The use of alizarin complexone for immersion marking of the otoliths of embryos and larvae of the turbot, Scophthalmus maximus (L.): dosage and treatment time

The purpose of this work was to determine the efficacy of marking turbot, Scophthalmus maximus (L.), eggs and larvae with alizarin complexone (ALC) for use in enhancement programmes and in studies of the ecology and behaviour of the species. The main aim was to determine the optimum dosage and treatment time for large batches of larvae destined for release into the sea.Eggs and larvae were immersed in a range of ALC concentrations (0–60 mg L^–1) for various times (3–24 h). On day 8 after hatching, eggs marked with the different ALC dosages had 99% marking success and had mainly good-quality or very-good-quality marks, whereas larvae achieved 100% success and had mainly very-good-quality marks. The best results were obtained after marking larvae with ALC at 60 mg L^–1 for 6–24 h.During the first months after marking, the fluorescent rings formed in the otoliths were easily detected when directly examined with a UV microscope, no prior preparation of the otoliths was necessary, but after 7 months the marks become faint as the otoliths get thicker, so it is necessary to cut or polish the otoliths before examination.