LAUSR

Abstract This study evaluated the sedimentation rates of nutrients and particulate matter in Nile tilapia (Oreochromis niloticus) net cage farming in the Umari, Brazilian semi-arid reservoir. Sedimentation chambers were installed under net cages which had stocking of 125 and 100 fish m−³ (experimental sites). The natural sedimentation rate of the reservoir (control site) was calculated using a sedimentation chamber installed three meters deep and about 200 m upstream from the net cages. Samples from all chambers were taken after 24 h of installation, at each sampling site. The average sedimentation rates for particulate matter, ammonia, total nitrogen, total phosphorus, total organic carbon, nitrite, nitrate and total inorganic carbon in both experimental and control sites were 8.31, 6.61, and 0.083 mg cm–² day–1; 60.82, 46.18, and 9.45 μg cm–² day–1; 0.14, 0.11, and 0.04 mg cm–² day–1; 189.42, 186.59, and 7.72 μg cm–² day–1; 1.99, 1.58, and 0.57 mg cm–² day–1; 0.35, 0.24 and 0.04 μg cm–² day–1; 6.84, 7.11 and 5.04 μg cm–² day–1; and 0.18, 0.18 and 0.18 mg cm–² day–1 respectively. The sedimentation rates in the experimental sites were significantly higher than those in the control site. The fish farming activity in net cages elevates the sedimentation rates of nutrients and particulate matter in this reservoir and could accelerate the eutrophication process, thereby hindering the fish farming activity itself.