Determination of squid age using upper beak rostrum sections: technique improvement and comparison with the statolith

Analysis of growth increments in beak rostrum sagittal sections (RSSs) has been increasingly used for estimating octopus age. In this study, we develop an effective method to process and read the RSS of four oceanic ommastrephid squid species (Dosidicus gigas, Ommastrephes bartramii, Illex argentinus and Sthenoteuthis oualaniensis) and validate the daily deposition of the increments by comparing to corresponding statolith-determined ages. The proposed method of processing yielded readable rates ranging from 42.9 % in I. argentinus to 71.7 % in D. gigas for samples from I. argentinus to D. gigas. The high precision of the increment readings with low independent counting coefficient of variation indicates that the processing and counting methods used are reliable. This study suggests that the RSS of the upper beak is an appropriate tool for estimating the age of D. gigas, O. bartramii and perhaps S. oualaniensis, although erosion of the rostral region may result in an underestimation of squid ages.