CCAMLR

We present developments towards spatially explicit age-structured population dynamics operating models for Antarctic toothfish in the Ross Sea. The operating models consider both a coarse-scale and medium-scale spatial resolution and consider scenarios where abundance can be present over the entire Ross Sea region or restricted to areas where the fishery has operated.

The models are implemented in the generalised Bayesian population dynamics model, the Spatial Population Model (SPM). The SPM program allows implementation of an aggregate movement model for use with large numbers of areas as a discrete time-step state-space model that represents a cohort-based population age structure in a spatially explicit manner. Models can be parameterised by both population processes (i.e., ageing, recruitment, and mortality), as well as movement processes defined as the product of a set of preference functions that are based on known attributes of spatial location.

The operating models considered were single sex age-structured models that categorised fish as immature, mature, pre-spawning, spawning, or post-spawning. Observations include spatially explicit commercial catch proportions-at-age, proportions mature and proportions spawning (based on GSI data), CPUE, and tag-release and tag-recapture observations.

Estimates of parameters when the operating models were used as estimation models with observations from the Ross Sea Antarctic toothfish fishery appeared to broadly reflect the hypothesised spatial distribution of Antarctic toothfish, suggesting that younger fish were found predominantly in the southern shelf areas, mature fish on the slope and spawning fish in the northern areas of the Ross Sea region. Fits to the commercial catch proportions-at-age observations were generally good in most models, although fits to the plus group of the proportions-at-age catch data were less than ideal. Model estimates of proportions-mature appeared to be sensible, with a clear pattern that the proportions mature were a function of location and age. Tag release and recapture data were less well fitted by the models due, in part, to the conflict with assumptions of known abundance in the model and the abundance information inherent in the tag-recapture observations. Whilst these models are an improvement on earlier versions, further work is required to improve these residual patterns and to better match the observations.

An age determination manual for Antarctic toothfish (Dissostichus mawsoni) was produced for CCAMLR in 2010. The manual has been updated to include the development and use of a reference set to provide training and refresher material for agers, and to formally track reader performance and reader drift. In addition, standardized analysis and data reporting are recommended to allow the evaluation of D. mawsoni aging data for use in stock assessment.

The exploratory fishery for Dissostichus spp. has now been operating for 16 years in Subarea 88.1 and for 10 years in Subarea 88.2. This report summarises the timing, depth, and location of fishing together with the catch of Dissostichus spp. and bycatch species by year for the period 1997–98 to 2011–12. During the 2011–12 fishing year most of the catch in Subarea 88.1 came from the slope SSRU 88.1K. About 70% of the catch in the north was taken from 88.1C, and about 85% of the catch on the shelf was taken from 88.1J. As in the past, most of the catch in Subarea 88.2 was taken from SSRU 88.2H in the north.

Unstandardised Antarctic toothfish CPUE (catch per hook and catch per set) in the Ross Sea and Subarea 88.2 fisheries have fluctuated over the past 10 years with no overall upward or downward trend. There is no evidence of any truncation of the overall length frequency distribution in any of the SSRUs, but there has been a marked reduction in median fish length in SSRUs 88.1H and 88.1I over the last 2–3 years. This appears to be at least partly a result of vessels carrying out more fishing in shallower parts of the slope, but could also reflect fishing on different parts of the slope, or a pulse of strong year classes. We conclude that there is no evidence for substantial changes in population structure or abundance of Antarctic toothfish at the regional (Subarea) or local (SSRU) level.

We also present a characterisation of the main six bycatch groups including macrourids, skates, icefish, eel cods, morid cods and rock cods & ice cods for the first time. For each bycatch group the main species are identified and the location and depth distribution of catches and catch rates rates are plotted.

At its 2011 meeting, the Scientific Committee agreed that a time series of relative recruitments from a well-designed survey could be a useful input into the Ross Sea stock assessment model and endorsed a proposal to carry out this work once the fishery had closed at the end of the 2011/12 season. The survey had two main objectives: (i) To establish the feasibility of developing a time series of longline surveys to monitor pre-recruit (<100 cm TL) toothfish in the south of SSRUs 881.J and 881.L in the southern Ross Sea using standardised gear in a standardised manner; and (ii) To carry out experimental depth-stratified fishing in 400–600 m depth adjacent to the survey boundaries to establish the most appropriate depth strata for future annual surveys.

The survey demonstrated it is possible to carry out a longline survey of pre-recruit toothfish from a commercial fishing vessel in Ross Sea using standardised gear in a standardised manner. Based on a total of 45 sets in the three core strata (A12-C12) the survey biomass estimate had an overall c.v. of 9%, which met the target c.v. of 10%. The survey caught mainly 70–110 cm TL, 5–10 year old, Antarctic toothfish. It provided new data on the depth distribution of pre-recruit fish in this area. Catch rates of pre-recruit toothfish were highest between 500 and 900 m and very low shallower than 450 m. The size distribution of toothfish was very similar between the four main strata suggesting no depth preference between 400 and 900 m. It is recommended that future surveys in this area should focus on depths of 500–900 m. The survey also demonstrated the feasibility of collecting samples for wider ecosystem monitoring.

Although the main aim of the 2012 survey was to monitor pre-recruit toothfish, the survey provided the opportunity to compare survey catch rates in 2012 with historical commercial catch rates by San Aotea II and its sister ship Janas in 1999, 2001, and 2004. The results of this analysis suggest that there has been no significant change in catch rates in the southern Ross Sea over the past decade.

The longline fisheries for Antarctic toothfish (Dissostichus mawsoni) in subareas 88.1 and 88.2 have provided more than a decade of observations to aid in understanding the life history and ecology of toothfish. Updated spawning ogives for males and females support the currently used values. Using histological samples, a summer GSI threshold of 1% appears to be a good indicator of vitellogenic fish developing to spawn in the upcoming season. Evidence is accumulating that almost all Antarctic toothfish in the northern SSRUs have spawned in the previous season and are preparing to spawn in the upcoming season. Observations of post-spawning fish on the slope indicate that some females either spawn on the slope or return to the slope in early spring. The absence of high condition fish in the north indicate that any northward migration occurs in late autumn or winter. Determination of fish movement during the winter period requires winter sample collection from the northern area.

In the Division 58.4.1 there are two stocks; one extends from the SSRU 58.4.1C to the SSRU 58.4.2A, and the other one to the SSRU 58.4.1H. The population sizes were vulnerable with a big range of about 1000-2000 t per SSRU in 58.4.1 and 100-1500 t per SSRU in 58.4.2. During the 2003-2007 fishing seasons, 3,434 toothfishes were tagged and released, but only 5 fishes among them have been recaptured which were not enough data to estimate stock biomass for considering the precautionary catch limit. The foodweb of toothfish stared to be studied recently only in the southern Ross Sea. Base on CCAMLR’s fishery report on 58.4.1 area as a whole is still data-poor fishery and is showing low recapture rate mentioned above. Consequently, providing catch and effort data, analyzing biological samples, and collecting recapture data focusing on SSRUs C, E, and G in Division 58.4.1 is very important to consider proper way for effective utilities of the population and ocean. Therefore, Korea notifies the participation in exploratory fishery for Dissostichus spp. in the SSRUs C, E, and G in Division 58.4.1 from 2012/2013 season to 2014/2015 using Korean commercial bottom longline vessel, NO.3 INSUNG in accordance with paragraph of CM 24-01 and paragraph 6(iii) of CM 21-02. The main objective of this scientific research plan is to assess the stock status of Dissostichus spp. in SSRUs C, E, and G in Division 58.4.1. We will collect the catch and effort, tagged and released, and recaptured dada and biological data to estimate biomass for Dissostichus spp. and assess the ecosystem-based fisheries. For the 1st year of this research, we will collect and analyze catch, effort, and biological data such as length, weight, gonadial development, otolith and muscle. During the 2nd year, we will collect the same data as the 1st year continuously. We will also submit the primarily results on the comparing catch rate between trot line and Spanish line, and estimations of biological parameters based on the collected data from the 1st research. For the 3rd year research, we will collect the same data consequently, and we will submit the results on the comparing results among the estimated biomass for Dissostichus spp. using three different analysis and ecosystem-based risk assessment.

We analysed the available data from vessel and scientific observer logbooks from the exploratory fishery for Antarctic toothfish (Dissostichus mawsoni) and Patagonian toothfish (D. eleginoides) in CCAMLR subarea 48.6 (South East Atlantic). This report summarises the catch, effort, timing, depth, location, size structure and maturity of Dissostichus spp.  Information on the composition of  bycatch species  caught in the fishery over the period 2003-4 to 2011-12 are also presented..  For the Antarctic toothfish, accumulated catch across years yielded 1,353 tons with the majority of the catch coming from SSRUs 48.6G and 48.6E. For the Patagonian toothfish the accumulated catch was 349 tons and almost all was reported exclusively in the SSRUs 48.6A and 48.6G. Most of the catches were reported by vessels from Japan (58%) and Rep. of Korea (31%) and to a lesser extent, from South Africa (7%) and Norway (4%).

Unstandardized raw catch and effort data suggested that the median catch per unit of effort in Antarctic toothfish has generally increased over the course of the fishery whilst it has decreased over time for Patagonian toothfish. In both species average length of catches is larger in females. Antarctic toothfish is caught at deeper waters and have a larger mean length than Patagonian toothfish. There is not evidence of truncation in the overall length frequency distribution of both species, although some evidence of reduction in the mean fish length has been observed in the last three fishing seasons. In both sexes, high frequencies of maturity stages 2 and 3 were observed in Antarctic toothfish whereas for Patagonian toothfish immature (stage 1) fish were  most frequently observed. The bycatch for both toothfish species is composed of few species. In term of biomass, the most important bycatch species are grenadiers and blue antimora. A total of 3,828 individuals of toothfish have been tagged in this area but only 19 have been recaptured. We concluded that from the data examined from the fishery to date there is not evidence for substantial changes in the population structure of both species in this subarea.

A reduced (three day) groundfish survey took place at Shag Rocks on the FV New Polar between the 26-28th January 2012. This survey was not intended to be a full biomass survey so only 20 random hauls were completed around Shag Rocks with 3 additional opportunistic sites at South Georgia. Reasonable catches of Patagonian toothfish were found around Shag Rocks (greater than in 2009 and 2010), with three size cohorts of fish identified including evidence of a new cohort of 2+ age fish (29-39cm). Catch-weighted length frequencies of mackerel icefish indicated that 2+ and 3+ sized fish dominated at Shag Rocks and Euphausiidae was the dominant prey item in the mackerel icefish diet. Strong acoustic marks, some of which may be attributable to icefish, were observed in the water column, and in only 23 hauls 15.2 tonnes of mackerel icefish were caught and following the survey mackerel icefish were caught commercially.

This paper presents an update of the assessment of toothfish (Dissostichus spp.) in Subarea 48.4. The Subarea is divided into two management regions and assessed using a CASAL assessment of D. eleginoides in the northern region and a simple Petersen biomass estimate derived for D. eleginoides and D. mawsoni in the southern region. The CASAL model structure used to assess the stock in 2011 has been retained although a small number of changes have been made to try to improve the fit of the model and to reduce the dependency on a relatively small amount of data. These changes comprise the addition of catch at age information for 2011, a minor change to the initial parameters of the selection pattern and the inclusion of additional information on size at age. CASAL estimates of stock size, harvest rate and recruitment are similar to those of last year’s assessment. B₀ is estimated at around 1,690 tonnes and B₂₀₁₂/B₀ at around 88%. Sustainable yield, calculated in accordance with the CCAMLR decision rules, is 63 tonnes for D.eleginoides in the northern region whilst sustainable yield calculated from the Petersen biomass estimates of Dissostichus spp. in the southern region and application of the gamma value (0.038) determined for toothfish in Subarea 48.3 was 52 tonnes. Options for the future development of the assessment of toothfish in Subarea 48.4 are considered.