CCAMLR

New Zealand will deploy four pop-off satellite transmitters on Antarctic toothfish along the Ross Sea continental slope in January 2012. They will be programmed to pop-off the fish, float to the surface, and transmit in December 2012. The purpose of this paper is to provide an announcement for members to distribute to vessels, crew and observers given the chance that these fish may be encountered in the fishery. The announcement details what to do if a satellite tagged fish is encountered, and who should be contacted. A second page provides specific instructions to observers and vessel crew that could be translated and provided directly to industry via the CCAMLR website.

Stock assessments rely upon estimates of the total mortality resulting from fishing activities. The level of mortality resulting from the loss of sections of bottom longline gear in the Ross Sea region and Subarea 88.2 Antarctic toothfish (Dissostichus mawsoni) fisheries is not currently known. Lost fishing gear should be monitored to provide information to evaluate how well CCAMLR meets its management objectives relating to human impacts on the Antarctic environment. We provide a method to estimate unaccounted fishing mortality from lost lines in the Ross Sea region and Subarea 88.2 Antarctic toothfish fisheries. Our estimates suggest that on average 175–244 t of Antarctic toothfish may be unaccounted annually. While these estimates may be improved through better data reporting practices, the current estimates have been provided as sensitivities in the 2011 stock assessments for the Ross Sea region and Subarea 88.2 fisheries.
 

Descriptive analyses of the toothfish tagging programme in Subareas 88.1 and 88.2 are updated, including summaries of data for the 2011 season. Overall, a total of 28 458 Antarctic toothfish have been reported as released and 1417 recaptured, and 1124 Patagonian toothfish released and 67 recaptured since 2001. In recent years, most vessels have achieved or exceeded the target 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 the north and slope regions have been observed. Long distance movements to the north were recorded for six fish that moved from SSRU 88.1H to 88.1C; two moved from SSRU 88.1H to 88.1B; and one from SSRU 88.1I to SSRU 88.1C. Long distance southwards movements were recorded for three fish that moved from SSRU 88.1B to SSRUs 88.1H; and one to SSRU 88.1I. In addition, several fish moved long distances on the slope and the shelf, with movements between SSRUs 88.1H, 88.1I, and 88.1K to grounds off Terra Nova Bay and Ross Island in SSRU 88.1J and vice versa..

The total number of recaptures in 2010 and 2011 was at a similar level to 2007 and 2008 and about double that in 2009. The reason for the low number of recaptures in 2009 is unclear but is more likely to be related to the different locations of fishing in 2009 rather than a lower detection rate.
 

The exploratory fishery for Dissostichus spp. has now been operating for fifteen years in Subarea 88.1 and for ten 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 2010–11. During the 2010–11 fishing year most of the catch in Subarea 88.1 came from the slope SSRUs 88.1H and 88.1I, and SSRU 88.1K. About 63% of the catch limit in the north was taken from 88.1C with the rest taken from SSRU 88.1B. The catch limit from the shelf was taken mainly from 881J. Most catch from Subarea 88.2 was taken from SSRU 88.2E but over 100 t was taken from each of SSRU 88.2D and 88.2F, and SSRU 88.2C was fished for the first time.

Unstandardised raw catch and effort data examined during this study suggest that the median toothfish catch per set has generally increased over the course of the fishery whilst the median number of hooks per set has remained relatively constant. Other indicators of CPUE including unstandardised catch per set and catch per hook have varied considerably over time in each of the fisheries showing no trend. There is no evidence of any truncation of the overall length frequency distribution, but some evidence for a reduction in median fish length in SSRUs 88.1H and 88.1I over the last 1–2 years as a mode of smaller (c. 100 cm) fish is selected by teh fishery, most likely  related to fishing depth. We conclude that from data examined from the fishery to date there is no evidence for substantial changes in population structure or abundance at the regional (Subarea) or local (SSRU) level.
 

We provide an update of the Bayesian sex and age structured population stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the SSRU 88.2E, using revised catch, catch-at-age and tag-recapture data from selected trips and all vessel trips. The 2011 reference case model resulted in a 30% higher estimate of initial biomass than for the 2009 base case. The main reason for this increase was likely to be because of the increase in the proportion of selected trips in the 2011 assessment.

Model fits to the data were adequate, with the tag-release and recapture data providing the most information on stock size, although there were conflicts between information in the recapture data from different years. Monte-Carlo Markov Chain (MCMC) diagnostics suggested no evidence of non-convergence in any of the models. The selected trips model run using the revised maturity ogive MCMC estimated initial (equilibrium) spawning stock abundance B0 d at 10 100 t (95% credible intervals 8 720–11 770 t), and current (B2011) biomass estimated at 82.7% B0 (95% credible intervals 80.0–85.1%). Estimated yields from this model were 484 t.

SSRU 88.2E has historically been assessed and managed as a separate area because it is geographically separated from the Ross Sea fishery and little fishing had been carried out in the other parts of Subarea 88.2. Furthermore, it is generally considered more conservative to manage small fisheries separately to avoid potential problems with localised stock depletion. However, over the past 2–3 years there has been an increase in fishing in SSRUs 88.2CDFG and it may now be more appropriate to carry out a combined stock assessment based on SSRUs 88.2C–G. This combined stock assessment is developed in a separate paper.
 

This paper describes the first Bayesian sex and age structured population stock assessment model of Antarctic toothfish (Dissostichus mawsoni) in Subarea 88.2 covering SSRUs 88.2C, D, E, F, and G (SSRUs 88.2C–G). For modelling purpose, we split the region into two fisheries: south of 70.2° S where smaller fish are typically caught and effort has been light; and north of 70.2° S where larger fish are caught and most of the effort has been concentrated. For our reference case we used a selected trips data set and fitted to tag and fishery data from the northern fishery only.
Model fits to the data were adequate, with the tag-release and recapture data providing the most information on stock size. Estimated initial equilibrium mid-season spawning stock biomass (B0) for model R1 (2011 reference case) ranged from 9820 to 13 350 t, with the initial biomass estimated to be 11 420 t. The yield was estimated at 544 t, slightly lower than the combined current catch limits for the region of 575 t.
We examined several sensitivities including (i) unaccounted mortality due to lost gear, (ii) catch and tag data from the southern fishery, and (iii) tag data from all trips. The inclusion of the unaccounted mortality from lost gear had little impact on estimates of biomass. The inclusion of tag data from the south fishery led to a 10% increase of biomass, due to the low numbers of tags there, whilst including tag data from all vessels led to a 20% increase in biomass.
We believe that the SSRU 88.2C–G model provides a more realistic representation of the observations and catch data for the area than the SSRU 88.2E only model. The SSRU 88.2C–G area included tags that may move between SSRU 88.2E and adjacent SSRUs, and it better accounts for the age frequency structure of the north and south areas.
 

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 2010–2011 seasons. The 2011 reference model using the selected trips tag data gave a similar, but slightly higher estimate of initial biomass than the 2009 base case. Retrospective analysis suggests that this is partly as a result of the increased number of vessels in selected data set and partly as a result of the 2010 and 2011 observations. Two sensitivity models are presented; the first considers the effect of including possible unaccounted mortality from lost gear, and the second uses tag release and recapture data from all vessel trips.

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 2011 reference model were estimated as 73 870 t (95% credible intervals 69 070–78 880), and current (B2011) biomass was estimated as 80.0% B0 (95% credible intervals 78.6–81.3).The estimated yield, using the CCAMLR decision rules, was 3282 t
 

359 Antarctic krill catches were analysed for by-catch, it took 48.5% of observed catches and 31.3% of total catches. Fish by-catches were usual in the sector 48.1 (89.1% of cases); the most of by-caught fishes belonged to families Nototheniidae (NOX) (66.3%). Fish by-catches in the sector 48.2 were recorded in 50% examined catches; the most frequent were fishes of families Myctophidae (LXX).