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

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 f ish were processed into a fine powder (trout meal) and pelleted to form a n utritionally balanced feed which was then fed to juvenile rainbow trout (On corhynchus mykiss). Fish were fed either a diet containing 3.1 mug/g biolog ically incorporated silver (an environmentally relevant concentration), or one of three control diets containing approximately 0.05 mug/g Ag for 128 d ays. All dietary treatments were fed to satiation once daily. Dietary silve r 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, he moglobin levels), plasma glucose, metabolism (oxygen consumption, ammonia a nd urea excretion rates) and intestinal Na/K-ATPase and amylase activities were all unaffected. Based on the physiological parameters investigated her e, this dietary silver exposure appeared to be physiologically benign to ra inbow trout. However, silver concentrations in the livers of the silver-fed fish were significantly elevated at day 16, and reached a steady-state lev el of similar to 20 mug/g Ag by day 36. The concentration specific accumula tion rate in the livers of fish fed biologically incorporated silver was ab out 4.6 orders of magnitude greater than when fed dietary silver sulfide, i ndicating much greater bioavailability. Despite this increase, hepatic meta llothionein concentrations remained unchanged, in contrast to waterborne ex posures, indicating that bioaccumulated silver behaves differently dependin g on whether it is taken up from the diet or from the water. Apart from a s ignificant reduction in hepatic Cu at day 16, liver concentrations of Cu an d Zn were not affected by dietary silver. Silver concentrations were also s ignificantly elevated (relative to control fish) in the kidneys of the silv er-treated fish on days 88 and 126, and in the gills and plasma at day 126. (C) 2001 Elsevier Science B.V. All rights reserved.