CCAMLR

WG-FSA-19 recommended an update on stock parameters, including recruitment indices from the trawl survey, and age-frequency data and tag-recapture data from the fishery be presented to WG-FSA-20 to evaluate whether recruitment and the stock trajectory were consistent with those estimated by the stock assessments for Patagonian toothfish (Dissostichus eleginoides) fishery in CCAMLR Division 58.5.2.

Here, we present an update on stock parameters, including recruitment indices from the trawl survey, and age-frequency data and tag-recapture data from the fishery. These data indicate that the stock trajectory is consistent with that predicted by the 2019 stock assessment model. Increases in the survey biomass and young fish in the survey catch composition also indicate the potential for a recruitment pulse in recent years.

During late March to mid-April 2020, the annual random stratified trawl survey (RSTS) around Heard Island and McDonald Islands (HIMI) was conducted in CCAMLR Division 58.5.2, with the completion of 151 stations. The survey was conducted on the FV Atlas Cove. Sampling protocols such as the design and the duration of the hauls were similar to recent surveys, but with a new set of randomly selected station points. However, only 18 of the 30 stations allocated in Plateau Deep East could be sampled due to damage to the trawl warps which prevented fishing on deeper stations.

The catch of Patagonian toothfish (Dissostichus eleginoides) () was 86.3 t which is the highest catch since the RSTS began and more than three times the average since 2015.  The catch of mackerel icefish (Champsocephalus gunnari) was 7.3 t which is about average.

The estimated biomass of the target species D. eleginoides and C. gunnari in the 2020 survey were the highest for the past 10 years. Biomass estimates for the managed by-catch species unicorn icefish (Champsocephalus rhinoceratus) and Macrourus spp. were also at their highest levels of the past 10 years and the estimate for grey rockcod (Lepidonotothen squamifrons) showed the first substantial increase since 2014. Amongst the three species of skate, biomass estimates show an upward trend over the last few years with of Bathyraja murrayi also being at the highest levels for the past 10 years.

Length and weight measurements were taken for more than 17,000 fish and for more than half of those, other biological measurements were also recorded. Otoliths were collected from 763 D. eleginoides and a number of other species, and 645 toothfish were tagged and released.

The research plan on Antarctic toothfish in Subarea 88.3 has been underway for four consecutive years. Two vessels have participated in tagging and releasing toothfish from 8 discrete areas. Subsequent fishing effort has recaptured some fish, which can now be used to transition the research to a “biomass estimation” phase based on catch limits for each research block following CCAMLR’s trend analysis rules. A total of 2309 tagged fish have been released and 11 recaptured, all in research block 883_4. As proposed in the research plan milestones, we applied the observed recapture rates to the projected catch limits to set expectations of future recaptures using the trend analysis rules to evaluate the likelihood of achieving enough recaptures for stock assessment purposes. We estimate that, at least for research block 883_4, obtaining recaptures of more than 6 tags per year is feasible at the current tagging rate of 5 fish per ton of catch, but only if vessel performance is maintained and within-season recaptures do not occur.

We review the stock structure of Antarctic toothfish Dissostichus mawsoni in Statistical Area 88 with the aim of characterising the likely stock boundaries relative to fishery management. We include information from studies examining genetics, otolith microchemistry, stable isotopes, tagging, size and age structures, growth dynamics, and particle dispersal simulations. A clear progression in the size and age of Antarctic toothfish from juveniles on the continental shelf to adolescents on the continental slope to adults on the Pacific Antarctic Ridge demonstrates the ontogenetic progression within the Ross Sea following the local bottom water currents in each area. Similar characteristics are observed in the Amundsen Sea and the Bellingshausen Sea but with some clear differences that suggest a high level of mixing of juveniles in the eastern Amundsen Sea and Bellingshausen Sea. The smallest fish (~60 cm) are not observed in the Ross Sea but are observed mainly near 1200 m in both the Amundsen and Bellingshausen Seas. Interestingly, 75–110 cm fish are not frequently observed in the Amundsen or Bellingshausen Seas. With few seamounts in the Bellingshausen Sea, adult fish are well represented on both the continental shelf and slope. Dispersal simulations from locations of adults suggest that Ross Sea juveniles recruit to the Amundsen continental slope, while Amundsen and Bellingshausen Sea juveniles are typically distributed to the continental slope of the Bellingshausen Sea. These patterns of pelagic advection during the first two years in the plankton followed by a slow, down-current ontogenetic migrations returning adults to deep seamount habitats over a 10–15 year period suggesting a metapopulation structure with largely resident regional adult populations with highly variable amounts of juvenile mixing.

This paper updates the research and monitoring activities for New Zealand relevant to the Ross Sea region Marine Protected Area, as encouraged by Conservation Measure 91-05 paragraph 16(i)–(ii).

In 2016 the Commission for the Conservation of Antarctic Marine Living Resources adopted Conservation Measure 91-05, establishing the Ross Sea region Marine Protected Area. Annex B of this Conservation Measure specifies the specific objectives and the Management Plan for this Marine Protected Area. In addition, Annex C specifies the Priority elements for Scientific Research and Monitoring, including research and monitoring priorities and research and monitoring questions that should be addressed. We also recall the objectives of the RSRMPA and the research questions for research and monitoring for the RSRMPA.

A programme was implemented in the 2019/20 fishing season in the Ross Sea region to tag and release skates for population size estimation and to validate the thorn ageing method for Antarctic starry skate (Amblyraja georgiana). Skate tagging is required under CM 41-01 in the 2020/21 fishing season as well. Skates that were tagged and released in the 2019/20 season on NZL, AUS, and GBR vessels were also injected with either oxytetracycline hydrochloride or strontium chloride in order to validate age marks incorporated into the caudal thorns. In 2020/21 (and future years), skates recaptured on all vessels in the Ross Sea region should be biologically sampled and a selection of caudal thorns removed and frozen, and returned to NIWA, Wellington for subsequent laboratory analysis.

Biology, ecology, and management of macrourids in the Ross Sea was last summarised for CCAMLR in 2010. Since then, the discovery of a previously cryptic species (Macrourus caml), two additional trawl surveys, and additional work on species-specific biological productivity has been progressed to better inform management options. Due to the COVID-19 pandemic causing impacts on the time available to progress the development of management advice, this paper describes the overall work plan and the topics of associated papers that are planned to be submitted to the CCAMLR Scientific Committee’s working groups. These topics include biomass estimates of macrourids on the Ross Sea slope, species identification, distribution and abundance, life history, interactions with the fishery, mechanisms to monitor macrourid status, and options for precautionary catch limits.

The second winter survey for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region was conducted by the FV Janas during September and October 2019. The survey followed the northern extent of sea ice from SSRU 882B through 881B. Antarctic toothfish sampled during this survey were in spent condition by the beginning of September, gonadosomatic indices were less than 5%, suggesting spawning had ended by mid-August. Eggs collected from the plankton were at the eyed-embryo stage, comparable with day 15 of D. eleginoides development. Buoyancy was similar to newly fertilised eggs indicating a near-surface distribution. Four D. mawsoni were tagged and released with Pop-off Satellite Archival Tags provided by NOAA to examine movement patterns of adults in the region. Four adult D. mawsoni released with conventional tags during commercial fishing on the Ross Sea slope were recaptured more than 2300 km away on a seamount in SSRU 882B. Species composition and density were correlated with oceanographic variables and structured by location with respect to the Pacific Antarctic ridge.

The time series of relative abundance and age structure from the Ross Sea shelf survey is an important input to the Ross Sea stock assessment, providing information about year class strength and variability. In 2020, the estimated relative biomass index of toothfish showed an increase, the highest recorded in the survey time series. Biological data and samples were collected from more than 3,000 toothfish and bycatch species. A total of 232 toothfish were tagged and released with a 91% tagging length overlap statistic. Although the catch limit of 45 t was not exceeded, the experimental stratum N could not be sampled. Otoliths collected will be aged for the upcoming stock assessment in 2021.

The 2019 stock assessment for Antarctic toothfish in the Ross Sea region estimated the precautionary yield using the CCAMLR decision rules and by  applying the total catch limit of 3140 t between north of 70° S (19%), south of 70° S (66%), and the Special Research Zone (15%)  in accordance with conservation measure 91-05 (2016) and 41-09 (2019). That stock assessment excluded data for vessels that had been previously quarantined by the Scientific Committee of CCAMLR due to data accuracy concerns. However, after the stock assessment had been completed, the Scientific Committee recommended that data from three additional vessels (Calipso, Koreiz and Simeiz 2015-2018) be quarantined pending an assessment of the implication of catch revisions from those vessels on the management advice.

Based on an analysis by Ukraine in 2020 the total DCD reported greenweight was 203 t greater than the reported C2 greenweight (12%) across Subareas 88.1, 88.2, and 48.2 from 2014–2018 for the three vessels combined. In the Ross Sea region, the difference totalled 78 t in 2014–2018 with the greatest difference occurring in 2018 (52 t).

A sensitivity model was run using the 2019 stock assessment incorporating the DCD estimated greenweight from the three vessels to revise the catch history in the assessment model. Both the MPD and MCMC estimates of initial biomass (B₀) and current biomass for models with revised catch for the three vessels in trips between 2014 and 2018 were almost identical to the 2019 base case assessment model. Therefore, the differences for these trips resulted in a negligible impact on the estimated precautionary yield.  We recommend that the Scientific Committee does not need to revise its 2019 advice for the toothfish catch limit for the 2020/21 season for the Ross Sea region.