1National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Hatsukaichi, Hiroshima 739-0452, 2An-naka, Gunma 379-0133, 3Seikai National Fisheries Research Institute, Fisheries Research Agency, Goto, Nagasaki 853-0508, Japan
We determined the time of formation of incremental zones (transparent light bands) and discontinuous zones (opaque dark bands) on larval sagittal otoliths of Japanese Spanish mackerel Scomberomorus niphonius through regular samplings over a day in captivity. First, distinctive rings were observed in 5-day-old fish when the larvae started feeding. The formation of the incremental zones on sagittal otoliths started between 05:00 and 08:00, and continued from the daytime to the nighttime. On the other hand, the discontinuous zones were formed during the following nighttime period. This result is helpful to measure the width of the outermost incremental zones and count the number of daily rings precisely in otolith analysis by considering the time when the samples of larvae were caught.