Oxytetracycline hydrochloride vital labelling revisited: the case of Dicentrarchus labrax and Diplodus puntazzo

Abstract

The main objective of this study was to determine the optimal oxytetracycline hydrochloride (OTC) dosage for otolith marking in Dicentrarchus labrax and Diplodus puntazzo. Adult reared individuals of these two species were marked with OTC intraperitoneal injections at three recommended doses (25, 45 and 60 mg kg(-1) fish) and a blank. Fishes were monitored for 36-49 days from when they were marked. The effectiveness of the otolith marking was inversely related to the OTC dosage. The injections were found to have deleterious effects at the highest dosages: (1) mortality was nearly 0% during the first 4 days but then it suddenly rose to 85-100% depending on the fish and treatment, (2) no daily growth increments were formed after marking and (3) no or negative somatic growth was observed. These results suggest that most of the current studies using OTC marking for estimating mortality of wild fishes use an observation period between marking the fishes and releasing that is too short to allow the deleterious effects of OTC to be detected. Species-specific experiments for assessing the long-term mortality should be undertaken when OTC marking is used on wild fishes.