CCAMLR

We provide an update of the Bayesian sex and age structured population stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (Subareas 88.1 and SSRUs 88.2A–B), using revised catch, catch-at-age, and tag-recapture data for the 2012–2013 seasons.  Several models were examined including an update of the 2011 base case (R1), a model using the revised data selection method, new maturity ogive, and new data weighting method (R2), and a model which used a logistic selectivity for the fishery in the North. We recommend model R2 be used to provide catch limits for the stock because this model reflects the best science and methodologies available.
 
Retrospective analysis and MPD profiles suggest that the tag recapture data from the last few years tend to push the model toward a higher biomass, in particular the data from the 2012 tagged fish. We expect this is due to higher catches in SSRU88.1K which has a comparably high biomass but a shorter catch history (therefore fewer tags available for recapture) than the other slope SSRUs 88.1H and I.  Fish released or recaptured in SSRU K have also consistently shown higher rates of movement between SSRUs and lower rates of recapture in the location of release, perhaps indicative of a more mobile population in this location.

In contrast the updated data selection algorithm resulted in less tag data being selected for input into the stock assessment, which tended to push the model toward lower biomass.  The updated maturity ogive a slightly positive effect, and the new data weighting had no effect on model biomass. 

Sensitivity runs showed that cryptic biomass is not an issue for this stock assessment, and that the data from the sub-adult survey series is expected to be helpful in estimating year class strengths in the future.

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 little evidence of non-convergence in the key biomass parameters, although there was some evidence of non-convergence in the annual shift parameters for the shelf fishery. MCMC estimates of initial (equilibrium) spawning stock abundance (B₀) for the 2013 reference model were estimated as 68 790 t (95% credible intervals 59 540–78 470); and current (B₂₀₁₃) biomass was estimated as 74.8% B₀ (95% C.I.s 71–78). The estimated yield, using the CCAMLR decision rules, was 3044 t.

Vessel-specific indices of tag detection rates and tagging mortality rates were developed in 2012 and further developed in 2013, 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.

Here we present the results of simulations which indicate that the index of tagging mortality rate obtained using this method is not as useful as is the index of tag detection rate. We recommend that in the absence of a better index of tagging mortality, the tag detection index be used to select data for use in the 2013 toothfish stock assessments for the Ross Sea and Subarea 88.2 fisheries, with respect to both tag releases and tag recaptures. This recommendation is in accordance with that of most of the participants at WG-SAM 2013 (paragraph 4.7).

We provide an update of the descriptive analyses of the toothfish tagging programme in Subareas 88.1 and 88.2, including summaries of data for the 2013 season. Overall, a total of 37 047 Antarctic toothfish have been reported as released and 1903 recaptured, and 1155 Patagonian toothfish released and 95 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 recorded as being recaptured only a short distance from their point of release, typically less than 100 km. However, several long distance movements of tagged Antarctic toothfish between slope and north regions have been observed. Of those that have changed areas (i.e. between Shelf, Slope, North) in the Ross Sea region, 44 have moved from the Shelf to the Slope, 31 have moved from the Slope to the Shelf, 13 from the Slope to the North, and 5 from the North to the Slope (although 4 of those are suspected of being poor tag links based on growth rates and ancillary linking data). Three recaptured fish moved between the Ross Sea region and Subarea 88.2; One moving from the Ross Sea (SSRU 88.1K - shelf) to 88.2H (north), and two moving from Subarea 88.2 to the Ross Sea slope (SSRU 88.1H), one from 88.2H - north, and 1 from 88.2F - slope).

The total number of recaptures in 2012 was lower than in 2010 and 2011, but the total number of recaptures in 2013 was very similar 2010 and 2011 and about double that in 2009. The reason for the low number of recaptures in 2009 and 2012 is likely a consequence of inter-annual changes in the main location of fishing effort arising from ice conditions in those years.

This report summarises the timing, depth, and location of fishing together with the biological aspects and catch of Antarctic toothfish for the period 1997–98 to 2012–13. In 2012–13, all the Ross Sea slope SSRUs were clear of ice and catches were evenly distributed across the three SSRUs. As in recent years, the remaining catches came mainly from SSRUs 88.1C, 88.1J, and 88.2H. Unstandardised Antarctic toothfish CPUE in the Ross Sea and Subarea 88.2 fisheries have fluctuated over the past 10 years with no trend. A standardised CPUE analysis in the Ross Sea fishery showed an increase to 2008 followed by a slight decline to 2013. Standardised CPUE indices in Subarea 88.2 showed a general decline from 2003 to 2013 in SSRU 882H, but a general increase from 2006 to 2013 in SSRUs 88.2C–G. However, these indices were very uncertain due primarily to the lack of a consistent set of vessels over time and may not be indexing abundance.

Length frequency distributions of Antarctic toothfish in the Ross Sea fishery have continued to be reasonably stable in the North and to have a strong mode of smaller fish on the Slope. Mean ages and ages of larger fish increased in the first few years of the fishery in each area but have declined since 2005. The trend on the Slope appears to be confounded by the trend of some vessels fishing shallower there in the last four years. There is some evidence for a similar 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 the age data are given low weighting in the 2013 assessment and that otolith readings for this area are given a high priority. There has been a marked change in sex ratio in the North of the Ross Sea fishery, with an increase in the proportion of males since 2001, but with little change in the rest of Subarea 88.1 or 88.2.

Proposed Research plan for the survey of Ukrainian longliner in 48.2 subarea

In the Division 58.4.1 there are two stocks of Dissostichus mawsoni, which is vulnerable with a big range of about 1,000-2,000 t per SSRU in 58.4.1. However, there is not enough data to utilize stocks properly, especially relate to reproductive mechanism. Therefore, the Korean scientists analyze the reproductive ecology of Dissostichus mawsoni in SSRU 58.4.1C based on the histological methods as a part of Korean research plan for Dissostichus spp. in SSRUs 58.4.1 C and E during 2012/2013 season. 

The ratio of female of D. mawsoni collected in SSRU 58.4.1C during the 2012/2013 season was 0.54 displaying the tendency increased with the total length became larger. The gonadosomatic index of the female was 2.6, and 1.1 for the male. The hepatosomatic index of the female (2.5) and male (2.3) were similar. The type of ovarian development of D. mawsoni was group-synchronous type. In the ovaries of growing stage, initial vitellogenic oocytes with approximately 150 ㎛ of egg size was mainly observed. The egg size of mature oocytes presented approximately 800 ㎛. Eosinophilic yolk materials and vacuolar oil droplets were interspersed within the cytoplasm, and the egg size of ripe oocytes were approximately 1,000 ㎛. Eosinophilic yolk materials are distributed within the medulla of cytoplasm, and combined multiple vacuolar oil droplets were existed in the cortical section of the cytoplasm. Histological testicular type of the fish was testicular lobule type, and the testicular lobule was composed of numerous testicular cysts. In the testis of growing stage the cysts became more definite gradually, and spermatogonia and spermatocytes occupied most of the cortex of the testicular lobule. It was possible to observe spermatids that illustrated basophilic reaction in H-E staining within the expanded testicular cysts in the mature stage. In the ripe stage, inner cavity of testicular lobules was occupied by spermatozoa of basophilic in H-E stain. Frequency of ovarian development stages of the toothfish showed the highest proportion with 46.3% at the mature stage, and followed by the growing stage with 29.3%, and ripe stage with 24.4%. Frequency of testicular developmental stages demonstrated the highest proportion with 58.3% for the growing stage, and followed by mature stage with 36.1%, and ripe stage with 5.6%. According to the ovarian developmental stages, spawning season of D. mawsoni may be deduced from May to June, and the fish may spawn two or three times during a spawning period in SSRU 58.4.1 C area. The group maturity in accordance with the total length (TL) categories of D. mawsoni displayed 100% for the female in lager than 90 cm categories, and almost 100% for the male in larger than TL 100 cm categories. Absolute fecundity on the total length 139.1 cm and total weight 39.944 kg was 1,130,178. And relative fecundity on the length and weight was 8,124/cm and 28/g, respectively. The results of this study were concluded from the limited number of samples and interpretation of reproductive ecology of D. mawsoni. Therefore, reanalysis of the reproductive ecology based on the much larger numbers of samples should be carried out in the near future.
 

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 1,000-2,000 t per SSRU in 58.4.1 and 100-1,500 t per SSRU in 58.4.2. During the 2003-2007 fishing seasons, 3,434 Dissostichus spp. 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 food-web of Dissostichus spp. started to be studied recently only in the southern Ross Sea. Some studies on biology of the fish have been studied, but those are still not enough to assess the stocks and consider proper management measures. 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 and E in Division 58.4.1 is very important to consider proper ways for effective managements of the population and ocean.

Korea notifies the participation in exploratory fishery for Dissostichus spp. in the SSRUs C and E 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 in 2012, and this plan is one of the processes of the notification.

The main objective of this scientific research plan is to assess the stock status of Dissostichus spp. in SSRUs C and E in Division 58.4.1. We will collect the catch and effort, tagged and released, recaptured and biological data to estimate biomass of Dissostichus spp. and evaluate migration and distribution of developmental maturity and size stage. For the 1st year (2012/2013) of this research, we collected and analyzed/will analyze the catch, effort, and biological data such as length, weight, gonadal development, otolith and muscle. However, the results were not good enough according to lack of data because of the bad sea-ice condition for the survey. Therefore, for the 2nd year (2013/2014), we will extend the sample size of catch and fishing efforts, and biological data, and also submit the primarily results on the comparing catch rate between trot line and Spanish line, and estimations of biological parameters relating to productivity based on the collected data from the 1st and 2nd researches. During the 3rd year, we will collect the same data as the 2nd year continuously. We will also submit the complete results for all three year researches such as biomass for Dissostichus spp. using CASAL model and ecosystem-based risk assessment, and consider proper measurements for the fishery management.

Stocks of Dissostichus mawsoni in Division 58.4.1 were vulnerable with a big range of about 1,000-2,000 t per SSRU in Division 58.4.1. However, there is not enough data and information to assess the robust stocks and deliberate proper measurements for sustainable utilize, especially relate to diet and feeding strategy which may lead to think about ecosystem based stock assessments and fishery managements. Therefore, the Korean scientists analyze diet composition and feeding strategy of Antarctic toothfish, Dissostichus mawsoni in SSRU 58.4.1.C-a as a part of Korean research plan for the exploratory longline fishery for Dissostichus spp. in SSRUs of 58.4.1 C and E during 2012/2013 season.  This study was carried out based on the results of stomach content identification of the Antarctic toothfish caught in SSRU 58.4.1 C-a in CCAMLR Conversion Area in March, 2013. The diet composition and feeding strategy of Dissostichus mawsoni were studied using 36 specimens (104 to 176 cm in total length).  D. mawsoni is a carnivore and piscivorous fish that mainly consumed fishes, especially Macrourus whitsoni with 14.72% of the diet by weight.  Its diet also included small quantities of mollusks, crustaceans, and seaweeds. In this study, fishes were the dominant prey item in all size classes (I, 104-140 cm TL, and II, 140-176 cm TL). The graphical method for feeding strategy revealed that D. mawsoni is an opportunistic and specialized predator on fishes and showed narrow niche width.

To identify the important prey items for pre-recruit Antarctic toothfish in the SSRUs 88.1.K of the southern Ross Sea and 58.4.1.C of the eastern Antarctic Sea, their fatty acid (FA) composition and stable carbon and nitrogen isotope ratios were determined and compared to those of bycatch species and stomach samples. Sampling efforts were made during a longline survey of pre-recruit toothfish from commercial fishing vessel in February-March 2013. Similarities in total FA compositions and the FA profiles in muscle tissue of Antarctic toothfish, bycatch specimen (Channichthyidae), and icefish in the stomachs indicated a trophic connection between the toothfish and icefish. Difference in the δ¹³C values of Antarctic toothfish between two areas showed their dependence on the different organic matter resources. δ¹⁵N values of Antarctic toothfish were higher than those of most specimens collected, indicating higher trophic postion of the toothfish. Similar δ¹⁵N values between two areas 88.1.K and 58.4.1.C suggested that they occupy the same trophic position irrespective of the area. The combination of stable isotope ratios and fatty acid profiles can be effectively used to trace the trophic transfer from organic matter sources to higher trophic levels through food chains. Further studies on the trophic relationship between Antarctic toothfish and other animals by collection and subsequent biomarker analyses for more pelagic and benthic biota are needed to better understand the carbon flow through the Antarctic ecosystems.

We reanalyzed data collected by the longline fishery in Subarea 88.1 to compare the bycatches of VME taxa made by vessels using Autolines and Spanish longlines. The fractions of sets with positive bycatch, as well as the weights of bycatch when bycatches were observed, were higher for Autolines than for Spanish longlines. Using a generalized linear approach with a Tweedie distribution to model the large number of zero-bycatch sets, we found that both the amounts of positive bycatches and the probabilities of observing positive bycatches decreased with depth for both gears. We also found that the rate at which positive bycatches decreased with depth was slightly greater for Autolines than for Spanish longlines. Since we only analyzed data from areas where there was substantial spatial overlap of sets made with each gear type, we attributed this difference to a difference in the rate at which VME taxa drop off Autolines relative to Spanish longlines while the gear is being hauled. We used a Bayesian approach and estimated that bycatch of VME taxa by Autolines is about 9 times greater per hook (about 5 times greater per set) than bycatch by Spanish longlines at 600 m. While there is considerable uncertainty about this ratio (the 95% credible interval for this ratio ranges from about 3 to 22 times greater per hook or about 2 to 12 times greater per set), there is near certainty that Autoline bycatch is greater than Spanish bycatch at all depths represented in our data. Limiting the use of Autolines, which are in more intimate contact with the sea floor than Spanish longlines, may decrease bycatches of VME taxa and thereby provide a precautionary approach to mitigating impacts on VMEs.