The spawning dynamics of Patagonian toothfish in the Australian EEZ at Heard Island and the McDonald Islands and their importance to spawning activity across the Kerguelen Plateau

In 2011, AAD, the Fisheries Research and Development Corporation and Australian fishing industry conducted a joint project to study the reproductive dynamics of Patagonian toothfish (Dissostichus eleginoides) around Heard Island and the McDonald Islands (HIMI, Division 58.5.2). Gonadosomatic data was available for 12 563 individuals. The histology of male and female gonads was also analysed from specimens collected in 2011. This data confirms that toothfish spawn between July and August across a large latitudinal range along the deep slope of the Kerguelen Plateau. A comparison of macroscopic staging of gonad development with histological analysis showed macroscopic characteristics often failed to distinguish spent or resting mature animals (as confirmed by the presence of post-ovulatory follicles) from first maturing animals, and that even large mature fish may not spawn every year. As a result, the conventional method of estimating size at maturity, with stage 3 or above (according to the Kock and Kellerman (1991) scale) being assessed as mature, did not provide realistic estimates. A modified method, defining stage 2 or above as  ‘mature’, offset by two years to account for the duration of the maturation process, provided more realistic estimates. The revised ogives were shown to have a significant impact on the estimates of stock status relative to those used previously that were derived from studies of other populations of toothfish.