LENGTH AND AGE AT SPAWNING OF ANTARCTIC TOOTHFISH DISSOSTICHUS MAWSONI IN THE ROSS SEA

This study uses a histological assessment of age and length at spawning for female and male Antarctic toothfish Dissostichus mawsoni from fish sampled in the Ross Sea spanning the 2000-2009 fishing seasons. The female spawning ogive incorporates the proportion of sexually mature fish that do not spawn in a given year and is therefore appropriate to evaluate spawning stock biomass relative to stock management decision rules. A characterisation of the oocyte developmental cycle of Antarctic toothfish shows that primary endogenous growth can occur for an extended period with oocytes accumulating at the cortical alveoli stage at least a year prior to spawning. Individual oocytes are then recruited into the vitellogenic phase over at least a 6–12 month period, resulting in a developed batch of oocytes accumulating at the final maturation stage by May. Oogenesis is characterised to provide the biological basis for interpreting the histology. Both forecasting (using the most advanced oocyte stage) and hindcasting (using remnants of previous spawning) yielded similar results. The age at 50% spawning for females on the Ross Sea slope region is estimated to be 16.6 yr (range 16.0–17.3) or 133.2 cm (range 130.9–135.7) by length. Males are estimated to spawn at a younger age; 12.8 yr (range 11.9–14.0) or 120.4 cm (range 114.8–126.7) by length. Evidence of skip spawning by females results in a right-shifted ogive, increasing the functional difference between male and female ogives. Comparison of the spawning age distribution on the slope with the age distribution in the northern area and the lack of evidence for skip spawning in the north indicates that all fish in the north are spawning. The degree to which the overall population ogive is biased right (older) by excluding the mature fish in the north depends on the proportion of the total population occurring in the northern area, which is currently unknown.