CCAMLR

This paper presents the data and assessment approach for the revised stock assessment for Patagonian toothfish, Dissostichus eleginoides, at Heard Island and the McDonald Islands (HIMI) in CCAMLR Division 58.5.2. Since the last assessment in 2013, an extensive program for ageing otoliths has been launched, length-at-age has been re-estimated, and research into tag-recapture data has started. We have therefore made substantive progress in addressing the main points raised during WG-FSA 2013 for the next stock assessment.

We propose a process by which to select the version of software generally used for stock assessments in CCAMLR, with the specific example of the CASAL software used for stock assessment. The latest published version of any stock assessment software approved by CCAMLR should be used by default, unless the latest development version is necessary, in which case it is the responsibility of the author to demonstrate that the latest development version performs as expected. As an example, the latest published version of CASAL should be used by default for stock assessments unless particular features available only in a more recent development version are necessary. If a different version is used, then the scientist proposing the stock assessment is responsible for ensuring the version used is performing as expected.

We present an updated spatially explicit age-structured population dynamics operating model for Antarctic toothfish in the Ross Sea region, for a medium scale spatial resolution (189 spatial cells) covering the Ross Sea region. We build on the previous working medium-scale model in which the population was restricted to those cells in which at least 5% of cell area was of a depth deemed suitable as habitat for toothfish (120 cells – semi-restricted model).

Model fits to the reproductive data were unsatisfactory in previous models, with a much flatter maturity ogive than that derived through histology. In the previous model reproductive parameters (proportion maturing and proportion spawning) were derived from two gonadosomatic index thresholds. In this model we keep only proportion spawning data, and retain the assumption that a proportion of the mature fish spawn each year. The model estimation of the maturity ogive is now similar to that estimated by histological analysis. This model represents our current best model of the spatial distribution and movement of Antarctic toothfish in the Ross Sea region given the movement hypothesis proposed.

Further data collection would be useful to improve the parameterisation of the model, in particular obtaining gonad weight measurements from all fished areas, biological characterisation of fish on the spawning grounds during winter, and characterising the fish population in areas not fished to date.

We propose that the modelling platform presented here can be used to test the likelihood and examine the consequences of alternative fish movement hypotheses given the data available. For a given movement scenario the model can also be used to examine the likely effects on the toothfish stock assessment of alternate spatial management options or alternate tagging programme designs affecting the distribution of fishing effort, data collection, and/or tagged fish releases.

Vessel-specific indices of tag detection rates and tagging mortality rates have been developed using a methodology which controls for the spatial and temporal variability of fishing operations by pairing each individual tag release or recapture event with all other fishing events which occurred in the same time and place (i.e. within a specific distance and in the same fishing season). The method showed that when the confounding effects of variable fishing location and time were controlled, there were nonetheless significant differences between vessels with respect to the tag detection index in the Ross Sea region. This index was subsequently used in 2013 to select the tagging data to be included in the stock assessment for the Ross Sea region and Amundsen Sea region, with the understanding that additional work should be undertaken to further develop this method.

Previous applications of the pairwise method utilised an index of tag detection with an arbitrary threshold to determine which tag data to include in the stock assessment, with respect to both tag release and tag recapture data. Here we present new results of simulations which indicate that the index of tagging mortality rate previously generated using this method is as useful as the index of tag detection rate. We further develop a method to estimate effective tag releases and effective numbers of fish inspected for tags based on those indices. This method allows the use in the stock assessment of tag data from all vessels, via direct application of the continuous indices, i.e. without reliance on arbitrary vessel selection thresholds. This method accounts for differences in tagging mortality and tag detection rates among vessels, and therefore is less likely to over-estimate the stock biomass than if tag data were used indiscriminately.

We recommend that the method proposed be used in the Ross Sea region stock assessment and in the Amundsen Sea region stock assessment.

This paper presents further investigations of the Bayesian sex and age structured population stock assessment of Antarctic toothfish (Dissostichus mawsoni) in Subarea 88.2 covering SSRUs 88.2C, D, E, F, G, and H (SSRUs 88.2C–H). Annual age-length keys were applied, and the time trends in the tag-derived biomass index were investigated. The impact of hypothesised stock structure, fish movement, tag mixing and the potential for localised depletion on the stock assessment were also investigated.

We recommend the following:

• That the Amundsen Sea region be considered as a single stock, with the stock assessment including all data from SSRUs 88.2C-H and the catch limit applied to the same region.
• That R3 be chosen as the base case model for providing management advice for the 2014/15 fishing year once it has been updated with the 2014 fishery data.
• That ways to obtain a better estimate of the biomass in SSRUs 88.2C-G are considered, in particular by improving the spatial overlap between tags released and subsequent fishing effort.
• That the recent high exploitation rates and localised depletion in SSRU 88.2H is recognised by allocating a greater proportion of the catch limit to SSRUs 88.2C–G and less to SSRU 88.2H.
• That all Members, where possible, contribute to the development of annual age-length keys, using approved otolith reading protocols and aging validation.

This paper frames a discussion for improving the assessment of toothfish abundance for SSRUs 88.2C–G. We initially provide a characterisation of the fishery and a summary of available tagging and length frequency data. Although more than 700 tagged fish have been released in this region, only 2 fish have been recaptured. It is likely that the lack of tag recaptures in this region has been caused primarily by the poor spatial overlap of released tagged fish with subsequent fishing effort.

By drawing on the success of tagging programmes in other CCAMLR fisheries, we develop an approach for improving the spatial overlap of the location of tagged fish and subsequent fishing effort. We identify four main grounds previously fished in SSRUs 88.2C–G. An upper catch limit could be set for each of these grounds based on historic CPUE and a maximum local exploitation rate. It should be noted that the catch limit for SSRUs 88.2C–G will be constrained by the total catch limit for the wider Amundsen Sea stock (SSRUs 88.2C–H) and a subjective decision regarding how it will be split between the 88.2H and 88.2C–G fisheries. We then explore mechanisms for encouraging vessels to complete some or all of their sets within these previously fished grounds. Noting the relatively small number of tags currently available for recapture in this region we further recommend that the tagging rate in this area be increased to 3 tags per tonne in the short term to obtain more precise estimates of abundance and improved data indicative of stock structure.

Catch, effort, and mark-recapture data for Antarctic toothfish from individual seamount features in SSRU 88.2H were analysed to compare multiple raw indicators of biomass trends in isolated local areas. These metrics were also calculated for SSRU 88.2H as a whole. The analyses indicate that:

• Fish seldom move among seamounts within the complex, and residence time on particular seamounts is typically 1-2 years before fish depart from these seamounts, presumably back to the slope SSRUs. Therefore, it is recommended that the stock assessment include only recaptures of tagged fish that have been at liberty for less than 2 years, and that data from the Amundsen Sea slope SSRUs (88.2C–G) are included in the stock assessment along with 88.2H.
• Fishing has occurred on almost every seamount in every year, and no evidence for serial depletion or spatial patterns that would produce a current bias in recapture probability among seamounts was found.
• Single-season tag recapture rates exceeding 20% on some seamounts suggest relatively high local exploitation rates and the potential for localized depletion on some seamounts.
• Results from opportunistic post-hoc Leslie depletion analyses conflicted with other biomass indicators and suggested that opportunistic CPUE-derived abundance measures derived from Olympic fishery catch data are unlikely to index abundance at small temporal and spatial scales.
• Although indicative biomass as estimated by the CPUE by seabed analogy method produces values that vary markedly depending on the choice of reference area, the method produced lower biomass estimates than mark-recapture estimates when based on adequate sample sizes for this seamount fishery.

We review the stock structure of Antarctic toothfish in Statistical Area 88 with the aim of characterising the likely stock boundaries relative to stock assessments. We include information from genetic studies, otolith microchemistry, stable isotopes, tagging, size and age structure, growth dynamics, and egg and larval dispersal simulations. Taken together, the information suggests two biological stocks with limited mixing, especially at adult stages: a Ross Sea Region stock and an Amundsen Sea Region stock. The Amundsen Sea stock likely includes juveniles and adults along the continental margin in the Bellingshausen Sea (SSRUs 88.3A–D), SSRUs 88.2C–G, and a spawning region in the seamount complex of SSRU 88.2H. Key information needs to further understand stock structure include characterising the movement patterns and linkages of Antarctic toothfish in the 80–120 cm size range, throughout the continental margin SSRUs of the Amundsen Sea (i.e. SSRUs 88.2C–G). We recommend that the Ross Sea Region stock assessment boundaries remain as Subarea 88.1 plus SSRUs 88.2A–B and that the Amundsen Sea Region boundaries include SSRUs 88.2C–H.  Further research to better elucidate the proper stock affiliation of adjacent portions of the Bellingshausen Sea (SSRUs 88.3A-D) and juveniles in SSRUs 88.2C–G is a priority.

A research proposal to continue the southern Ross Sea subadult toothfish survey is presented. The survey will focus on estimating relative abundance in the core strata (A, B, C) in SSRUs 88.1 J and 88.1L. It will also carry out 15 exploratory sets in a newly defined stratum K in Terra Nova Bay. Survey timing and methods will remain the same as in previous years, and the number of sets will be retained at a total of 60 sets. A nominal catch limit of 68 tonnes, to be taken from the shelf catch limit for the 2014/15 fishing year is requested for the survey.

At its 2011 meeting, the Scientific Committee agreed that a time series of relative abundance from a well-designed survey could be a useful input into the Ross Sea stock assessment model. The first survey was completed in February 2012 (), and the second survey was completed in February 2013 (Parker et al. 2013). In this paper we provide preliminary results of the third survey in the time series.

The objectives of this third survey were:
(i)    To carry out a longline survey to monitor subadult toothfish in the southern Ross Sea (strata A–C) using standardised gear in a standardised manner; and
(ii)    To sample additional experimental stations in an adjacent area to identify areas of high subadult abundance which could be included as strata in future annual surveys.

The survey was successful in completing all the planned stations. The catch composition was similar to the previous surveys, although raw catch rates were down in strata B and C. Standardised catch rates for the core strata were similar to 2013 and down on 2012, although not significantly. Experimental stratum I showed larger fish than in the other strata, in particular close to Ross Island in the south and in the north-eastern side of that stratum.

Further results will be provided at WG-FSA-14, including age data and comparison with commercial catches in 2014.