The physiological effects of a biologically incorporated silver diet on rainbow trout (Oncorhynchus mykiss).

ABSTRACT

Silver was biologically incorporated into a diet by exposing rainbow trout for 7 days to 100 mg/l of waterborne silver as silver thiosulphate. These fish were processed into a fine powder (trout meal) and pelleted to form a nutritionally balanced feed which was then fed to juvenile rainbow trout (Oncorhynchus mykiss). Fish were fed either a diet containing 3.1 microg/g biologically incorporated silver (an environmentally relevant concentration), or one of three control diets containing approximately 0.05 microg/g Ag for 128 days. All dietary treatments were fed to satiation once daily. Dietary silver did not significantly affect mortality, growth, food consumption, or food conversion efficiency. Furthermore, ion regulation (plasma Na(+) levels and Na(+) influx rates), hematological parameters (hematocrit, plasma protein, hemoglobin levels), plasma glucose, metabolism (oxygen consumption, ammonia and urea excretion rates) and intestinal Na/K-ATPase and amylase activities were all unaffected. Based on the physiological parameters investigated here, this dietary silver exposure appeared to be physiologically benign to rainbow trout. However, silver concentrations in the livers of the silver-fed fish were significantly elevated at day 16, and reached a steady-state level of approximately 20 microg/g Ag by day 36. The concentration specific accumulation rate in the livers of fish fed biologically incorporated silver was about 4.6 orders of magnitude greater than when fed dietary silver sulfide, indicating much greater bioavailability. Despite this increase, hepatic metallothionein concentrations remained unchanged, in contrast to waterborne exposures, indicating that bioaccumulated silver behaves differently depending on whether it is taken up from the diet or from the water. Apart from a significant reduction in hepatic Cu at day 16, liver concentrations of Cu and Zn were not affected by dietary silver. Silver concentrations were also significantly elevated (relative to control fish) in the kidneys of the silver-treated fish on days 88 and 126, and in the gills and plasma at day 126.