A comparative tissue distribution study of oxytetracycline in rainbow trout, Oncorhynchus mykiss (Walbaum), and chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Oxytetracycline (OTC), a broad-spectrum antibiotic, is used widely to treat bacterial diseases in farmed fish. In the present study, the time course of OTC concentrations in freshwater rainbow trout, Oncorhynchus mykiss (Walbaum), and seawater chinook salmon, Oncorhynchus tshawytscha (Walbaum), were compared, tissue by tissue, after receiving a bolus dose of the antibiotic (5 mg kg^–1 or 50 mg kg^–1) intra-arterially (i.a.). The OTC concentration–time profiles of rainbow trout tissues were found to be very similar to those of the corresponding tissues in chinook salmon. Therefore, neither water salinity nor fish species seemed to play an important role in the disposition and elimination of OTC in these salmonids. In a separate experiment, rainbow trout were implanted surgically with a urinary cannula and received a single dose of OTC (50 mg kg^–1) i.a. Urine was collected from the cannula daily for 13 days. The amount of OTC excreted into the bile was found to be larger than that eliminated by the urine. These results show the similarity of OTC pharmacokinetics in freshwater rainbow trout and seawater chinook salmon and render support in using a single fish species to study the pharmacokinetics of a drug for other species in the same taxon.