Size at maturity and histological procedures explored to determine spawning activity of female Dissostichus mawsoni from samples collected from the Ross Sea in January 2004, December 2004 and January 2005

Gonad samples from female Antarctic toothfish (Dissostichus mawsoni), collected during the 2003/04 and 2004/05 commercial fishing seasons in the Ross Sea to improve estimates of size of maturity, were analysed macroscopically and histologically. Progress was, however, limited because few samples of toothfish less than 80 cm and over 120 cm total length were obtained. The first part of the study used classic histological techniques to classify ovary stages by the most advanced state of oocyte development visible in histological sections of the 2003/04 samples. Using this methodology, it is theoretically possible to determine the proportion of fish maturing to spawn, and from that, determine the mean size at maturity. The calculated Lm50 of 113.0 cm was very close to the value of 115.2 cm estimated in 2000/01. GSI data collected from across the fleet, however, still raise doubt about the true Lm50.
The second part of the study examined histological sections from the 2004/05 season for indicators of spawning activity that may also be useful in ascertaining the maturity ogive of D. mawsoni in the previous season. A range of staining techniques was used to determine variations in the presence of PAS bodies (macrophage aggregates), encapsulated residual eggs, ovary wall thickness, atretic oocytes, the configuration of lamellae tips and oocyte packing. The latter three attributes showed the greatest potential in identifying post-spawners, but require calibration over a much wider range of size samples than obtained for this study. Recommendations for further sampling and character measurements are given.