Age determination and validation using internal growth rings in shell cross-sections of Ezo abalone Haliotis discus hannai, Iwate Prefecture, Japan

¹Iwate Fisheries Technology Center, Kamaishi, Iwate 026-0001, ²Tohoku National Fisheries Research Institute, Fisheries Research Agency, Shiogama, Miyagi 985-0001, ³Miyako laboratory, Tohoku National Fisheries Research Institute, Fisheries Research Agency, Miyako, Iwate 027-0097, Japan

We developed an age determination method for Ezo abalone Haliotis discus hannai using the internal growth rings in the cross-section of the inner lip shell. A laboratory experiment showed that 1-year-old juveniles had one internal growth ring that was deposited during the season of lowest seawater temperature. Tagged juveniles (2 years old) were released into their natural habitat and recaptured after 2-5 years from the date of release. The recaptured individuals (n=3) had the same number of internal growth rings as their age in addition to one disturbance ring. Results of stable oxygen isotope analysis on the shell cross-sections confirmed that the growth rings were deposited between winter and spring when the temperature decreased below 7℃. Furthermore, the disturbance rings that were observed only in the released individuals were formed at the time of release into the natural habitat. Comparing the validity of the age determination method developed in this study with the conventional burning shell method, the age estimated by the conventional method was less than that by the present method for ≥8-year-old abalone. Age determination using internal growth rings is more accurate than the conventional method, leading to differences of 1-2 years in estimates.