Effects of dietary protein levels and environmental zinc exposure on the growth, feed utilization, and biochemical variables of Nile tilapia, Oreochromis niloticus (L.)

Abstract

This study was based on a 3 × 2 factorial design with two levels of dietary protein (25% or 45%) and three zinc (Zn) concentrations (0.0, 3.5, and 7.0 mg L−1). This study was conducted to evaluate if dietary protein levels could influence effects of water-borne Zn on Nile tilapia, Oreochromis niloticus (L.). Fish (7.2–7.8 g) were fed on 25% or 45%-crude protein (CP) diets and under each dietary protein level, fish were exposed to 0.0 (control), 3.5, or 7.0 mg L−1 Zn over 6 weeks. The results showed that the unionized ammonia in water increased significantly with the increase of dietary protein levels and/or Zn concentrations. The growth, survival, and feed utilization of the fish significantly increased when dietary protein level was high and Zn concentration was low. CP and total lipids contents in whole-fish body were significantly affected by protein levels and Zn concentrations; but there was no interaction between the two factors. Ash content and Zn residuals in whole-fish body increased significantly with increasing Zn concentrations only. Glucose, aspartate aminotransferase, alanine aminotransferase, and creatinine increased significantly, while protein and lipids in serum decreased significantly with increasing protein levels and/or Zn concentrations. The cortisol value in fish serum increased significantly with the increase of Zn concentrations; while it was not affected by dietary protein levels. This study concluded that the increase in dietary protein levels could minimize the deleterious effects of Zn on Nile tilapia performance.