CCAMLR

This note describes the activities of an alliance of New Zealand government, fishing industry and environmental groups that has formed to promote fishing practices that avoid seabird capture by fishing fleets in the southern hemisphere.

Data on maturity, gonadosomatic index (GSI) and length weight of Antarctic Toothfish Dissostichus mawsoni were collected from Sub areas 88.1 and 88.2 in the 2002-2003 season by the New Zealand bottom longline vessels Janas, Avro Chieftain and San Aotea II operating in the Exploratory Fishery. The first fully mature female fish were found in December, three months earlier than in the previous season.
The onset of spawning in D. mawsoni is in December and continues until at least June, in both Sub-areas 88.1 and 88.2, and spawning occurs on isolated geographic features north of the main Antarctic shelf areas. One of the mature fish caught in Sub Area 88.2 had originally been tagged in McMurdo Sound Sub Area 88.1 in 1986.
A new 7 stage recording system is proposed for D. mawsoni to be agreed at WG-FSA.v

The CCSBT Ecologically Related Species Working Group held its 5th meeting in Wellington in February 2004. This report provides a summary of the discussion and outcomes of that meeting from the IMAF nominated observer at the meeting.

Two molecular methods, mitochondrial DNA and introns, were used to determine genetic relationships among Antarctic toothfish, Dissostichus mawsoni, samples from three CCAMLR areas 48.1, 88.1, and 58.4.2. D. mawsoni appeared to be characterized by low diversity; no genetic variation was detected with restriction enzyme digests of nine sub regions of the mitochondrial genome. Polymorphisms were found in four out of seven introns digested with the restriction enzymes, but there was no population differentiation among the three sea areas. While direct sequencing of cytochrome b (665 base pair) showed virtually no variation, sequences for ~600 bp of the left domain of the control region showed nucleotide variation with some haplotypes restricted to sea areas. The weak genetic differentiation is supported by oceanic gyres, which may act as juvenile retention systems, and by limited movement of adult tagged fish. It is recommended that the Ross Sea D. mawsoni be treated as a separate stock unit.

The stomach contents of 496 Antarctic toothfish (Dissostichus mawsoni) captured by bottom longline in the western Ross Sea during the 2003 summer fishing (from Dec 2002 to Jun 2003) season were analysed. Fish sampled were caught in 770-1890 m depths and ranged between 72–212 cm in total length. D. mawsoni feeds on a wide range of prey but is primarily piscivorous. The most important prey species was Whitson’s grenadier (Macrourus whitsoni), which dominated the overall diet. In continental slope waters, M. whitsoni, the icefish Chionobathyscus dewitti, eel cods (Muraenolepis spp.) and cephalopods predominated in the diet, while on oceanic seamounts M. whitsoni, violet cod (Antimora rostrata) and cephalopods were important.

Icthyoplankton sampling was carried out by the New Zealand bottom longliner Avro Chieftain in the Ross Sea Sub Area 88.1 in the 2002-03 fishing year. Four direct haul samples were taken with a UNESCO WP2 net to a maximum depth of 1620 m in areas where mature D. mawsoni had been observed. No Dissostichus eggs or larvae were observed. Larvae of Kreffichthys anderssoni and Electrona antarctica, and two unidentified egg types were found.

X-rays of vertebral half-centra, and X-rays and transmitted white light examination of caudal thorns, were used to estimate the ages of Amblyraja georgiana from the Ross Sea. Caudal thorns viewed with transmitted white light provided the clearest growth bands, but reading precision was low, producing uncertain age estimates. Furthermore, annual deposition of thorn bands has not been validated. The maximum age was 14 years, but this should be regarded as a conservative estimate of longevity because of the possibility that thorn growth ceases in large individuals. Von Bertalanffy growth curves are provided for females, males, and both sexes combined. There was no obvious difference between the sexes, so the curve for both sexes provides the best available representation of growth in the species: Lt = 70.8(1-e-0.308[t+1.10]), where Lt is the pelvic length (PL) in centimetres at age t. Applying this growth curve to the estimated lengths at maturity for males and females (64 cm and 66– 69 cm PL respectively) produced estimated ages at maturity of 6–7 years for males and 8–11 years for females. These estimates are near the middle to upper end of the range of ages at maturity for skates worldwide.

A number of morphological and reproductive measurements made on Antarctic toothfish Dissostichus mawsoni over the four summer seasons from 2000-01 to 2003-04 aboard the autoliner San Aotea II have been analysed. Summer seasons are defined here as the period from mid December to the beginning of April. Results of this study indicate measurable differences in a number of indices from toothfish found on the Ross shelf proper, as distinct from those sampled on the more isolated seamounts and features to the north.. These differences are fish body condition factor, reproductive development, sex ratio, and length modal distribution. D. mawsoni samples from the northern region showed that this section of the population was more advanced in reproductive development, were in poorer condition, showed a consistent and significant higher ratio of males to females, and showed a unimodal length distribution at a consistent peak over all seasons in comparison with the southern group in which the distribution was multimodal.

Two regions of the mitochondrial DNA, part of the cytochrome b gene and the control region, were sequenced in specimens of Antarctic skates from the Ross Sea (Pacific Ocean sector), the Australian Antarctic Territory (Indian Ocean sector), and around Elephant Island and the South Shetland Islands (Atlantic Ocean sector). Based on the DNA results, the species of Bathyraja referred to as B. eatonii in the Ross Sea appears to be a discrete sister species to B. eatonii from the Atlantic sector. In contrast, the DNA results for the undescribed species Bathyraja n. sp. would suggest that this species is common to the Atlantic and Pacific (Ross Sea) Ocean sectors. Two specimens of B. maccaini from the Atlantic sector and Indian Ocean sector appeared similar.