Dietary copper exposure and recovery in Nile tilapia, Oreochromis niloticus

There are few reports of dietary copper (Cu) toxicity to warm water species of freshwater fish, and little is known about recovery from dietary Cu exposure. In this study Nile tilapia (Oreochromis niloticus) were fed to satiation on a Cu-loaded diet (2000 mg Cu kg(-1)dry weight (dw) feed), or a control diet (3 mg Cu kg(-1)dw feed), for 42 days. All fish were then fed the control diet for a further 21 days to assess recovery. Nutritional performance, haematology, histology, and tissue ion content (Cu, Na(+), and K(+)) were measured. No mortalities occurred during the experiment. Dietary copper exposure was confirmed by elevated Cu concentrations in the intestine (30-fold), liver (three-fold) and gills (2.7-fold) of Cu-exposed fish compared to controls after 42 days (ANOVA, P < 0.05). Copper-exposed fish showed a reduction in food intake, and weight gain by day 21 of exposure, compared to controls (ANOVA, P < 0.05) and this persisted throughout the experiment. There were no treatment-dependent effects on food conversion ratio or hepatosomatic index, and all fish showed normal tissue Na(+) and K(+), and haematology throughout the experiment. Gill and intestine did not show overt pathology, but fatty change was observed in the liver of Cu-fed fish during exposure. The recovery phase on normal food was characterised by a reduction in intestinal and branchial Cu levels back to control values. However, the liver of the Cu-fed fish showed a further 1.7-fold rise in Cu content and marked hepatic lipidosis (increased intracellular fat stores) post-exposure, suggesting redistribution of Cu to the liver and delayed hepatotoxicity.