CCAMLR

We provide extensive diagnostic plots for models R1 and R2 for Antarctic toothfish in the Ross Sea as recommended by SAM-2015 (2015). The stock assessment rationale and models as well as their main diagnostics are described in Mormede et al. (2015). The CASAL files for models R1 and R2 were provided to the CCAMLR Secretariat as a zip file.

In this paper we present the results for first model R1 and then model R2. The diagnostic plots cover input data, MPD and MCMC outputs. Key tables are given in Mormede et al (2015) including process error weightings and the MPD objective function components.

We provide an update of the Bayesian sex and age structured population stock assessment for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (Subareas 88.1 and SSRUs 88.2A–B), using catch, catch-at-age, and tag-recapture data from 1997–98 to 2014–15 and including the results from the Ross Sea shelf survey from 2012 to 2015. The model results were similar to the 2013 assessment suggesting that recent tag-recapture and proportion-at-age observations were consistent with previous observations. Sensitivity runs showed that the results were relatively insensitive to the use of logistic selectivity ogive, inclusion/exclusion of quarantined data, and restricting tag data to the most recent six years.

Model runs showed that the data from the Ross Sea shelf survey were required to reliably estimate relative year class strengths. Year class strengths were estimated from 2003 to 2009 and showed one strong year class and two weak year classes.

Overall, model fits to the data were adequate, and, as in previous assessments, the tag-release and recapture data provided the most information on stock size. Monte-Carlo Markov Chain (MCMC) diagnostics suggested no evidence of non-convergence in the key biomass parameters. The precautionary yield, using the CCAMLR decision rules and current relative catch distribution between the shelf, slope, and north areas of the Ross Sea region, was either 2855 t or 2870 t from the two reference case model runs R1 and R2.

We provide an update of the descriptive analyses of the toothfish tagging programme in Subareas 88.1 and 88.2 through the 2015 season. A total of 44 827 Antarctic toothfish have been released and 2473 recaptured, and 1191 Patagonian toothfish released and 103 recaptured since 2001. In recent years, most vessels have achieved or exceeded the required tagging rate of one toothfish per tonne of catch in the Ross Sea region.

Tag recapture data showed that most fish are recaptured only a short distance from their point of release, typically less than 100 km. However, a small proportion of tagged Antarctic toothfish have moved long distances with exchange observed between Shelf, Slope and North habitats. The 308 recaptures in the Ross Sea region in 2014 was the highest observed, which then reduced to 183 in 2015, close to the recent average. SSRU 88.2H showed a similar annual number of tag returns despite a 25% reduction in quota for the area for 2015. The research plan implemented in SSRUs 88.2C–G in 2015 resulted in 1 128 tagged fish released, 20 in-season recaptures and 2 recaptures from 2009 releases.

This report summarises the timing, depth, and location of fishing effort together with biological characteristics of the catch of Antarctic toothfish up to and including the 2015 season. In 2015, the Ross Sea slope SSRUs were significantly constrained by sea ice and catches were unevenly distributed across the three SSRUs (73% in 88.1H, 22% in 88.1K and 5% in 88.1I). As in recent years, the remaining catches came mainly from SSRUs 88.1C, 88.1J, and 88.2L. A standardised CPUE analysis showed changes in CPUE that were not consistent with biologically driven changes in abundance and are considered not to be indexing abundance.

Length frequency distributions of Antarctic toothfish in the Ross Sea fishery were stable in the North up until 2014, then showed a decrease in mode, particularly for males (to less than 150 cm TL). Distributions in the Shelf have become unimodal and less variable in the last three years. The strong mode of smaller (90–120 cm TL) toothfish present in the Slope fishery from 2010–2013 was not present in the 2014 and 2015 seasons, and the size composition in 2014 and 2015 was similar to that in the early period of the fishery – the reason for this is unclear. There has been a slight reduction in mean age in SSRU 88.2H, but the data are very uncertain due to the paucity of otolith readings and it is recommended that additional otolith readings for this area are given a high priority. There has been a marked increase in the proportion of males in the Ross Sea North fishery throughout the series but little change in sex ratio in the other areas. 

The effect of sea ice is acknowledged as a major influence on fishing operations in high latitudes but not widely understood in detail other than by fishers and specialists in Antarctic fisheries research and management. A retrospective analysis over a 15 year period up to the 2013/14 season was presented in WG-FSA-14/55 to quantify the level of impact that ice conditions have on fishing by creating an annual fishability index. We update the analysis for the 2015 season, a challenging ice year for fishing ranking overall as the third worst in the sixteen-year series. We recommend that the fishability index be incorporated into the Ross Sea fishery report.

A research proposal to continue the southern Ross Sea sub-adult toothfish survey for the next two years, 2016 and 2017, is presented. The survey will continue to focus primarily on estimating the relative abundance of sub-adult (<110 cm TL) toothfish in the core strata (A, B, C) in SSRUs 88.1J and 88.1L so as to provide a time series of recruitment of toothfish. This is a continuation of the time series of CCAMLR-sponsored research surveys of these strata carried out from 2012–2015.

Following the recommendations of WG-FSA and WG-EMM, an additional secondary survey objective has been developed to monitor larger (sub-adult and adult) toothfish in McMurdo Sound and Terra Nova Bay, where toothfish are believed to form an important part of the diet of Type C killer whales and Weddell seals. This survey objective complements ice-based ecosystem monitoring work undertaken on these three species by US, Italian and New Zealand scientists in these two areas. It is proposed to survey McMurdo Sound in 2016 and Terra Nova Bay in 2017 to match the timing and location of the sea-ice based work.

Survey timing and methods will remain the same as in previous years, and the number of sets will be retained at 45 sets for the core strata but reduced to 10 sets for the McMurdo/Terra Nova Bay strata making 55 sets in total. A nominal catch limit of 40 tonnes is requested for each survey. It is envisaged that the results of the 2016 survey, and trends in the time series, will be presented to WG-FSA for review in 2016 and that a full review be completed and presented to WG-EMM, WG-SAM and WG-FSA in 2017.

We propose to implement a random, depth-stratified vertical longline survey conducted through the annual sea ice in McMurdo Sound to monitor the age composition, diet, maturity, and local abundance of Antarctic toothfish. A pilot season was carried out in 2014, where sampling gear and methodology for a standardised hook and line survey were trialled in McMurdo Sound. Data collected will be integrated with ongoing collaborative research programmes to determine the importance of toothfish to mammalian predators in the area. Age composition, physiology, and diet objectives can be met with the current design. However, power analysis based on historical catch data suggests that the sample size (number of stations sampled per season) needed to detect a moderate change in the abundance index is likely to be higher than feasible at this stage. Therefore, techniques and additional infrastructure will be developed and tested to obtain the sample size and experimental design necessary to index abundance in the future.

A dramatic reduction in catch rates of Antarctic toothfish in McMurdo Sound, Antarctica, has led to conclusions that the commercial bottom longline fishery for toothfish in the Ross Sea has drastically altered the toothfish population with cascading effects on the McMurdo Sound ecosystem. However, results from a new monitoring programme for Antarctic toothfish and other top predators carried out in McMurdo Sound in 2014 have shown toothfish catch rate, fish size and fish age similar to those observed prior to 2002. These results suggest that either large and old fish have returned to McMurdo Sound following a temporary environmentally driven absence or that they remained locally present but were not detected in the areas sampled. These findings highlight the importance of continued standardized monitoring for detecting the potential effects of fishing on the Ross Sea ecosystem.

The first year of a two-year and multi-member research survey using standardised longline gear to sample toothfish populations was carried out in the northern areas (60°‑ 66° S) of SSRUs 88.2A–B. The sampling strategy and data collection adhered to standards set in CM 41-10 (2014). A total of 109.1 t of Antarctic toothfish and two Patagonian toothfish (0.029 t) were caught. Toothfish catch rates, size, sex ratio, and reproductive maturity status were similar to existing data from SSRU 88.1C. Age composition of the catch as predicted by the spatial population model was similar to that observed in the survey. The gonadosomatic index showed a decreasing trend from west to east across the research blocks. The percentage of fish with empty stomachs was high, but prey diversity and quantities of prey items were similar to those reported from the northern Ross Sea. Bathymetry data were collected during the survey and during transit across research blocks and have been provided to the CCAMLR Secretariat. We recommend continuing the survey with minor modifications to aid in the achievement of the objectives including specification of data collection requirements, bathymetric survey requirements, research block-specific catch limits to ensure a greater spread of effort, and a higher level of scientific oversight of survey operations to ensure optimal scientific design and data collection.

A proposal for a Joint SC-CCAMLR IWC Workshop to develop multi-species models of the Antarctic marine ecosystem was proposed at the SC-CCAMLR 2014, and a steering group for the Workshop was formed.

WG-EMM 2015 agreed on the benefit of dividing the workshop into two; the first to review data availability, and the second as a comprehensive discussion of approaches in relation to modelling and monitoring. The steering group was tasked to develop a draft terms of reference for the workshops to be tabled to the meetings SC-CAMLR 2015 and SC-IWC 2015 for both committees to consider.