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.
Highlights
Heavy metals pollution is a major ecological concern due to its high persistence in the environment
The concentration of unionized ammonia was increased significantly with increasing Zn concentration, from 0.71 mg L-1 in the control aquaria to 0.98 mg L-1 in the 7.0 mg Zn L-1 treatment after 6 weeks (Table 1), suggesting that ammonia excretion may be induced by Zn stress, e.g., as noted by Wendelaar Bonga (1997), who reported that, during stress, elevation of cortisol stimulated the production of ammonia
The water-born Zn exposure regimes employed in the present study were well tolerated by Nile tilapia as portrayed by the high fish survival (96.7–100.0 %)
Summary
Heavy metals pollution is a major ecological concern due to its high persistence in the environment. The agricultural and industrial activities are the main source of heavy metal pollution, which adversely affect the aquatic ecosystem (Rashed 2001; Yilmaz 2003; Khare and Singh 2002). Int Aquat Res (2016) 8:197–206 equipped with a variety of physico-chemical and biological mechanisms to eliminate or reduce adverse effects of toxic substances, toxicants may evoke changes in development, growth, reproduction and behavior, or may cause death of freshwater organisms (see Eisler 1993). Zinc is one of the most common contaminants in aquatic systems and is associated with urban runoff, soil erosion, industrial discharges, pharmaceuticals, pesticides and a variety of other activities and sources (Schmitt 2004; Bowen et al 2006). The danger of Zn is aggravated by its almost indefinite persistence in the environment because it cannot be destroyed biologically and is only transformed from one oxidation state or organic complex to another (Everall et al 1989)
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