CARRYING CAPACITY LIMITS OF NET CAGE AQUACULTURE IN BRAZILIAN RESERVOIRS

Abstract

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.