic data) were collected from technical reports and specialized bibliographies. A daily organic matter sedimentation rate of 5 g/ m2 was used to indicate complete degradation of the seabed (Díaz-Almela et al. 2008).
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