Contrasting effects of tetracycline and calcein in otolith marking of adult turbot (Scophthalmus maximus (Linnaeus, 1758))

Reliable chemical marking of otoliths is essential for age validation, because accurate age estimates form the basis for a sustainable management of wild fish stocks. We evaluated the suitability of two chemical markers, tetracycline hydrochloride and calcein, for marking otoliths of adult turbot (Scophthalmus maximus, Linnaeus, 1758) in the western Baltic Sea. A total of 124 wild-caught turbot were injected intraperitoneally with three concentrations of tetracycline hydrochloride (25, 50, or 100 mg/kg), two concentrations of calcein (12.5 or 25 mg/kg), or saline controls. Survival and otolith mark quality were assessed after 98 days. Mortality in all treatments occurred within the first 30 days. Tetracycline hydrochloride produced high survival at low to moderate doses (100% at 25 mg/kg, 94% at 50 mg/kg) but dropped to 57% at 100 mg/kg. Tetracycline hydrochloride produced clear, consistent otolith marks across all tested concentrations. In contrast, all calcein-treated fish died, demonstrating that calcein is unsuitable for intraperitoneal application at the chosen doses in this species. We therefore recommend tetracycline hydrochloride at 25–50 mg/kg as an effective otolith marker for wild, adult turbot. This method offers a practical tool for mark-recapture and age validation studies, contributing to more accurate stock assessments and the sustainable management of this commercially valuable flatfish species.