This paper represents the annual report of a multi-member longline survey on Antarctic toothfish (Dissostichus mawsoni) at Subarea 48.6 in the 2019/20 fishing season by Japan, Spain, and South Africa. The data set, C2 and Observer data, was provided by the CCAMLR Secretariat on the 14th July, 2020. In this paper, the data set during current fishing season (2019/20) was used for reporting the quantity of data and samples collected. The research operations at 48.6 have not yet been completed in the 2019/20 fishing season.

This paper updates the knowledge of egg and larval transport of Dissostichus mawsoni in the East Antarctic region as a background document of the research plan for the multi-Member research on the Dissostichus mawsoni exploratory fishery in East Antarctica (Divisions 58.4.1 and 58.4.2) from 2018/2019 to 2021/2022. Using particle tracking technique, this experiment assessed successful transport pathways and percentage of egg and larvae released from the Banzare Bank and continental slopes to identify which recruitment scenario hypothesized by the CCAMLR scientific report appears to be in action in the East Antarctic region. The result of this paper suggested that the majority of successful transport was supplied from continental slopes. Although the southeast part of the BB was the source of the successful transports, the level of successful transports was overall low. Thus, the 3rd scenario of originally hypostasized three scenarios seemed to work in the East Antarctic region. The local recirculation played an essential role to cause successful transport from continental slopes to continental shelves. In addition to this, the position of the southern branch of the ACC against sources on continental slopes was also vital to determine successful egg and larval transports.

In this paper, we report on fish by-catch during exploratory fishing activities undertaken in Divisions 58.4.1 and 58.4.2 during the period 2014 to 2020. Fish by-catch comprised 12 species or groups of species. In 2019 and 2020, exploratory fishing occurred exclusively in research block 58.4.2_1 where by-catch represented 4.8% of the total catch in both years. 98% of the biomass was represented by two families: Macrouridae and Channichthyidae. The other most common by-catch species or families were Muraenolepis spp., Antimora rostrata and Artedidraconidae. Raja and Bathyraja were rarely caught. Species composition varied between research blocks except for Macrourus spp. which dominated by-catch composition everywhere. The ratio by-catch to target catch was higher in the eastern part of Division 58.4.1.

In research block 58.4.2_1, the ratio of Macrourus to Dissostichus decreased in the last two years compared to 2017, while the ratio of Channichthyidae in the total catch increased. None of the bycatch thresholds set in CM 33-03/A were reached. Macrourus catch rates was relatively low in 2019 and 2020 (9.7 and 9.4 kg/1000 hooks) compared to the average value across the two divisions (23 kg/1000 hooks).

As found in others areas of the Convention, reported Macrourus catch rates was 2 times higher for autolines than Spanish lines and trotlines, and it peaked at depths between 900 and 1300m. Catch rates of other by-catch species were much lower and highly heterogeneous in space. Macrourus catch was dominated by females in all research blocks and their length frequency distribution did not reveal any temporal changes within research blocks.

The Antarctic toothfish, Dissostichus mawsoni, serves as a valuable fisheries resource around the Antarctic Continent since 1997, managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Although defining genetic or stock structure of populations is crucial for improving fishery management of this species, its number of populations or stocks and genetic diversity levels remain unclear. In the present study, we assessed the population genetic and phylogeographic structure of the Antarctic toothfish populations across 20 geographic localities spanning from the two main Subareas 88 and 58 (58.4, 58.5), based on a combined analysis of mitochondrial DNA (mtDNA) cytochrome oxidase I (COI) and 16S rRNA (16S) sequences and seven nuclear microsatellite loci. MtDNA revealed a low level of polymorphism (h=0.571, π=0.0006) with 40 haplotypes in 392 individuals, connected only by 1-5 mutational steps. Nonetheless, microsatellites showed much higher variation with allelic richness (AR) values of 6.328 (88.3 RB3) to 7.274 (88.3 RB6) within populations. Levels of genetic diversity were generally higher for the 58 Subarea populations than for the 88 Subarea. Eight of 15 populations showed a genetic signal of inbreeding, despite no sign of population bottlenecks detected. Population structure analyses of microsatellites suggest that the sampled 88 and 58 Subareas are likely to comprise a well-admixed single gene pool (one genetic stock), probably due to high contemporary gene flow during the prolonged epipelagic larval phase of this fish. However, given weak, but significant microsatellite differentiation found between six population-pairs (58.4.2 A vs 88.3 RB4; 58.5.2 vs 88.1 RBK; 58.5.2 vs. 88.3 RB1; 88.1 RBI vs 88.2 RB1; 88.1 RBI vs 88.3 RB4; 88.1 RBH vs 88.3 RB4), the possibility of existence of multiple stock lineages could not be excluded. The mtDNA AMOVA also indicated a significant difference in the population structure between the 88 and 58 Subarea groups (F_CT=0.011, P=0.004). To clarify these issues, further study with additional polymorphic markers (such as microsatellites or SNPs) using more samples from other areas, particularly the 48 Subarea will greatly help to determine the population or stock structure of an entire population of D. mawsoni more concretely. The findings of this study will inform conservation efforts on the stock (unit) management for this valuable fisheries resource. Genetic monitoring for the Antarctic toothfish populations will be essential to understand how well these valuable fisheries resource will sustain in response to ongoing climate changes.

The Antarctic toothfish, Dissostichus mawsoni, is a species of Family Nototheniidae of Order Perciformes native to the Southern Ocean and one of the top predators in the Antarctic ocean. We were able to obtain 72 haplotypes (10 orders, 16 Families, 26 Genera, and 35 species) as the five-year metabarcoding analyses about the stomach contents of D. mawsoni in subarea 88.3. Based on the proportions of fish reads (99.61 %) compared with those of the cephalopods (0.27 %), D. mawsoni was piscivorous, which is consistent to the previous studies. Chinobathyscus dewitti (59.09 %), Macrourus caml (20.91%), Magnisudis prionosa (10.69 %), and Lepidonotothen squamifrons (4.52 %) turned out to be the four main preys of D. mawsoni in subarea 88.3. The metabarcoding results in subarea 88.3 were clustered together distinctly from other subarea such as 88.1 and 58.4.1. Those results strongly indicated that metabarcoding analysis of stomach contents of D. mawsoni can reflect the fish assemblage of the specimens collected. Besides regional clustering of stomach contents indicated that prey species of D. mawsoni are regionally different. Finally, the positive correlations between IRI and the proportions of NGS reads were identified with the different R values, which suggested the proportions by the metabarcoding can reflect the biomass of each prey species, albeit with a large degree of variations. For the more accurate estimation of each prey’s biomass, qPCR will be the better method, which would be further used for the long-term surveys with large research areas.

Antarctic toothfish (Dissostichus mawsoni) have been found in regions sampled throughout Subarea 88.3 at depths from 550–2000 m, with size distributions and catch rate (i.e. abundance), differing by area, and especially by depth. Size distributions along the slope were typically bimodal, comprised of both very small and adult fish. Toothfish catch rates were highest on the central portion of the slope (88.3 B). Consistent with observations from other areas, growth of D. mawsoni differed between sexes, with females being larger on average for a given length than males. Resulting von Bertalanffy growth curves were similar to those derived for Antarctic toothfish in the Ross Sea region. Based on histology, length at 50% maturity of female and male Antarctic toothfish was estimated to be 124.5 cm and 124.0 cm, respectively, with length at 95% maturity estimated to be 145.2 cm and 166.3 cm for females and males, respectively. Age at 50% maturity was estimated to be 13.06 years for females and 13.27 years for males. Age at 95% maturity was estimated 18.64 years for females and 21.16 years for males, however greater sampling is required to confirm these parameters. Further sample collection and examination of spatial patterns in the biology and stock structure of D. mawsoni and key bycatch species in Subarea 88.3 are recommended.