A fluorometric procedure has been designed to quantify the uptake and release of submicrogram quantities of the antibiotic oxytetracycline (OTC) by young sockeye salmon Oncorhynchus nerka. Juveniles and smolts were homogenized in 5 trichloroacetic acid, the solute containing OTC was extracted with ethylacetate, and the OTC within the ethylacetate phase was shaken with calcium to form a fluorescent complex. Fluorescence intensity was linearly related to OTC concentration for both OTC standard solutions and samples extracted from fish. Efficiencies of OTC extraction averaged 36 for standard solutions and 32 for spiked biological material and were independent of OTC concentration for sample spikes ranging from 0.47 to 9.4 #x3BC;g OTC; the minimum quantity that could be detected against background was 0.17 #x3BC;g OTC. The method allowed OTC to be detected in 100 of test fish when they were 33 mm long (0.4 g) after 29 d feeding on treated food; usual standard methods allowed 100 detection only after 54 d of treatment, when fish were 44 mm long (0.8 g). Oxytetracycline, which chelates to calcium, was taken up both by bone, where calcium turnover is slow, and by skin, scales, and other sites of rapid calcium turnover. The amounts of OTC in the latter tissues declined once treatment was stopped, but OTC quantities remained stable in bone. The longer OTC-impregnated feeds were given, the more OTC was taken up by bone, which allowed sublots of fish to be uniquely tagged. We were able to distinguish two differentially marked groups of fish 9 months after treatment stopped, by which time the fish had overwintered and become smolts. The smallest fish that secreted reliably detectable OTC into bone were 40 mm long (0.6 g).
展开▼
