SUMMARY
Juvenile rainbow trout Oncorhynchus mykiss were exposed to diets with low (12.6 nmol g–1), normal (50.4 nmol g–1) or elevated (4437.5 nmol g–1) Cu concentrations in combination with either low (5.8 nmol l–1) or normal (48.5 nmol l–1) waterborne Cu levels over a 50-day period, during which body mass increased up to fivefold. A nutritional requirement for Cu was demonstrated based on growth response and whole body and tissue Cu status. Simultaneous low Cu levels in both the water and the diet depressed growth by 31 % over 7 weeks. There were reductions in both specific growth rate (SGR, 1.95 versus 2.55 % day–1) and food conversion efficiency (FCE, 53–59 % versus 75–80 %) over weeks 0–4, but these effects disappeared in weeks 4–7. Elevated concentrations of dietary Cu did not affect SGR or FCE. Low levels of dietary and waterborne Cu decreased, and high levels of dietary Cu increased, the Cu concentrations in whole body, liver, carcass, gut and gills. Copper levels in the liver strongly reflected the exposure conditions with a corresponding fivefold decrease and a 22-fold increase in Cu concentration. Restricting available Cu caused an exponential decline in whole body Cu concentration from 0.0175 to 0.0069 μmol g–1 and increased the uptake of waterborne Cu (measured with 64Cu) by the gills. Conversely, high levels of dietary Cu caused a linear increase in whole body Cu concentration to approximately 0.170 μmol g–1 and depressed the uptake of waterborne Cu. Waterborne Cu uptake contributed the majority (60 %) of the body’s Cu accumulation under Cu-deficient conditions while dietary Cu contributed the majority (99 %) at high dietary levels of Cu. True bioavailability of dietary Cu decreased with increasing levels of dietary Cu concentration, although the absolute amount retained increased. These findings demonstrate an important interaction between dietary and waterborne Cu uptake in fish and provide compelling evidence of a key role for the gill in Cu homeostasis.
