Marking Otoliths of Adult European flounder ( Platichthys flesus Linnaeus, 1758) With Tetracycline and Calcein

Mark‐recapture studies using fluorochrome dyes are a valuable approach for validating the age determination of fish, yet the optimal chemical marker and its dosage for adult European flounder ( Platichthys flesus Linnaeus, 1758) remain unclear. We assessed the effectiveness of tetracycline hydrochloride and calcein at different concentrations for marking otoliths and the survival rates. A total of 117 adult European flounder were injected intraperitoneally either with one of the chemical markers or with a saline solution as a control. They were held in stationary net pens in the western Baltic Sea over a 69‐day period. Fish were monitored for survival, growth, and body condition, and otoliths were examined for fluorescence mark quality. Survival was highest in the tetracycline groups: 90% at 25 and 50 mg/kg, and 82% at 100 mg/kg, whereas calcein treatments were associated with intermediate survival: 55% at 0.5 mg/kg and 50% at 5 mg/kg. A total of 76% of otoliths in the tetracycline hydrochloride‐treated fish showed usable fluorescence marks (good: 12%, poor: 64%), while marks of calcein‐treated fish were only observed in 10% of the otoliths. The differences in survival outcome between the three tetracycline dosages were not significant, supporting the use of the lowest tested tetracycline dose (25 mg/kg) for effective marking. These results indicate that tetracycline hydrochloride is a suitable and well‐tolerated chemical marker for adult European flounder, whereas calcein was unsuitable due to the recorded mortality and low marking success. The findings provide a methodological basis for mark‐recapture studies of European flounder in the Baltic Sea and may inform similar applications in other areas and in related flatfish species.