CCAMLR

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.

Catch and effort data recorded by all vessels were analyzed using the latest CCAMLR C2 and observer data.

The stock size estimate for a research block (5843a) was revised following advice in the last WG-SAM meeting. We tentatively recalculated a sample size of Dissostichus spp in each block in such a way that the numbers of tag recoveries in the 2016 season shows an approximately 0.3 of coefficient variance of biomass estimate from the bootstrapped procedure, under the condition that the exploitation rate (sample size / estimated stock size) would not exceed 3.5 %. The tentatively estimated sample sizes turned out to be larger in many blocks and smaller in a few blocks over the data poor-fisheries. Since we have yet to obtain enough evidence to estimate plausible stock size, and accordingly have yet to estimate appropriate catch limit, we propose to conduct the current research operation for at least 3 years with the same sample size as decided at the last CCAMLR meeting for the current research block, in order to promote successful stock assessment.

The separation rule was one of the major inconveniences for successful research, as it may excessively limit the available area for research hauls. For this reason, we propose the mitigation rule so that 50% of lines are required to be separated by not less than 3 n miles from each other but the rest of lines can be set freely between the separated lines. In this way, over-concentration of catch and effort can be avoided while reducing the inconveniences for operation.

Similarly, strict application of the move-on rule could compromise success in the research. We propose the relaxation of rule by increasing the current trigger level of 1 500 kg for the catch of Macrourus spp. in two 10-day periods provided in the paragraph 6 of CM 33-03 to 2 000 kg.

Catch and effort data recorded by all vessels were analyzed using latest CCAMLR C2 and observer data.

The stock size estimates for the research block (5842E) was revised following advice in the last WG-SAM meeting. We tentatively recalculated a sample size of Dissostichus spp in each block in such a way that the numbers of tag recoveries in the 2016 season shows an approximately 0.3 of coefficient variance of biomass estimate from the bootstrapped procedure, under the condition that the exploitation rate (sample size / estimated stock size) would not exceed 3.5 %. The tentatively estimated sample sizes turned out to be larger in many blocks and smaller in a few blocks over the data poor-fisheries. Since we have yet to obtain enough evidence to estimate plausible stock size, and accordingly have yet to estimate appropriate catch limit, we propose to conduct the current research operation for at least 3 years with the same sample size as decided at the last CCAMLR meeting for the current research block, in order to promote successful stock assessment.

We tentatively examined the degree of coverage of the austral summer sea-ice in the current research blocks in past ten years. The sea-ice condition during the summer season was generally favorable for operations for the research block.

The separation rule was one of the major inconveniences for successful research, as it may excessively limit the available area for research hauls. For this reason, we propose the mitigation rule that 50% of lines are required to be separated by not less than 3 n miles from each other but the rest of lines can be set freely between the separated lines. In this way, over-concentration of catch and effort can be avoided while reducing the inconveniences for operation. 

Similarly, strict application of the move-on rule could compromise success in the research.  We propose the relaxation of rule by increasing the current trigger level of 1 500 kg for the catch of Macrourus spp. in two 10-day periods provided in the paragraph 6 of CM 33-03 to 2 000 kg.

Catch and effort data recorded by all vessels were analyzed using latest CCAMLR C2 and observer data.

The stock size estimates for research blocks were revised following advice in the last WG-SAM meeting. We tentatively recalculated a sample size of Dissostichus spp in each block in such a way that the numbers of tag recoveries in the 2016 season shows an approximately 0.3 of coefficient variance of biomass estimate from the bootstrapped procedure, under the condition that the exploitation rate (sample size / estimated stock size) would not exceed 3.5 %. The tentatively estimated sample sizes turned out to be larger in many blocks and smaller in a few blocks over the data poor-fisheries. Since we have yet to obtain enough evidence to estimate plausible stock size, and accordingly have yet to estimate appropriate catch limit, we propose to conduct the current research operation for at least 3 years with the same sample size as decided at the last CCAMLR meeting for the current research blocks, in order to promote successful stock assessment.

We tentatively examined the degree of coverage of the austral summer sea-ice in the current research blocks during past 10 years. The sea-ice condition during the summer season was generally favorable for operations for most of research blocks.

The separation rule was one of the major inconveniences for successful research, as it may excessively limit the available areas for research hauls. For this reason, we propose the mitigation rule that 50% of lines are required to be separated by not less than 3 n miles from each other, but the rest of lines can be set freely between the separated lines. In this way, over-concentration of catch and effort can be avoided while reducing the inconveniences for operation.  

Similarly, strict application of the move-on rule could compromise success in the research. We propose the relaxation of rule by increasing the current trigger level of 1 500 kg for the catch of Macrourus spp. in two 10-day periods provided in the paragraph 6 of CM 33-03 to 2 000 kg.

Catch and effort data recorded by all vessels were analyzed using the latest CCAMLR C2 and observer data.

The stock size estimates for research blocks were revised following advice in the last WG-SAM meeting. We tentatively recalculated a sample size of Dissostichus spp in each block in such a way that the numbers of tag recoveries in the 2016 season shows an approximately 0.3 of coefficient variance of biomass estimate from the bootstrapped procedure, under the condition that the exploitation rate (sample size / estimated stock size) would not exceed 3.5%. The tentatively estimated sample sizes turned out to be larger in many blocks and smaller in a few blocks over the data poor-fisheries. Since we have yet to obtain enough evidence to estimate plausible stock size, and accordingly have yet to estimate appropriate catch limit, we propose to conduct the current research operation for at least 3 years with the same sample size as decided at the last CCAMLR meeting for the current research blocks, in order to promote successful stock assessment.

We tentatively examined the degree of coverage of the austral summer sea-ice during past 10 years in the current research blocks and an additional research block (486e in SSRUs B and C) which Japan and South Africa proposed in the last WG-SAM meeting. The sea-ice condition during the summer season was generally favorable for operations for the research blocks. Based on this examination, 486e block appears to be a promising research block and hence we propose the addition of the research block 486e, where Shinsei-maru No.3 released a total of 587 tagged D. mawsoni in 2011, in order to maximize the use of tagged fish. 

We propose the extension of the east boundary of research block 486d from the current 13°E to 15°E to examine whether the fish migration could be active in this continental-shelf block.  

The separation rule was one of the major inconveniences for successful research, as it may excessively limit the available area for research hauls. For this reason, we propose the mitigation rule that 50% of lines are required to be separated by not less than 3 n miles from each other but the rest of lines can be set freely between the separated lines. In this way, over-concentration of catch and effort can be avoided while reducing the inconveniences for operation.  

Similarly, strict application of the move-on rule could compromise success in the research. We propose the relaxation of rule by increasing the current trigger level of 1 500 kg for the catch of Macrourus spp. in two 10-day periods provided in the paragraph 6 of CM33-03 to 2 000 kg.

We revised stock status assessments of D. eleginoides in SSRUs C and D using comparative CPUE, modified Lincoln-Petersen and CASAL models following advices during the last WG-SAM meeting. Detailed results using CASAL models in SSRU C are described in a separated document.

The catch and tagging data in SSRU D were not enough to be applied to modified Lincoln-Petersen and CASAL models. Thus the stock sizes of D. eleginoides in SSRU D is estimated only by using CPUE comparison with the stock size estimates in SSRU C obtained from each method.

The estimated stock size in SSRU C from CPUE comparison (548 tonnes) was similar to that (542 tonnes) from modified Lincoln-Petersen. However, the stock size estimated from CASAL base model (1040 tonnes) was about 2 times higher than those from the two methods.

Since we have not obtained enough evidence to estimate plausible stock size, and accordingly have not been able to determine catch limit yet, we would like to conduct research repeated for at least one more year in the same locations as in the 2012/13 season, with the same survey design and sample size of a total of 50 tonnes in SSRUs C and D for the 2013/14 as in the 2012/13, thereafter, the phase of the survey will be moved from effort limited to catch limited.

I revised the CASAL catch-at-length models for stock status assessments of Dissostichus eleginoides in SSRUs C in Division 58.4.4 a & b (WG-SAM-13/21) following several advices during the last WG-SAM meeting as follows: 1)I included 2012 tag data (which was not used for model runs in WG-SAM-13/21), and a combined value of over-dispersions were used for all tag-release years (2008 and 2010‒12) instead of separated value for each year, 2)I calculated effective sample size for the proportions-at-length using regression line between coefficient variances estimated by a bootstrap procedure and expected proportions, 3)I changed the tagging mortality rate estimate of 0.1 instead of 0.2 because Japanese trotline system showed the captures of an adequate number of single-hooked fish in a state suitable for tagging. I made following four sensitive runs; 1)base, 2)25%_IUU, 3)100%_IUU and YCS models. The base model assumed the constant year class strength (YCS) and no IUU fishing occurrence in SSRU C. I changed the base model from that used in WG-SAM-13/21 which assumed estimated variability of YCS and a quarter of observed IUU fishing occurrence in SSRU C because estimated variability of YCS was inconsistence with the estimated vulnerable biomasses, and stock size estimate with an assumption of no IUU occurrence was conservative.

Fits of tag data were a little improved compared to those in the WG-SAM-13/21 in the base model. However, there was some evidence of lack of convergence in MCMC posterior trace and in the median MCMC jump statistics for the shape of the right hand limb of the double normal selectivity in each model. Hence, the MCMC results are not yet robust.

   Median MCMC estimates (95 % credible intervals) of initial and current biomasses in SSRU C were 1100 tonnes (530-3900 tonnes) and 1040 tonnes (470-3840 tonnes), respectively in the base model. The CCAMLR yield was 28 tonnes in the model.

A survey was conducted to collect data of stock status and various biological information of toothfish in SSRUs C and D of Division 58.4.4a & b (Ob Lena Banks) from April 25 to May 22, 2013. A total of 64 research hauls using normal trotline system were made in SSRUs C and D. Total catch of Dissostichus eleginoides (excluding those tagged and released) was 31.1 tonnes in the survey area from 64 longline sets. Mean CPUEs in terms of weight of D. eleginoides was 70.6 kg / km (CV = 79.1%) and 44.2 kg / km (CV = 91.9%) in SSRUs C and D, respectively. Both nominal and standardized CPUEs were relatively higher in 2011 but showed no clear tendencies with the years in SSRU C. Fish in single-hooked and good physical condition constituted 31.6 and 27.1 % of the total catch in SSRUs C and D, respectively. It seems that larger fish > 140 cm are more damaged than smaller ones, which is consistent with the results in 2011/12 season. The possible maximum tagging rate was calculated to be 30.1 fish per tonnes and 23.5 fish per tonnes in SSRUs C and D, respectively. Accordingly sufficient numbers of fish suitable for tagging was captured by using the normal trotline system. A total of 233 fish of D. eleginoides in good physical condition and single-hooked were tagged and released during the survey, and 3 fish of D. eleginoides, which were tagged and released in SSRU C in 2007/08 and 2010/11, were recaptured in the same SSRU. No set was affected by killer whales in both SSRUs.