In the Division 58.4.1 there are two stocks of Dissostichus mawsoni, which is vulnerable with a big range of about 1,000-2,000 t per SSRU in 58.4.1. However, there is not enough data to utilize stocks properly, especially relate to reproductive mechanism. Therefore, the Korean scientists analyze the reproductive ecology of Dissostichus mawsoni in SSRU 58.4.1C-a based on the histological methods as a part of Korean research plan for Dissostichus spp. in SSRUs 58.4.1 C and E during 2012/2013 season.
The ratio of female of D. mawsoni collected in SSRU 58.4.1C-a during the 2012/2013 season was 0.54 displaying the tendency increased with the total length became larger. The gonadosomatic index of the female was 2.6, and 1.1 for the male. The hepatosomatic index of the female (2.5) and male (2.3) were similar. The type of ovarian development of D. mawsoni was group-synchronous type. In the ovaries of growing stage, initial vitellogenic oocytes with approximately 150 ㎛ of egg size was mainly observed. The egg size of mature oocytes presented approximately 800 ㎛. Eosinophilic yolk materials and vacuolar oil droplets were interspersed within the cytoplasm, and the egg size of ripe oocytes were approximately 1,000 ㎛. Eosinophilic yolk materials are distributed within the medulla of cytoplasm, and combined multiple vacuolar oil droplets were existed in the cortical section of the cytoplasm. Histological testicular type of the fish was testicular lobule type, and the testicular lobule was composed of numerous testicular cysts. In the testis of growing stage the cysts became more definite gradually, and spermatogonia and spermatocytes occupied most of the cortex of the testicular lobule. It was possible to observe spermatids that illustrated basophilic reaction in H-E staining within the expanded testicular cysts in the mature stage. In the ripe stage, inner cavity of testicular lobules was occupied by spermatozoa of basophilic in H-E stain. Frequency of ovarian development stages of the toothfish showed the highest proportion with 46.3% at the mature stage, and followed by the growing stage with 29.3%, and ripe stage with 24.4%. Frequency of testicular developmental stages demonstrated the highest proportion with 58.3% for the growing stage, and followed by mature stage with 36.1%, and ripe stage with 5.6%. According to the frequency of gonadal development stages, spawning season of D. mawsoni may be deduced from May to June, and the fish may spawn two or three times during a spawning period in SSRU 58.4.1 C area. The group maturity in accordance with the total length (TL) categories of D. mawsoni displayed 100% for the female in lager than 90 cm categories, and almost 100% for the male in larger than TL 100 cm categories. The results of this study were concluded from the limited number of samples and interpretation of reproductive ecology of D. mawsoni, therefore reanalysis of the reproductive ecology based on the much larger numbers of samples should be carried out in the near future.