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A nitrogen-based assessment of aquaculture : shrimp farming in northwest Mexico Gomez-Galindo, Carlos

Abstract

Understanding the interactions between aquaculture and the environment requires the consideration of production inputs and outputs, ecological processes, as well as the identification and application of quantitative approaches. A conceptual and analytical framework to assess shrimp farming systems was developed based on nitrogen (N); nitrogen is an important element appearing in those interactions both as a nutrient and as a toxicant. The assessment relied on Nitrogen Productivity (NP), an indicator developed by merging Nitrogen Efficiency (NE) and Ammonia Assimilative Capacity (AAC). These indicators, calculated through model simulations, were used in the screening for optimal operating conditions. The simulation model was based on N fluxes, the processes influencing shrimp growth, and the production and loss of ammonia. The model simulated changes in stocking density (Do) and water exchange rate (W), two of the main management variables associated with the level of intensification in aquaculture. The model predictions were compared to a typical, semi-intensive shrimp farming pond in Northwest Mexico. The analysis indicated that optimal operating conditions for the Mexican case were attained by increasing Do towards 30 shrimp•m ⁻², if survival was maximized, and by using no water exchange. This management scheme conveyed a significant reduction (>90%) in water usage, but increased N deposition in sediments. Simulated shrimp farming also performed well only at W > 0.7 d ⁻¹ ; such high rates may have significant economic and biophysical costs that merit analysis. Moderate increments in W (to 0.12 - 0.13d ⁻¹) resulted in a significant level (95% or higher) of effluent N in dissolved form, with a higher potential for its reutilization, but increased ammonia levels in the pond. Formulated feeds represented more than 75% of total N inputs for Do higher than 15 Shrimp•m ⁻², while N from fixation and fertilizer contributed importantly only on the extensive side (Do < 5 shrimp•m ⁻²) of the farming spectrum. Phytoplankton growth and sedimentation rates represented important processes influencing ammonia levels in the pond. This nitrogen-based analysis identified dominant processes and the effects of management practices associated with the intensification of shrimp farming. Nitrogenbased tools appear to have great promise for analyzing the biophysical and economic aspects of aquaculture.

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