Ecological carrying capacity for intensive tilapia (Oreochromis niloticus) cage aquaculture in a large hydroelectrical reservoir in Southeastern Brazil
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Cage culture of tilapia (Oreochromis niloticus) in large reservoirs is an emergent aquaculture practice in Brazil. Due to the availability of large amounts of suitable quality waters in hydroelectric dams, there is a large but still undetermined potential for cage aquaculture in the upper Parana River basin. Sustainable aquaculture production should consider assessment of ecological carrying capacity for rational use of natural resources such as water bodies. The present survey estimates the ecological carrying capacity for tilapia cage culture in several sites on a large reservoir of "Ilha Solteira" upper Parana River basin, Southeastern Brazil. Ecological carrying capacity was estimated based on the Dillon and Rigler (1975) mass balance model, considering limnological and farming field data to evaluate area-specific Phosphorus loads that can be assimilated in these environments. Using average farming data of feed composition, tilapia (O. niloticus) whole body composition and Feed Conversion Rate (FCR), the estimated emission of Phosphorus per ton of fish produced was 14.8 kg P ton(-1). Modeling provided evidence for the importance of feed Phosphorus content and availability for determination of total allowable production, as well as the relevance of proper inputs of limnological field data. When field data was collected, only two sites (Ponte Pensa and Dourados) had aquaculture activities; as aquaculture is expanding, limnological information provided in the present study is a registry of conditions found before the massive aquaculture development. Production estimated to the reservoir as a whole (156,000 ton) is more than fivefold the pooled production (30,000 ton) of the fifteen selected sites; if production estimated to the reservoir as a whole cluster around a few best sites, than effects on water quality is expected to be even more drastic. The limit of 1% occupancy by aquaculture posed by Brazilian government is not an effective safeguard against excessive eutrophication, and detailed limnological studies are demanded for each inlet assigned for cage aquaculture. The impact of cage fish farming on the aquatic environment by the release of nutrients that affect water quality can not only bring about conflict with multiple users, but also primarily exert a negative feedback effect in the cage operations themselves.