Mass-marking of ayu eggs and larvae by tetracycline-tagging of otoliths.

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Abstract
The object of this study is to develop a method of mass-marking the egg and larval stages of fish in order to carry out field surveys of the early life history and population dynamics of the fish. In this method the eggs or newly hatched larvae are immersed in tetracycline hydrochloride (TC) solution and marked-fish or fish from treated eggs are detected by the presence of a fluorescent mark on otoliths under UV light. To determine the optimum TC concentration and treatment time for marking the eggs and larvae of the ayu Plecoglossus altivelis, a diadromous fish, various combinations of TC concentration and treatment time (40-44 lots) were tested. As a result, 200-300 mg/l for 24-48 h was found to be appropriate for eggs and 200-300 mg/ml for 3-24 h for the larvae. Mark retention and the effect of TC treatment on survival and growth were examined by rearing marked larvae in 5001 plastic tanks for more than 5 months. The mark was easily descernible (100%) until 105 days after hatching without any special preparation for detection. Even 164 days after hatching the percentage of retention was 100% after grinding or etching in weak acid, compared with 83% before such treatment. The treatment had no effect on the survival and growth of the ayu. These results strongly suggest that this method can be applied to field surveys on the ayu during early life stage in the sea.

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