CCAMLR

Revised weight-length and length-length regressions are provided for Amblyraja georgiana and Bathyraja cf. eatonii. Significant differences between males and females occurred for all comparisons except for B. cf. eatonii length-length regressions. Some observer maturity staging data were unreliable, as indicated by comparison with lab staging, and the presence of very large skates scored as immature. For A. georgiana, this resulted in flatter maturity ogives for observer data than lab data, and different estimates of median length at maturity. The lab estimates were based on small datasets that did not cover the maturation range adequately, so using them instead of the observer estimates is not necessarily a good option. Logistic regressions differed significantly between the sexes for both datasets, but observer data suggested that females matured at a larger size than males, whereas lab data suggested the reverse. The differences between sexes may therefore be spurious, and an artifact of insufficient and inaccurate data. Until better data are available, length at maturity should be regarded as the same for both sexes. The median lengths at maturity estimated for both sexes combined were similar for both data sources: 65.9 cm for lab data and 66.5 cm for observer data, suggesting a reasonable overall value of 66 cm PL. There are still insufficient reliable data to estimate the median length at maturity of B. cf. eatonii, but it is probably around 65–75 cm for males and 70–80 cm for females. Reasonable point estimates are therefore 70 cm for males and 75 cm for females.

A dataset for the 2009 assessment of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea was selected on the basis of data quality metrics for individual trips. Initial informative datasets were selected comprising trips with high (above median) rates of recovery of previously released tags, and/or where tags released on the trip were subsequently recaptured at a high rate. These trips were used to define a range for various data quality metrics considered to be informative with respect to tagging data. Other trips with data quality metric values within these ranges were added to the dataset. A retrospective analysis of this data selection method was undertaken to evaluate whether trip selection was stable over time. The 2009 dataset was reduced by omitting data from the two most recent years, and the data selection method applied to these 2007 and 2008 datasets. Results from this three year period indicate that additional, historical trips may be added to the data set as new tagging data become available. However, once selected, trips are also selected in subsequent years. This analysis indicates that the tagging data selection methodology developed for the 2009 assessment yields generally stable selections over time. The retrospective analysis period is, however, short, and stability of trip selection in future years as further data are added will also be of interest.

The toothfish fishery in the Ross Sea region (CCAMLR Subareas 881 and 882) has been operating since the 1996–97 fishing season. The fishery has increased to an annual Antarctic toothfish (Dissostichus mawsoni) catch of about 3000 t. Skates form a small proportion of the total catch (typically 2% or less). In this paper we summarise the current available fisheries and biological information for skates in the Ross Sea Region, including the data collected in the two “Year of the Skate” fishing seasons in 2008/09 and 2009/10. The composition of the skate catch by species is uncertain; it is estimated that about 33 000 starry skates were landed, and 55 000 released in the Ross Sea region by all vessels to the end of the 2009–10 fishing season, and about 4300 Eaton cf. skates were landed and 4600 released in the same region over the same period of time. There were also differences in the distribution of the two species, starry skates generally being found deeper and more to the west than Eaton cf. skates in the Ross Sea region. Scaled length frequencies showed no change in distribution between 2003 and 2008 for landed starry skates, whilst tagged starry skates had a lower proportion of large individuals than landed starry skates. Eaton cf. skates had a different length frequency, with a narrower distribution centred around a larger average size than starry skates. This larger distribution is consistent with other studies suggesting Eaton cf. skates might grow to larger sizes than starry skates. During the “Years of the Skate”, a total of about 3300 starry skates and 700 Eaton cf. skates were tagged and 13 starry skates and 3 Eaton cf. skates were recaptured and successfully linked to a tag event. In total there have now been a total of 179 tags recaptured but only 128 have been successfully linked. There was no evidence of growth retardation linked with the capture and tagging event. Tag loss rates of T-bar tags were similar to those calculated for toothfish. The “Years of the Skate” have been instrumental in collecting further data on skates, in particular length and tagging data. The quality of the tagging database has also improved since it has been centrally managed by CCAMLR. It has shown the value of collecting large amounts of data in specific years rather than small amounts of data in many years. An updated risk assessment of the skate population in the Ross Sea region should be carried out.

A species profile, covering aspects of the biology, fisheries and stock assessment of both toothfish species was first completed by Everson (2002). A new species profile, specifically for Antarctic toothfish (Dissostichus mawsoni, Norman), was developed by Hanchet (2006). That document concentrated on drawing together mostly unpublished data on D. mawsoni biology including distribution and overlap with D. eleginoides, stock structure, reproduction, age and growth, food and feeding, predation, and depredation. It focused primarily on the biology of D. mawsoni based on information collected from the Ross Sea fishery because that was where most of the work had been carried out. The aim of the current work was to update and revise the species profile of Hanchet (2006). It updates, collates, and summarises new biological data on D. mawsoni. Although the report still focuses primarily on data collected from the Ross Sea fishery, it also presents data from other areas and fisheries where available. It is intended that this report form the basis of a species profile of D. mawsoni as requested at the 2009 CCAMLR meeting (SC-CAMLR XXVIII).

The exploratory fishery for Dissostichus spp. Has now been operating for 14 years in Subarea 88.1 and for 9 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 2009–10. During the 2009–10 fishing year most of the catch in Subarea 88.1 came from the slope SSRUs 881H and 881I, with a reduced catch from 881K due to ice conditions. The catch limit in the north was all taken from 881C, whilst the catch limit from the shelf was taken mainly from 881J. Most catch from Subarea 88.2 was taken from SSRU 882E, with little fishing occurring in the other SSRUs, and the catch limit was under caught by 250 t. Unstandardised catch per set and catch per hook have varied considerably over time in each of the fisheries showing no trend in the main fisheries in Subarea 88.1 and SSRU 88.2E, but a slight decline in SSRU 882CDFG over the last two years. We carried out a more detailed characterisation of Patagonian toothfish catches for the first time. A total of 124 t of Patagonian toothfish has been reported from the fishery, with catches mainly coming from the northwest of the Ross Sea region (SSRUs 881A and 881B). Some observers appear to have problems distinguishing the two toothfish species, and many small (<100 cm TL) toothfish which have been reported as Antarctic toothfish should in fact be Patagonian toothfish. We recommend that observers be made aware of this issue, and that the species identification be independently checked through examination of a subsample of otoliths from these small fish. A major source of uncertainty in the current stock assessment concerns the recruitment dynamics. It has been suggested that a pre-recruit longline survey be developed to collect data to better estimate these parameters. We recommend WG-FSA consider whether such a survey would be worthwhile and identify several fishing grounds on the Ross Sea shelf where such a survey could be carried out.

The stomach contents of 273 Dissostichus mawsoni captured by bottom longline in the Ross Sea region during the 2010 summer fishing season were analysed. The D. mawsoni sampled were caught in 640–2127 m depths and ranged between 78 and 181 cm TL. A total of 198 stomachs contained prey items, of which 13 contained only digested prey, leaving 185 stomachs for analysis. The majority of these stomachs were collected from the continental slope (n = 164) where Whitson’s grenadier (Macrourus whitsoni) dominated the diet occurring in 37% of stomachs and accounting for 45% by weight and 52% by IRI. De Witt’s icefish, eel-cods, and glacial squid were also important occurring in about 20% of stomachs and by IRI 23%, 13%, and 8% respectively. The large onychoteuthid squid, Kondakovia longimana dominated the diet.of D. mawsoni collected from the northern area (n = 21). The results of the current study were compared to a similar study based on samples collected in 2003. Preliminary examination of the data suggested there had been no change in the diet of large (> 100 cm TL) D. mawsoni between the two years. It is planned to carry out a more rigorous analysis of these data in the coming year.

Production of an age determination manual for Antarctic toothfish (Dissostichus mawsoni) is part of the focus of the SC-CAMLR workplan for 2010. The aim of the current work was to produce a manual that presents standard protocols used by the National Institute of Water and Atmospheric Research Limited (NIWA) for estimating age of D. mawsoni. It uses otoliths collected from the Ross Sea region and aims to guide researchers to maintain common procedures. This will increase the precision and accuracy of age estimates contributing to stock assessment.

Antarctic silverfish (Pleuragramma antarcticum) were sampled during a trawl survey in the Ross Sea, Antarctica. Biological data, including fish length, weight, sex, gonad maturity, liver weight and diet analysis were collected from 311 specimens. Standard length and weight were well correlated (r2 = 0.99).

Counts of growth zones in 304 thin-sectioned otoliths were used to estimate ages and von Bertalanffy growth parameters. The species is relatively slow-growing with a moderate longevity; the maximum estimated age was 14.3 years. Von Bertalanffy parameters derived for both sexes combined are: L, 22.1 cm SL; K, 0.167 y-1; t₀, -0.4 years. Parameter estimates were also derived for the sexes separately. Female Antarctic silverfish appear to reach a larger size than males, but none of the estimated von Bertalanffy parameters were statistically significantly different between sexes. All parameter estimates are preliminary as the ageing method is unvalidated and about two-thirds of the sampled fish could not be sexed.

Precision estimates and age bias plots indicated that there was good within-reader and between-reader agreement, so the otolith sections appear able to be consistently interpreted.

The standard lengths of the sampled Antarctic silverfish ranged from 4.6 cm to 22.9 cm. Pronounced modes in the length-frequency distribution occurred at 7.1–7.5 cm, 10.6–11.0 cm, and 15.1–15.5 cm. The age-frequency distribution exhibited a mode from age 6 to 9 years.

Samples of an eel cod (Muraenolepis sp.) and violet cod (Antimora rostrata) were obtained from the bycatch of the longline fishery for toothfish (Dissostichus mawsoni and D. eleginoides) in the Ross Sea, Antarctica. Counts of zones visible in sectioned otoliths were used to estimate growth parameters for these species. The estimates must be considered preliminary because they are unvalidated, the otoliths are quite difficult to interpret, and the sampled fish do not represent the full length or sex distributions of the populations (i.e., small fish and males were absent). The Muraenolepis samples were not identified to species. However, they were obtained from a relatively confined areal and depth range, so probably comprise a single species. They are relatively short-lived, with a maximum estimated age of 9.5 years. Von Bertalanffy parameters derived from female and unsexed fish only are: L., 42.8 cm TL; K, 0.408 y-1; t0, -0.1 years. Violet cod are relatively long-lived, with a maximum estimated age of 41.5 years. Von Bertalanffy parameters derived from female and unsexed fish only are: L., 78.4 cm TL; K, 0.053 y-1; t0, 0.1 years.

Ration and daily food consumption in 3 low-Antarctic (Champsocephalus gunnari, Chaenocephalus aceratus, and Pseudochaenichthys georgianus), and 3 high-Antarctic (Chionodraco rastrospinosus, Cryodraco antarcticus, and Chaenodraco wilsoni) ice fish species were estimated around islands of the southern Scotia Arc in 1996, 1999, 2001, 2002, 2003, 2007 and 2009, and in 2006 off the north western part of the Antarctic Peninsula. Variability of food consumption was comparatively low between years in the krill – feeding C. gunnari and C. wilsoni, both within an area and between areas. Food intake was more variable in C. aceratus and C. antarcticus which, as larger fish, rely heavily on fish as their primary dietary component. Their food intake varied by a factor of 2 or 3 between years. Most estimates of daily food intake of demersal Antarctic fish both available in the scientific literature and from our study range from 0.5 – 2.5% body per day. These values may be exceeded locally and temporarily in summer when krill is abundant and likely forms dense aggregations in frontal zones such as the Weddell – Scotia Confluence. It is still unknown how long digestion time of various food items takes and if fish empty their stomachs completely before taking in new food. If we assume that digestion of krill takes about 48 hours while the digestion of fish prey takes likely in excess of 96 hours, our data are well in line with those from previous more limited studies.