Re-evaluating age estimation using daily increments on otoliths of the skipjack tuna Katsuwonus pelamis

This study compared the microstructures of transverse sections and surface of the otoliths of skipjack tuna. Pairs of otoliths were extracted from the same individuals (fork length [FL]: 6.5-77.5 cm) to examine the difference in the counts of the daily increments between the sections. The increments counted on the transverse section were less than those on the surface section, and the difference between the two sections increased with body size. Even high-resolution scanning electron microscopy did not enhance the difference between the two sections. Increments in the transverse section would not occur daily, which makes otoliths from this section unsuitable for age estimation. Subsequently, we re-evaluated the age estimated using the surface reading with new criteria and compared them with the results of a reference study. Despite successful age estimates using surface otoliths exposed by an etching method, practical difficulties in the method and vague criteria for reading increments have been raised against their reliability for age estimation. By reproducing detailed procedures of the etching method and using reading criteria, we estimated the ages of individuals at various life stages. In applying the criteria, the coefficient of variation (CV) for increment counts among readers decreased. In particular, the criteria reduced the CV for the transition area where the width between increments gradually narrows after the 50th-90th increment. The age estimates (FL < 40 cm) using the criteria are reasonably fit to the growth curve reported in the reference study with validation of daily increment deposition for surface reading. Thus, the new criteria are suitable for estimating age estimation in skipjack tuna up to an FL of 40 cm.