The Fate of Cu, Zn and Mn in an Intensive Fish Aquaculture (Tilapia - Oreochromis niloticus) in an Artificial Reservoir in Northeastern Brazil

Abstract

Emissions of Cu, Zn and Mn and their fate in a typical tilapia (Oreochromis niloticus) cage farm in an artificial reservoir in northeastern Brazil were estimated through the analysis of their concentrations in the various compartments of the farm. Major source of Cu and Zn, but not of Mn, to the farm is from aquafeeds, with annual inputs of 3.83, 57.50 and 10.95 kg.ha −1 , respectively. Notwithstanding, only a small fraction of the added Mn and Zn (0.13%, 2.61%, respectively) is incorporated into fish biomass, resulting in a larger proportion of the introduced metals to be exported to reservoir environment. For Cu, a much larger fraction (47.4%) is exported within the fish biomass. Once released, metals are incorporated into bottom sediments within the farm area, resulting in higher metal contents compared to sediments far from the farm area. However, the analysis of sediment cores and the respective enrichment factors showed that a significant fraction of the net input of Zn (70%) and of Cu (30%) is exported out to the open reservoir.

Limnology of the largest multi-use artificial reservoir in NE Brazil: The Castanhão Reservoir, Ceará State

“…Upon reaching the shore, currents loop back into the reservoir, washing out through the bottom and export effluents from fish cages to depths below the thermocline, accumulating nutrients in the hypolimnion and keeping most of the water column oligotrophic. Details on this hydrodynamic pattern can be seen in Oliveira et al (2015Oliveira et al ( , 2017. This is facilitated by the dynamic of aquafeed pellets, which floats long enough to be transported prior to sedimentation, as demonstrated by Molisani et al (2015).…”

Impacts of the Residual Trace Metals of Aquaculture in Net Cages on the Quality of Sediment

“…Oliveira et al [ 19 ] analyzed fish, sediment and fish feed in the Castanhão reservoir, semiarid region of Brazil, and observed that only 0.13% of Mn and 2.61% of Zn are incorporated by cultivated tilapia, that Cu absorption is 47.4% and that the excess percentage of these metals are sedimented and/or dispersed to other areas of the environment. The results pointed out by Oliveira et al [ 19 ] are indicators of the concentrations and dispersion of trace metals verified in the Rosário reservoir, occurring between the fish farming area (CTV) and the postculture area (PCTV).…”

“…In addition to the increase verified during the cultivation period in the Rosário reservoir, we can estimate the possibility of the future accumulation of residual metals from fish farming according to Oliveira et al [ 19 ]. For these authors, the technological package of tilapia production in the semiarid region of Brazil is predominantly standardized both in terms of cultivation management and the quality of the feed for tilapia cultivation.…”

“…Among the factors that allow for the dispersion of metals from the sediment, pH variations were the most significant [ 78 ], as the increase in pH favors the adsorption and precipitation of metals [ 9 ] since the release to water can occur by pH decrease [ 81 ] and sediment oxygenation [ 19 ].…”

“…Feeds for fish farming have been identified as a source of metals for the sediment: Cu and Zn [ 19 ]; Fe, Mn, Zn, Cu, Cd, Cr and Pb [ 20 ]; and Cr, Ni, Co, Mn, Cu, Zn, Fe and Mo [ 21 ]. Feed as a source of metals for sediment and native fish was observed by Anim-Gayampo, Kumi and Zango [ 22 ] in the Tono Reservoir in Ghana and Xie et al [ 18 ] in the Changshou Reservoir, China.…”