Abstract
Reliable discrimination between wild and cultured fish is essential for traceability in aquaculture. This study reports the first successful application of intramuscular oxytetracycline (OTC) injection for otolith marking in juveniles of Atlantic bluefin tuna ( Thunnus thynnus ). Fifteen individuals (initial weight ≈ 100 g) were injected with either 100 mg kg −1 ( n = 8) or 200 mg kg −1 ( n = 7) OTC. All otoliths examined exhibited clear and persistent fluorescent bands under ultraviolet light, from 24 h up to 98 days post‐injection, with no significant differences between concentrations. These preliminary results indicate that intramuscular OTC injection is technically feasible as an otolith‐marking method in juveniles of this species.