CCAMLR

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.

Application of lead-radium dating was attempted as a feasibility study for otoliths of blackfin icefish (Chaenocephalus aceratus) and ocellated icefish (Chionodraco rastrospinosus). Because otoliths from these fishes were small and the number of otoliths available low, a whole otolith application was necessary as a first look at lead-radium levels. The application was successful as a first step in determining the parameters of future applications that would lead to a validation of age. In the current study, sample mass for the pooled otolith samples was low (~0.4 g). This finding, coupled with low radium-226 levels, led to age determinations that had a high margin of uncertainty; however, the findings led to an educated recommendation for future lead-radium dating studies for these species. Based on the measured lead-radium levels, the future direction may include further exploration of the whole otolith approach. This approach has some disadvantages, such as the circularity associated with developing a mass-growth model, but would provide the opportunity to make an age determination that does not reply on growth zone counting. Furthermore, a follow-up lead-radium dating study using cored otoliths (extraction of the first few years of growth) is possible for these species, provided there is enough sample mass and perhaps an application of micromilling. This approach would require ~150 otoliths per sample, given an arbitrary target core weight of 0.01 g, near the low end of the range used in this study. It is recommended that the collection of otoliths from these species be given a priority whenever possible to facilitate future age and growth studies and potential applications of lead-radium dating.

We asked whether silverfish (Pleuragramma antarcticum) are distributed in independent, discrete populations along the Western Antarctic Peninsula (WAP), or whether the shelf circulation leads to connectivity, by testing between rival hypotheses of population segregation and linkage. If independent, reduction in sea-ice along the WAP may lead to extinction of local populations through drastic mortality of early stages which are ice-dependent. Alternatively, if linked by the large-scale circulation, abundance in areas subsidized by migration may be more robust to declining sea ice coverage. We provide preliminary data from sampling along the WAP on board the R/V Nathaniel B. Palmer, including initial simulations using a circulation model. Differences in length distributions and maturity suggested two discrete populations at 1) the north tip of the Peninsula, and 2) the southern WAP encompassing Charcot Island and Marguerite Bay, with no mixing between. The presence of a single length mode at Charcot Island and Marguerite Bay corresponding to the year class found during GLOBEC 2001 suggested a dominant cohort and no local recruitment since 2001. The collapse in silverfish abundances previously found off Anvers and Renaud Islands, without similar collapses to the north and south, suggest a third population in the central WAP. Analyses of otolith chemistry are scheduled over the next year.

At the 2000 Fish Stock Assessment Working Group Meeting (WG-FSA-2000), differences were noted in the growth parameters being used for assessments of toothfish. To assess whether these differences were real or else due to methods of otolith preparation and reading, scientists from CCAMLR member nations participated in a Workshop in 2001 at Old Dominion Unversity with the primary aim of seeking conformity in estimating age of Patagonian toothfish (Dissostichus eleginoides). Arising from the workshop, the Center for Quantitative Fisheries Ecology at Old Dominion University has developed a manual to address recommendations for: the use of otoliths for estimating age; key features to be taken into account in reading otoliths; quality assurance and control; and the use of reference sets to monitor precision.

Presented some data for Dissostichus eleginoides fishery in the southwestern Atlantic Ocean beyond EEZ in statistical divisions 41.3.1 and 41.3.2.

For the determination of the age of Dissostichus mawsoni used otoliths and scales. All samples were collected in the Indian Ocean sector of the Southern Ocean. Methods are described. Otoliths were measured and weighed. All major life periods in corresponding ecological conditions are reflected well in the sections of otoliths as certain groups of the annual rings.

A survey of mackerel icefish, Champsocephalus gunnari, was undertaken in Division 58.5.2 in the vicinity of Heard Island in March-April 2010 to provide the information for an assessment of short-term annual yield in the 2010/2011 CCAMLR season. This paper provides a preliminary assessment of yield for the area of Division 58.5.2 to the west of 79°20’ E using standard CCAMLR methods. The strong year class detected in the last three years’ surveys is now fully recruited as the 4+ cohort, and dominates the population, however it is assumed this cohort will be unavailable to the fishery in 2010/11. A new 2+ cohort has been detected, and it is expected that the fishery in 2010/11 will focus on this cohort. A new growth model is also estimated which shows an improved fit to observed sizes at age of icefish from recent surveys.