The use of calcium pre-exposure as a protective agent against environmental copper toxicity for juvenile Nile tilapia, Oreochromis niloticus (L.)

This study was carried out to evaluate the response of Nile tilapia, Oreochromis niloticus (L.), pre-exposed to calcium oxide for 4 days, and exposed to copper for 6 weeks. Fish (8.3 ± 0.1 g) were stocked in 1.5 m3 fiberglass tank and exposed to calcium oxide at a rate of 100 mg Ca2+ L-1 for 4 days. After that, fish were transferred and randomly distributed at a rate of 15 fish per 100-L aquarium. Then, fish were post-exposed to concentrations of 0, 0.503, or 1.25 mg Cu2+ L-1 (T1, T2 and T3, respectively). Three other groups were not pre-exposed to Ca and exposed to the same Cu concentrations (T4, T5 and T6, respectively). A fish diet containing 30% crude protein was offered to fish at a daily rate of 3% of live body weight twice daily; 5 days a week for 6 weeks. The final weight, weight gain and SGR were affected significantly by both Cu concentrations and Ca pre-exposure (P < 0.05). Fish pre-exposed to Ca (T2 and T3) exhibited better growth compared to non-exposed groups (T5 and T6). Fish mortality increased significantly with increasing the Cu concentration (P < 0.05). Feed intake reduced, while FCR increased significantly with increasing the Cu concentration (P < 0.05). The feed intake and FCR in T2 and T3 were better than those of T5 and T6 groups. Moisture content was not significantly differed at different treatments (P > 0.05). Crude protein decreased significantly at T6 (P < 0.05). Total lipids in T2 and T3 were higher than those of T5 and T6 (P < 0.05). Ash content and Cu residues in T2 and T3 were significantly lower than those of T5 and T6 (P < 0.05). Fulton condition factor, liver somatic index, and viscera somatic index were affected significantly by Cu toxicity (T5 and T6; P > 0.05), while they exhibited non-significant differences in T1-T4 groups. RBCs counts, Hct and Hb values were significantly affected by Cu toxicity (P < 0.05), while they were similar to control group in Ca pre-exposed groups (P > 0.05). Uric acid, creatinine, and AST were significantly affected by both Ca pre-exposure and Cu toxicity (P < 0.05). There was no significant changes in uric acid among T2, T3 and T5 (P > 0.05). The highest value of uric acid was obtained at T6 (P < 0.05). Creatinine, AST and ALT in T2 and T3 were lower than those of T5 and T6 (P < 0.05). Lipids in plasma and liver were high, while protein in plasma and liver were low in T5 and T6 (P < 0.05). Histological sections were done in fish's gills, liver and kidney in all treatments (T1-T6). The pathologic lesions were observed due to Cu toxicity. These damages became severe with increasing Cu concentration. Tissue alterations in T2 and T3 were less than that in T5 and T6. The present study displayed that Ca pre-exposure may play a factor then it reduced Cu toxicity resulting in a better fish growth.