Daily increment formation in otoliths of juvenile skipjack tuna Katsuwonus pelamis

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: In the present study, we verified daily increment formation within sagittal otoliths of juvenile skipjack tuna Katsuwonus pelamis by analysis of diel changes in marginal increment width. Specimens were collected by midwater trawl in the tropical western Pacific from November to December 1995 and in February 1999. The sagittal plane of the otolith was embedded in enamel resin and polished with lapping films. The otolith-measuring system was equipped with a light microscope and was used for marginal increment analysis. The index of completion of the marginal increment increased with time of day, approximately 40% at 07 : 00–10 : 00 h, 66% at 13 : 00–16 : 00 h and 80% at 19 : 00–23 : 00 h. Therefore, growth of the marginal increment progressed from morning to evening and was completed during the night. We conclude that increments are formed daily in juvenile skipjack otoliths and these increments have a width of 15–40 µm.