This study investigated maturation and spawning of Patagonian toothfish Dissostichus eleginoides in the Heard Island and McDonald Islands (HIMI) fishery on the Kerguelen Plateau in the Indian Sector of the Southern Ocean based on gonads and otoliths collected between years 2004–2015, and using histological analyses and calibration of macroscopic staging criteria. Dissostichus eleginoides at HIMI spawn throughout the late autumn and winter months of May–August, and spawning activity is concentrated on slopes along the west and south of the plateau around HIMI at depths of 1500–1900m. Comparison between histological analyses and macroscopic gonad staging indicated that many fish that had spawned, as indicated by the presence of post-ovulatory follicles, returned to a resting stage which was macroscopically indistinguishable from maturing fish. Furthermore, the occurrence of females of all size classes with low gonadosomatic index and low macroscopic gonad stage during the spawning season suggested that a proportion of mature females did not spawn every year. Age-at-maturity estimates, obtained based on the assumption that fish of macroscopic stages ≥ 2 were mature, decreased between the 2004–2009 and 2010–2015 periods for both sexes. However, the magnitude of this temporal variation in age-at-maturity varied between gear types and fishing depths, and variable sampling regimes likely influenced these variations. This study highlights the importance of correct interpretation of macroscopic gonad stages, and understanding the influence of fishery operations on estimations of life-history parameters.

The annual random stratified trawl survey was undertaken in Division 58.5.2 in the vicinity of Heard Island and the McDonald Islands during April 2017. This paper provides an updated growth model and a preliminary assessment for mackerel icefish (Champsocephalus gunnari) population structure, abundance and yield in Division 58.5.2 to the west of 79^o 20’ E using standard CCAMLR methods (CMIX and Generalized Yield Model).

The 2017 survey showed a large 3+ cohort dominating the population. An updated growth model based on recent cohort progressions indicated an increase in size-at-age compared to the estimates of Welsford (2010), however Generalized Yield Model projections indicated that the updated growth parameters resulted in only a small impact on yield.  Catches of 526 t in the 2017/18 season and 395 t in the 2018/19 season respectively satisfied the CCAMLR decision rules. Given their apparent plasticity in growth, we recommend that growth parameters used in assessments be updated routinely.

This paper includes updated local biomass estimates of toothfish populations with bootstrapped confidence intervals using both CPUE-by-seabed area and Chapman biomass estimation methods agreed at WG-SAM-16 with revisions to the CPUE-by-seabed area parameter values agreed at WG-SAM-17. The analysis includes Research Blocks in Subareas 48.2, 48.6, 58.4 and 88.3. Given the changes in CPUE-by-seabed area method parameter values that were agreed at WG-SAM-17, the estimates of biomass in the relevant Research Blocks have increased relative to the estimates that were presented at WG-FSA-16. The higher CPUE-by-seabed area estimates has also resulted in a greater overlap in confidence intervals with the Chapman estimates in some, but not all, of the Antarctic toothfish Research Blocks. 

Fatty acid (FA) profile and stable isotope (SI) signature analyses has been used to examine feeding ecology of Antarctic toothfish and food webs of Antarctic marine ecosystem. Specimens of Antarctic toothfish, by-catch and their stomach contents were collected for the past five years in the eight locations of he Antarctic Waters. We investigated ontogenetic and spatial variability in resource utilization of Antarctic toothfish and distribution patterns of FA profiles in various organisms because of the particular importance of accumulation of lipid deposits in their tissue. Toothfish trophic niche indices was calculated using both FA profiles and SI values. Results shows spatial variation in their resource utilization between the Ross Sea shelf (RSS) and the deep-sea slopes the western Indian Ocean sector (IOS) and the slope towards the Pacific Ocean sector (POS). FA profiles of toothfish were characterized by high proportions of monounsaturated FAs and low proportions of polyunsaturated FAs, differing from other organisms in their habitats. Distribution patterns of FA profiles among various organisms were well established by self-organizing map (SOM) analysis. Similar FA profiles and SI values of toothfish between ISO and POS indicate a depth-dependent dietary difference from that of RSS. This appears to be closely associated with ontogenetic and spatial variation in resource utilization during their migration. The combination of both tracers explained well their dietary utilization pattern. Indeed, a recently developed mixing model calculation for FA profiles and SI values, which follows the Bayesian machiner, highlights the nutritional importance of Noththenidae fish compared to Channichthyidae species to whole population level. In contrast, the model calculations provided an estimation of dietary shift during ontogeny and thereby migration to deep water. Finally, the model results clearly explained spatial shifts on the nonmetric multidimensional scaling plot and the SOM map by realistic diet proportions.

Japan and France revised the next season’s (2017/18) research plan in research blocks 58.4.4b_1 and 58.4.4b_2 using the latest CCAMLR C2 and Observer data, following several advices during WG-SAM meeting in 2017. We have used extracted data provided by the CCAMLR Secretariat on 27th July, 2017.

The estimated median stock size in block 58.4.4b_1 and 58.4.4b_2 was 1452.03 and 921.91, tonnes respectively, in Chapman method considering tags released in the last three years as effective for the biomass estimation. The estimated median stock size in block 58.4.4b_1 and 58.4.4b_2 was 484.51 and 498.76 tonnes, respectively, in CPUE analogy method (reference area: HIMI) using the relevant information of reference area (recent median CPUE, estimated biomass and updated area size) recommended in the WG-SAM-16/18 Rev.1.

We propose to continue the current research operation for the next fishing season with the same survey design and total catch limit of 70 tonnes to further strengthen the stock assessments in the area.

The ratio of female among the 600 specimens of Antarctic toothfish collected from the 58.4.1 area and 88.3 area within the CCAMLR Conservation Zone during February through March, 2013 was found to be 0.50, and displayed the tendency of increase with increase in the total length 140 cm. The gonadosomatic index of the fish was 2.5 for the female, and 1.7 for male.  The hepatosomatic index of female (2.5) and male (2.7) were similar. The type of ovarian development of Antarctic toothfish was group-synchronous type. In the ovaries of growing stage, initial vitellogenic oocytes with approximately 150 ㎛ of egg size was observed mainly. The egg size of mature oocytes presented approximately 800 ㎛e. As the result of H-E staining, eosinophilic yolk materials and vacuolar oil droplets were interspersed within the cytoplasm. 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 Antarctic toothfish 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 Antarctic toothfish showed the highest proportion with 45.4% at the ripe stage, and followed by the growing stage with 25.9% and mature stage with 28.7%. Frequency of testicular developmental stages demonstrated the highest proportion with 67.0% for the matuer stage, and followed by growing stage with 24.2% and ripe stage with 9.9%. The group maturity in accordance with the total length (TL) categories of Antarctic toothfish displayed 80% for female in larger than TL 100 cm categories and almost 100% for male in larger than TL 150 cm categories.

South Africa and Japan revised the next season’s (2017/18) research plan in Subarea 48.6 using the latest CCAMLR C2 and Observer data, following the recommendations and notification from WG-SAM meeting in 2017 (e.g., WG-SAM-17 report, paragraph 4.4, 4.10, and 4.18, 2017) and data extracts provided by the CCAMLR Secretariat on the 27th of July, 2017 were used.

The estimated median stock sizes in research blocks 48.6_2E (southeast part of 48.6_2), 48.6_3, and 48.6_4 are 4864.30, 3132.55, and 8044.33 tonnes, respectively, when applying the Chapman method. Chapman method considers tags that were released in the most recent year as effective for the biomass estimation, except for 48.6_4 which considered the last three years when estimating biomass. The estimated median stock sizes in blocks 48.6_2W, 48.6_2E, 48.6_3, and 48.6_4 were 189.56, 306.31, 125.05, and 969.14 tonnes, respectively, when CPUE by sea bed area methods are used considering the relevant information of reference area (recent median CPUE, estimated biomass and updated area size) recommended in the WG-SAM-16/18 Rev.1.

We propose to continue the current research operation in Subarea 48.6 for the next fishing season with the same survey design. 

Following the note by the Working Group in paragraph 4.21 in WG-SAM-17 report on the ice concentration analysis by Namba et al. (WG-SAM-17/10), the further analyses for the ice concentration was conducted to investigate correlations between sea-ice coverage and wider global weather phenomena such as El Niño/El Niño Southern Oscillation or rising temperatures/increased variability in the sea surface temperature (SST) anomaly (variance from average) of ERDDAP of NOAA.