Natural mortality (M) is a key parameter in fisheries stock assessments, relating directly to the productivity of a stock, stock status, and estimated precautionary yields. In the Ross Sea region stock assessment for Antarctic toothfish (Dissostichus mawsoni), M is assumed to be 0.13 y^-1 and constant across age and sex.

In 2018, the independent review of the Ross Sea region D. mawsoni stock assessment recommended that CCAMLR consider the robustness and reliability of estimating natural mortality in the stock assessment model. We undertake a preliminary investigation of estimating natural mortality using the assessment model from the Ross Sea region from 2017.

We estimated natural mortality as a single value for males and females combined, with a uniform prior within the base model used in 2017 for Antarctic toothfish in the Ross Sea region. The model MCMCs estimated M=0.11 y^-1 (0.09–0.13 y^-1) with B0 estimated to be 92 140 t (70 330–121 950 t); a higher B0 72 620 t (65 040–81 050 t) when compared to the 2017 base case where M=0.13 y^-1. The precision of the estimate of B₀ when estimating natural mortality was lower and estimated the precautionary yield higher (3419 t compared with 3258 t). MPD simulations suggested that when the rate of natural mortality was assumed in an operating model, it was estimated with relatively high precision and with no bias.

Further investigations will need to be carried out to consider the effect on model fits and residuals and if robust estimates of the rate of natural mortality can be achieved using MCMC simulations. We recommend that analyses exploring the effect of the model-based estimate of M, including values of natural mortality that vary by age and sex, should be considered in future stock assessment sensitivity analyses

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. The annual survey has been conducted since 2012, with a revised proposal presented in 2017 for five years (2018–2022). The objectives of the survey included monitoring the abundance and age structure of sub-adult (≤ 110 cm TL) toothfish in the south of SSRUs 881.J and 881.L in the southern Ross Sea (Strata A–C) using standardised gear in a standardised manner; and monitoring trends in larger (large sub-adult and adult) toothfish in two areas (both situated in SSRU 881.M) of importance to mammalian toothfish predators: McMurdo Sound in 2018, and Terra Nova Bay surveyed in 2019. The estimated relative biomass index of toothfish showed an increase to the highest in the survey time series. Among the other survey objectives, the survey also released Popoff Satellite Archival Transmitters (PSATs) on toothfish throughout the survey area and tested a remote underwater camera to document benthic habitats where fishing occurs. Notification of research for the 2020 fixed-effort survey is included with the agreed catch limit of 45 t.

A research plan for Subarea 88.3 was commenced by Korea in 2016 and New Zealand joined the proposal in 2017. The joint research proposal was for 3 years covering the period 2017/18, 2018/19 and 2019/20. It was designed to build on Korea’s previous research by continuing to focus on research blocks where tagged fish have previously been released on the slope, while also prospecting two of the northern seamount complexes and two boxes on the continental shelf, where no research has occurred. Following a proposal from the Ukraine to join the research plan in 2018, the Scientific Committee requested an integrated proposal be developed for all three Members (SC XXXVII paragraph 3.191).

This proposal updates the final year of the research plan with the recommendation from the Scientific Committee to integrate the Ukrainian research with the Korean and New Zealand research plan. The objective of this integrated proposal is unchanged, i.e., to determine the abundance and distribution of Antarctic toothfish in Subarea 88.3. Secondary objectives are to improve understanding of stock structure of toothfish in Statistical Area 88, to carry out calibration trials among the vessels, to collect data on the spatial and depth distributions of bycatch species, and to trial scientific electronic monitoring technologies.

This coordinated proposal includes a catch distribution plan for the 2019/20 season, recognizing a full review and new proposal will be required for the 2020/21 fishing season.

Fishing activities targeting Patagonian toothfish (Dissostichus eleginoides) occurs in Division 58.4.4 and killer whales have been observed to interact with the vessels until 2014. Interaction rates between areas and seasons vary greatly (from 0 to 70%). Depredation rates were updated for subdivisions 58.4.4bB (16%), 58.4.4bC (1.5%) and 58.4.4bD (0%). Photo-identification effort has been conducted in 2012 and 2014, analyses show that 13 individuals have been interacting with the vessel. The three individuals observed in 2014 were observed again in 2018 on the west part of Crozet’s EEZ along with two other individuals including a young calf. This document provides an update of the interaction and depredation rates and the first insight into individual’s identifications for Division 58.4.4.