ALBERT

Notes: Rainbow trout Oncorhynchus mykiss fry in moderately hard water were exposed to control or high levels of dietary Cu (c. 6 and 580 ug Cu g food−1) at one of three levels of Na (1·5, 3·0 or 4·5%) in the diet, i.e. six experimental groups. Fish were fed a 4% body mass ration daily for 28 days and 10 individuals from each group were sampled every 7 days. Concentrations of Cu and Na were measured in the gills, liver, gut and remaining carcass of sampled fish. Growth was not affected and no consistent differences were found in mass, total lengths (LT) or indices of body condition among any of the groups on any sampling day. Copper concentration was significantly higher in tissues of Cu-exposed groups, although within treatment types (control Cu v. high Cu diet), it did not differ consistently among groups that received different levels of dietary Na. Tissue Na concentration did not differ among any of the groups and did not show any marked changes over time. In Cu-exposed groups, the proportion of total body Cu burden contained in the liver approximately doubled over time, from c. 30% on day 7 to c. 60% on day 28. In unexposed fish, the liver maintained c. 25% of the total Cu burden throughout the experiment. In contrast, the proportion of the total body Cu burden contained in the gut decreased somewhat over time in Cu-exposed fish, from c. 40% on day 7 to c. 30% on day 28, and remained fairly stable at c. 25–30% in control groups, i.e. approximately equal to liver values. In all groups, the carcass contained by far the largest portion of the total Na content (〉80%). Measurements made 36 h post-feeding indicated that all six groups had much higher Na efflux relative to influx, suggesting that the fish were eliminating excess Na taken up from the diet, and differences in Na influx rates were small. Na efflux rate was significantly higher in the high Cu and high Na group than in the high Cu and low Na group. The results indicate that at the concentrations used in this experiment, dietary Na has little effect on dietary Cu uptake by juvenile rainbow trout, and dietary Cu has little effect on Na homeostasis.