Abstract.Larvae and juveniles of two species ofCoregounssp. were immersed in tetracycline (TC) solutions (200–800)mg/1) and fluorescent marks in the otoliths from fish were identified under a UV‐light microscope. Whitefish.Coregonus Iavaretus(L.). juveniles (average weight 191 mg) immersed in TC solution retained their marking until the 127th day, hut 40 of the population lost their TC marking after wintering. Whitefish and peled.C. peledGmelin, larvae retained TC marking from the TC solution (400 mg/1) if water temperature was higher than 5°C, Whitefish juveniles retained clear marking in the otolith after 120 days of post‐immersion rearing. Peled larvae immersed in TC solution were grown in a pond and the fluorescent otolith centre of 55·4‐g fish was well visible after processing. Nearly 100 of peled larvae grown for 176 days can he identified based on fluoresceent marks observed under a UV‐lighl
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