Variations in patterns of daily changes in otolith increment widths of 0+ Pacific saury, Cololabis saira, off Japan by hatch date in relation to the northward feeding migration during spring and summer

Daily changes in otolith increment widths from the core to the edge in 0+ Pacific saury, Cololabis saira, were examined. There appeared a first peak of increment width at 22.9 +/- 9.6 (73) [mean +/- SD (sample number)] days old, followed by the bottom of the first trough [88.6 +/- 25.3 days old (73)] and a second peak [143.8 +/- 24.6 days old (62)]. The ages at the second peak formation were strongly related to the estimated hatch dates from 30 October to 7 April (N = 62, r(2) = 0.661) with a negative coefficient of elapsed days from 30 September, corresponding to the beginning of the spawning season. Thus, fish hatched earlier in the spawning season formed the second peak at an older age. The second peak was estimated to occur from 23 April to 14 August, mainly in June and July, at 152.5-216.8 mm (excluding one exception of 96.9 mm) in the earlier period (before 15 June) and at 87.5-177.5 mm in the later period (after 17 June). The peak of otolith increment width is concluded to approximate the peak of daily growth because the relationship between otolith radius and body size was expressed as an allometric curve. The occurrence of the second peak of increment width (growth) can be theoretically explained by descending temperature and ascending feeding condition, which Pacific saury experiences through the northward feeding migration from the warm and food-poor Kuroshio (subtropical) waters to the cold and food-rich Oyashio (subarctic) waters during spring and summer. The clear relationship between the age at the second peak and hatch date suggests the possibility of estimating hatch dates even of fish that have translucent zones in their otoliths by examining the increment widths from the core to the inner edge of the first translucent zone and specifying the age at the second peak.