Carrying capacity limits of net cage aquaculture in Brazilian reservoirs

Finfish aquaculture in net cages is widely used in Brazilian reservoirs, mainly for tilapia production. There is a large and increasing potential for production in the São Francisco river basin, and particularly the Itaparica reservoir. Tilapia production amounts to 24,000 t y-1, with a licensed amount of 43,267 t y-1. This intensive fish production in net cages is responsible for a significant biological oxygen demand, and phosphorus and nitrogen load on the reservoir, which promotes eutrophication. Particulate organic matter released from the net cages accumulates beneath the net cages, and a minimum water depth beneath the net cages of 10 m is required to limit the sediment increase to a few millimetres per year. Modeling of Icó-Mandantes bay has identified a reduced water exchange within the bay. Modeling of the effect of net cage aquaculture within the Icó-Mandantes bay points out clearly the significant increase in dissolved phosphorus and the accumulation inside the bay area. The carrying capacity of the reservoir was determined using the P load model, with a critical P concentration based on the phosphorus use efficiency. The critical P concentration amounts 25 μg L-1, and the critical P load of the reservoir amounts 2.84 g m-2 y-1; the actual load is already 3.30 g m-2 y-1, such the reservoir is already overcharged by nutrients. A sustainable “blue” aquaculture must be implemented based on use of advanced systems, species selection, fish feed, and linked production systems.