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Mass labelling techniques have great potential for the study of larval fish dynamics in closed habitats (lakes, ponds and flooded quarries) . Different methods of mass labelling of bone tissue were tested : bathing in tetracycline solutions with or without osmotic shock, and modification of otolith microstria by temperature, photoperiod or feeding manipulations using different batches of eggs, eleuteroembryos, larvae and prefed fry of Coregonus lavaretus L . from Lake Leman . A short period of immersion with osmotic shock produced better results than longer bathing (6 to 18 h) in low concentration (400 mg 1 -1 ) tetracycline solutions . At the `eyed' stage of eggs when otoliths appear, it is possible to use the immersion technique in a hyperosmotic solution (5 to 12% sodium chloride) with 1 % tetracycline (as the Hydrochloride or Oxytetracycline) . Optimal immersion times (maximal labelling with minimal mortality) were determined as follows : 10 to 15 minutes for `eyed' eggs, 3 .5 min . for eggs just before hatching and eleuteroembryos, 1 .5 min. for prefed larvae and less than 1 min . for fry of more than 20 mm total length . Microscopic examination of otoliths and caudal vertebrae from coregonid larvae and juveniles reared for two years has allowed us to determine the effectiveness and persistence of fluorescent tetracycline makers . 399

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