Preliminary studies on age, growth and size at sexual maturity of Dissostichus eleginoides in the Ob-Lena Bank in the 2007/08 season

Preliminary studies on the age, growth and size at sexual maturity of Dissostichus eleginoides sampled in the 2007/08 season in the Ob-Lena Bank were carried out. The ages of 214 of 3,013 fish sampled by Observers were estimated last year. Then, we took several advice and comments about the methods of otolith readings by experienced readers of AAD. One of these is that the first ring was generally identified within the primordium and thus many of age counts were overestimated. The Japanese experienced reader (who is different from the last year’s reader) recounted annuli for the 214 otolith samples and count annuli for the newly added 338 samples (thus total of 552 samples) referring these advice and comments. The estimated ages ranged from 5 to 48 years for males and from 4 to 48 years for females. Ten to fifteen years gave large contribution to the total age groups for both sexes. Females grew at a faster rate and reached a larger size than males. The individuals with 80 cm total length, which are generally thought to begin to mature, corresponded to 10.6 year for females and 11.4 year for males. On the other hand, the age at 50 % maturity for females and males was estimated to be 12.3 and 12.5 year, respectively, assuming animals with maturity stage > I.

However, the results in reading for 40 reference samples provided by AAD were less consistent with those read by the AAD readers. The Japanese reader tended to overestimate for < 30 years, but underestimate for > 30 years, as compared to AAD readers. This means that the present results on age, growth and size at sexually maturity is less reliable. The main reason for the underestimation for older ages was due to the consistently counting on the ventral side of the section.

We would like the AAD readers and CCAMLR member give further advice on the present results during the WG-FSA or intersessionally. We would recount the annuli for the present 552 otolith samples following their advice. Furthermore, we would count annuli for the newly added 300 samples from the 2007/08 survey and depict the average image of growth pattern in Ob-Lena Bank using total of ca. 800 samples in the next WG-FSA.