In the present study we provide new insights on the spatial distribution and population structure of juvenile D. mawsoni caught during bottom trawl survey on the shelves around the South Shetland Islands (CCAMLR Subarea 48.1). Fish samples were collected during five summer cruises conducted from 2001 to 2007. Juveniles were relatively evenly distributed on the shelves around Elephant Island and northern lower South Shetland Islands. Although they were caught in all depth strata, they were most abundant between 100 and 300 m depth. The sampled population was composed of 150 specimens measuring between 22 and 78 cm TL, equally distributed between sexes, most of them with gonads at immature stage of maturity. Both males and females showed a positive allometric body growth, suggesting a good condition factor. Based on counts of annual growth increments from sagittal otolith sections, age estimates ranged between 2 and 7 years in males and between 2 and 8 years in females, respectively. Annual growth rate was estimated from the age-length keys, spanning from 10 to 5 cm TL. Present data have been discussed in comparison with the biological traits of adults reported elsewhere.

A research plan for this subarea was commenced by Korea in 2016 and a revised collaborative proposal was submitted to WG-FSA-17 by Korea and New Zealand. This joint research proposal was for an additional 3 years covering the period 2018, 2019 and 2020. It was designed to build on Korea’s previous research by continuing to focus on those research blocks where tagged fish have previously been released on the slope, whilst also prospecting two of the northern seamount complexes and two boxes on the southern shelf, where little or no fishing has occurred. The main objective of the proposal is to determine the abundance of Antarctic toothfish in Subarea 88.3. Secondary objectives are to improve understanding of stock structure of toothfish in this area, to carry out calibration trials between the two vessels, to collect data on the spatial and depth distributions of bycatch species, and to trial electronic monitoring using video cameras. This paper updates the proposal with an amendment to include a catch redistribution mechanism and to incorporate a milestone table as recommended by WG-SAM-18.

This paper proposes to continue the Ross Sea shelf toothfish survey for the next five years, 2018–2022. The first objective of the survey is to monitor toothfish recruitment in the southern Ross Sea (core strata). This is a continuation of the yearly time series of research surveys of these strata carried out since 2012. It is important to continue the time series as the data collected provide essential input to the stock assessment to estimate year class strength which is used to inform management decisions. The second objective is to monitor trends in larger (large sub-adult and adult) toothfish abundance in alternate years in McMurdo Sound and Terra Nova Bay where toothfish may form an important part of the diet of Type C killer whales and Weddell seals. The third objective is to collect and analyse a wide range of data and samples from these areas (e.g. benthic invertebrates, stomach and tissue samples, and acoustic data). Both objectives two and three are specified as high priority research topics in the research and monitoring plan for the Ross Sea region MPA. The survey design will remain consistent with previous years. Different catch limits are requested on alternate year to account for the different catch rates in McMurdo Sound and Terra Nova Bay. The results of each survey and trends in the time series will be presented to WG-FSA for review each year and the results included in the stock assessment. A full review will be completed and presented to WG-SAM, WG-EMM, and WG-FSA in 2022. This revised proposal incorporates the milestone tracking table recommended by WG-SAM-18.

We propose to test three hypotheses to describe the reproductive ecology of Antarctic toothfish (Dissostichus mawsoni):

1) Antarctic toothfish eggs are buoyant and accumulate under sea ice. If true, this would retain eggs near the spawning locations under the vast sea ice extent and once broken up in the spring, may provide access to a productive pagophilic ecosystem for feeding as well as a transport mechanism for subsequent advection patterns, all of which could be impacted by climate change. This would have implications on the understanding where recruiting fish originate and how those patterns may be influenced by changes in sea ice or circulation patterns that affect observed recruitment patterns.

2) Antarctic toothfish spawn throughout the Pacific Antarctic fracture zone. Evidence to date only exists from the west of the region (SSRU 881B), yet adult Antarctic toothfish are found much further east and north of the CCAMLR Convention Area, which is bounded by latitude 60°S. Obtaining a better understanding of the location and movement of adult spawning toothfish has direct implications on the understanding of those parts of the adult stock that contribute to recruitment, and hence the productivity of the stock assumed in the stock assessment.

3) Biological characteristics of the northern spawning population change as younger, fatter, female fish move to the north for spawning during winter. Evidence to date found no change in these characteristics in June, suggesting sampling later in the spawning season is needed.

We propose to conduct a scientific survey during the austral winter in the northern Ross Sea region to test these hypotheses. The longline and plankton survey is designed to cover key gaps in the knowledge of the life cycle of Antarctic toothfish in the Ross Sea by collecting biological samples from a range of locations in the northern regions of Subarea 88.1 and 88.2 and begin in September 2019. The survey will be coordinated with a corresponding survey targeting Antarctic toothfish spawning dynamics in the southern area of the South Pacific Regional Fisheries Management Organisation (SPRFMO) area at a similar time. This revised proposal details the catch accounting method proposed by WG-SAM-18 and incorporates the milestone tracking table.

The New Zealand vessel San Aspiring carried out an exploratory research program for toothfish in the South Pacific Regional Fisheries Management Organisation (SPRFMO) Convention Area during August 2016 (Fenaughty et al. 2016) and September 2017. Analysis of the information collected showed high catch rates of post-spawning Antarctic toothfish, similar in magnitude to catch rates in the north region of CCAMLR subareas 88.1 and 88.2. The toothfish catch was almost entirely Antarctic toothfish except for two juvenile Patagonian toothfish. Sex ratios showed a high proportion of males to females. Fish were in poor body condition as assessed using Fulton’s condition factor indicative of full or partial starvation which would be consistent with a spawning event prior to the sampling in 2016 and 2017. Body condition was slightly better in 2017 when sampling occurred about 5 weeks later than in 2016. Fish length and mass, body condition, sex ratio, and gonad condition were consistent with previous observations from the northern Ross Sea region in CCAMLR Subareas 88.1 and 88.2 (Fenaughty 2006, Fenaughty et al. 2008, Parker & Marriott 2012, Stevens et al. 2016).

These results give a strong indication that Antarctic toothfish also spawn north of 60° south latitude in the Southern Ocean. Biological measurements collected from fish sampled in this SPRFMO area of study are consistent with previous information and analyses from the northern regions of CCAMLR Subarea 88.1, indicative of spawning in that region suggesting that Antarctic toothfish spawning may extend over a wider geographic area than initially hypothesised.

This paper presents further progress towards an integrated stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Amundsen Sea region, defined here as SSRUs 88.2C–H. The region is split into two areas: the North (SSRU 88.2H) comprising large mature fish, and the South (SSRUs 88.2C–G) comprising a mix of large mature fish and small immature fish.

Two-area stock assessment models were first developed for the region in 2014 and refined in 2015 and 2016. Results showed the need to collect mark-recapture data in the South to inform the estimation of biomass in the South. Simulation work undertaken in 2017 showed that if tag recaptures continued in the south, and were spread among research blocks, a model may be developed for management advice.

We present an update on these two-area stock models including data from four years of recapture data from the South. The assessment models were fit to the proportions-at-age in the catch, and the mark-recapture data from the two areas. The results suggest that data from the research plan are starting to inform the model, especially with respect to the size of the population in the south and migration rates between areas. Biomass estimates are in agreement with those calculated using local Chapman and CPUE by seabed area analogy methods. At this stage the model should only be used as indicative due to issues including poor fit to the age data in the south, the lack of year-specific age frequency data to inform these fits, and the limited spatial extent of the tag recaptures in the South (almost all recaptures in research block 882_2).

We recommend that the analyses be developed to incorporate the spatial distribution of the tagged fish and subsequent fishing effort, as well as further investigations of the distribution of fish in relation to their length. We also recommend that all Members contribute validated age data from otoliths collected in Subarea 88.2 to further inform the understanding of the stock, and that the research data from vessels that have lower effective tag survival and detection be significantly improved to ensure the best use of the available information.

The Amundsen Sea region toothfish fishery has been operating since 2003. In 2015, a research plan was developed to estimate the toothfish biomass the area, particularly in the South.

The biological characterisation of the fishery shows a truncation of the right-hand limb of the age distribution between 2004 and 2014. No age data are currently available after 2014. We recommend that further ageing of toothfish in the ASR be made a priority to develop annual age-length keys and age frequencies. We further recommend that catch limits in the ASR be calculated following the trend analysis rules developed by WG-FSA for research blocks (CAMLR-XXXVI 2017, Annex 7, paragraph 4.33). However, the calculations are slightly different as the effective tag release and recaptures (to account for different levels of tag survival and tag detection by individual vessels) can be applied:

• 882H: 177 t based on a one-year Chapman estimate using effective tagging survival and tag detection rates and allowing a maximum of 20% change in the catch limit from 200 t.
• Research block 882_1: 240 t based on CPUE by seabed area and allowing a maximum of 20% change in the catch limit from 200 t.
• Research block 882_2: 240 t based on a one-year Chapman estimate using effective tagging survival and tag detection rates and allowing a maximum of 20% change in the catch limit from 200 t.
• Research block 882_3: 160 t based on CPUE by seabed area and allowing a maximum of 20% change in the catch limit from 200 t.
• Research block 882_4: 214 t based on CPUE by seabed area and allowing a maximum of 20% change in the catch limit from 200 t.

We note that research blocks 882_1, 3 and 4 use CPUE by seabed area analogy and can also be calculated by CCAMLR Secretariat using the agreed method for research blocks in other areas. As 882H and research blocks 882_2 use effective tagging survival and effective tag detection rates, we have presented Chapman estimates for these research blocks.

We note that consideration of the spatial overlap of tag releases and subsequent effort to recapture tagged fish remains an issue for tag-based estimations of biomass and we note that improved analytical methods need to be developed. Individual research block catch limits (rather than an overall catch limit for all southern research blocks) should improve the ability for information to be obtained from these research blocks, especially blocks 1 and 4, where sea ice dynamics can prevent access early in the season.

This report summarises the toothfish fishery in the Ross Sea region (Subarea 88.1 and SSRUs 88.2AB) together with biological characteristics of the catch of Antarctic toothfish through the 2018 season. Despite 2018 being the first year of implementation of the Ross Sea region Marine Protected Area, most of fishing effort was carried out in the historically fished areas. There was a small amount of effort in the northern area of SSRU 88.2A, which was opened to the exploratory fishery with the introduction of the Ross Sea region MPA. A small amount of the catch historically taken from areas bounded by the SRZ and the area inside the General Protection Zone of the Ross Sea region MPA was taken in the slope area outside the MPA and south of 70°S.

The median and 90^th percentile scaled length distributions showed no trend over time in the shelf, slope or northern areas. Patterns in the age frequency of the catch showed no consistent pattern of trend through time, but showed interannual variability. This was likely driven by spatial variability in the locations of fishing effort, most likely driven by ice conditions and vessel choice of locations. The ratio of males to females showed some suggestion of a weak trend with a small increase in the proportion of males over time in all three areas. The number of fish recaptured in 2018 was similar to that recaptured in previous years, with some fish recaptured after 14 years at liberty.

Total numbers of Antarctic toothfish collected in the CCAMLR Conservation Zone were 600 fish which consist of 306 fish from the 58 area and 294 fish from the 88 area during February through March, 2016. Female ratios were found to be 0.49 in both areas, showing higher female occurrence with higher total length. The ratio of female among the 857 specimens of Antarctic toothfish collected from the 58.4.1 area and 88.3 area within the CCAMLR Conservation Zone during December, 2016 to March, 2017 was found to be 0.53, and displayed the tendency of increase with increase in the total length 140 cm. The type of ovarian development of Antarctic toothfish was group-synchronous type. The results in 2016 are as follows. The average gonadosomatic index of the fish was found to be 2.16, with 2.57 for female and 1.77 for male. Frequency of ovarian development stages of Antarctic toothfish showed the highest proportion with 41.9% at the mature stage, and followed by the growing stage with 40.6% and ripe stage with 17.5%. The group maturity in accordance with the total length (TL) categories of Antarctic toothfish displayed 50% for female in larger than TL 120.1 cm categories and almost 100% for male in larger than TL 180 cm categories. And the results in 2017 are as follows. The average gonadosomatic index of the fish was found to be 3.88, with 4.18 for female and 3.53 for male. Frequency of ovarian development stages of Antarctic toothfish showed the highest proportion with 62.7% at the mature stage, and followed by the ripe stage with 19.9% and growing stage with 17.5%. The group maturity in accordance with the total length (TL) categories of Antarctic toothfish displayed 50% for female in larger than TL 130.1 cm categories and almost 100% for male in larger than TL 170 cm categories. The main spawning season is estimated after May.