The effect of recirculating aquaculture systems on the concentrations of heavy metals in culture water and tissues of Nile tilapia Oreochromis niloticus

To date, farming fish in recirculating aquaculture systems (RAS) is one of the most environmentally friendly ways of producing fish. However, with the trend towards intensification, and consequently decrease in water exchange rates, these systems may accumulate substances, such as heavy metals, in the water and fish. Inductively-coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectroscope (ICP-MS) were used to determine Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn, in the water and fish (Nile tilapia Oreochromis niloticus). Three RAS were used, differing in daily water exchange rates (30, 70 and 1500 l/kg feed/d). The concentrations of As, Fe, Mn, Ni and Zn in the water increased with decreasing water exchange rates, suggesting an accumulation of heavy metals as more water was re-used. Such accumulation in the water was, however, not translated into accumulation in the liver and muscle. Accumulation of heavy metals was always higher in the liver than in the muscle; however, As reached 1.61 mg/kg wet weight in the muscle of fish farmed in RAS-70 l/kg feed/d. However, these levels are considerably lower than permissible safety levels for human consumption.