An investigation into oxytetracycline residues in Atlantic salmon, Salmo salar L

. An outbreak of clinical furunculosis in commercially reared Atlantic salmon, Salmo salar L., broodfish in fresh water was successfully treated with a single intraperitoneal injection of oxytetracycline. The mean serum oxytetracycline residue in these fully mature fish did not decline and, at 8 weeks post‐injection, was 3·7 μg/ml. In contrast, the mean serum residues in other fully mature but healthy fish declined during an 8‐week experimental period to 1·3 μg/ml. The mean concentration of oxytetracycline in the liver of healthy mature fish was approximately two‐fold that of similar aged but non‐mature fish, and approximately four‐fold the concentration found in post‐smolts. Oxytetracycline residues 8 weeks after injection exceeded the minimum inhibitory concentration required for oxytetracycline sensitive Aeromonas salmonicida isolates in all groups, therefore under these circumstances, this antibiotic may be effective at a lower dose than that currently recommended. The histology of the liver from the furunculosis‐surviving broodfish showed an increase in tissue lesions when compared with the other mature and non‐mature fish in fresh water. The combination of prior infection and the physiological status of fasting broodfish may have influenced the rate at which this antibiotic is removed, and, in cases where these residues persist, there may be further damage to the liver.