Age validation of Patagonian toothfish (Dissostichus eleginoides) from Heard and Macquarie Islands

Patagonian toothfish, (Dissostichus eleginoides) have traditionally been aged by counting presumed growth increments on the sectioned face of sagittal otoliths. While there is a general consensus that the observed increments represent annual events, no direct validation has been provided for this species. This study attempts to validate the annual increments using strontium (Sr) marked otoliths from tagged-recaptured samples. Since 1996, a large scale tag-and-release program on D. eleginoides in the Heard Island and Macquarie Islands fisheries has included injection of most fish with Strontium Chloride (SrCl2) on release. One hundred and forty two of these strontium marked fish were selected for analysis. Strontium (Sr) marks were detected in 139 of the 142 otoliths examined. Using the number of observed increments after the mark in relation to the time-at-liberty between tagging and recapture, we determined that for age classes 5 to 18, one increment is laid down per year. The expected number of increments was the same as the observed increments for 88% of cases (when the time-at-liberty was known at the time of reading and 52% when the time-at-liberty was not known These results indicate that while interpretation difficulties of increment structure in sectioned otoliths of D. eleginoides may result in variability from the absolute age, reliable estimates can still be produced.