First increment validation of Patagonian toothfish (Dissostichus eleginoides) from Heard Island

Patagonian toothfish, (Dissostichus eleginoides) have traditionally been aged assuming one opaque and one translucent increment is formed each year on the sectioned face of sagittal otoliths. It has been recorded that differences in otolith interpretation exist between institutes routinely ageing D. eleginoides, resulting in differences of ageing data presented. One area that was assumed to cause interpretation differences was the position of the first increment that corresponded with the end of the first year of growth. While there is a general consensus that the observed increments represent annual events, no direct validation has been provided for the early growth of this species. In an attempt to determine the position of first increment formation daily increments in 7 sagittal otoliths were counted. An average daily increment count between the hatch mark and the outer edge of the opaque nucleus of 229 days was recorded. Due to the resolution of light microscopy, increments could not be confidently counted in the first translucent zone immediately succeeding the opaque nucleus. It was assumed that the outer edge of the first translucent zone would correspond with the end of the first year’s growth. The average measurement from the primordium to the outer edge of the first translucent zone on the ventral side of the otolith section was 0.630mm. This is consistent with the measurements in previously aged samples from the Heard and Macquarie Islands fisheries. These data suggest that the protocol currently used for the determination of the first annual increment is valid.