Abstract
Finfish aquaculture in net cages is widely used in Brazilian reservoirs, mainlyfor tilapia production. There is a large and increasing potential for productionin the Sao Francisco river basin, and particularly the Itaparica reservoir. Tilapiaproduction 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 significantbiological oxygen demand, and phosphorus and nitrogen load on the reservoir,which promotes eutrophication. Particulate organic matter released from thenet cages accumulates beneath the net cages, and a minimum water depthbeneath the net cages of 10 m is required to limit the sediment increase toa few millimetres per year. Modeling of Ico-Mandantes bay has identified areduced water exchange within the bay. Modeling of the effect of net cageaquaculture within the Ico-Mandantes bay points out clearly the significantincrease in dissolved phosphorus and the accumulation inside the bay area.The carrying capacity of the reservoir was determined using the P loadmodel, 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 thereservoir 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, speciesselection, fish feed, and linked production systems.
Highlights
Fisheries in lakes and reservoirs are of interest due to the success and increasing significance of finfish aquaculture, with the aquaculture yield exceeding that of artisanal fisheries in many countries
The environmental impact of net cage fish culture in tropical reservoirs was studied in the Itaparica reservoir in São Francisco river, Northeast Brazil
The use of net cage culture systems for tilapia aquaculture is an inappropriate low-cost technology, because the potential for net cage emission treatment is very restricted; in pond systems and raceways, wastewater treatment can be done with a simple sedimentation tanks or with secondary treatment steps by aeration and denitrification
Summary
Fisheries in lakes and reservoirs are of interest due to the success and increasing significance of finfish aquaculture, with the aquaculture yield exceeding that of artisanal fisheries in many countries. The significance of fish aquaculture has been increasing in Brazil for several decades, especially freshwater fish, and artisanal fisheries have been restricted due to the limited capacity of the water bodies and seasonal effects of low water levels. The use of reservoirs for aquaculture is still a common practice in many countries, with restrictive regulations implemented to limit the environmental impact. More advanced fish aquaculture systems use ponds, tanks, and raceways on the border of a reservoir, with a flow through of pumped lake water. Fish aquaculture systems in lakes and reservoirs have to be evaluated in terms of both their benefits, such as food production and economic development, and their impact on nature, mainly due to contamination of water bodies, leading to eutrophication, and sediment, leading to anoxic conditions. Organic matter inputs lead to the consumption of oxygen by mineralization, as well as the contamination of sediment and the promotion of anoxic conditions
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