Growth, biochemical variables, and zinc bioaccumulation in Nile tilapia, Oreochromis niloticus (L.) as affected by water-born zinc toxicity and exposure period

Abstract

The present study was carried out to investigate the effect of sublethal zinc (Zn) concentrations on growth performance, biochemical variables, and Zn residues in various organs of Nile tilapia, Oreochromis niloticus (L.). Fish (25 ± 0.5 g) were exposed to 0.0, 3.5, or 7.0 mg Zn L−1 for 1 or 6 weeks. Fish growth was significantly reduced with increasing Zn concentrations. However, fish exposed to 7.0 mg Zn L−1 grew less quickly than those of the control group. Likewise, best feed intake and feed conversion ratio were obtained at the control group. Furthermore, glucose, aspartate aminotransferase (AST), alanine aminotransferase (ALT), creatinine, and cortisol increased significantly with increasing Zn concentrations and exposure time, with maximal values in the 7.0 mg Zn L−1 treatment after 6 weeks. Meanwhile, highest values of serum protein and lipids were obtained in the control fish reared for 6 week, whereas their lowest values were observed in fish exposed to 7.0 mg Zn L−1 for 1 week. There was no significant change in whole-body moisture content of fish due to Zn exposure, although crude protein and total lipid contents decreased significantly with increasing Zn concentrations. In addition, Zn exposure increased total ash contents and Zn residues in different investigated fish organs. The Zn concentrations in all fish organs were time-dependant and the Zn residues after 1 week were found to follow the order of gills > liver > kidney > muscle, meanwhile after 6 weeks it followed the order of liver > kidney > gill > muscle. The present findings revealed that liver and kidney tissues are the prime sites of Zn bioaccumulation, while Zn load in the muscle was for low as compared to other organs.