A Technique for Tetracycline Injecting and Tagging Billfish

Various techniques are available for validation of aging techniques for fishes. Modifications are usually required to suit the immediate needs. While promising results have been presented for the determination of age in species of billfish, the periodicity of incremental marks in hardparts has not been established. I describe the equipment developed and applied to achieve such validation through the injection of oxytetracycline into Pacific sailfish and black marlin in east coast Australian waters. I have outlined the practical constraints of the exercise and the utility of the tagging system highlighted by the results. Over 400 sailfish and black marlin were tagged and simultaneously injected by the crews of gamefishing vessels undertaking normal charter operations. Five black marlin were recaptured of which three were available for study. Each of these fish displayed easily discernible oxytetracycline marks in the otoliths and fin-spines. There has been no rigorous validation of aging techniques in any species of billfish and the success of this project and the utility of the technique offers a means to achieve this necessary proof of age.