Immersion mass marking of pikeperch (Sander lucioperca) larvae in oxytetracycline hydrochloride and its detection using fluorescence microscopy

Fish marking is essential to distinguish between stocked and naturally reproduced fish. This procedure is challenging, especially in early life stages. Therefore the objective of this study was to investigate the possibility of oxytetracycline hydrochloride (800 mg L−1 solution) immersion marking on 3–5 days old pikeperch larvae, with focus on durability and readability of marks on the otolith of marked individuals. The immediate mortality after marking was low (˂5%) and growth of the pikeperch was continuous during the eight week study period. The fluorescent marks were detectable during the whole study period, but after the sixth week grinding of the otoliths was necessary. With grinding otoliths the processing time and the probability of damaging the mark increased significantly. Another critical period was the necessity of sample storage in a dark place due to the marks’ instability in light, in all phases before the final mark detection. The study confirmed the suitability of oxytetracycline hydrochloride marking for use in studies focused on pikeperch juveniles’ early growth, stocking success, distribution etc.