CCAMLR

This review paper summarises available information on the Somniosus (sleeper shark) subgenus, comprising three species; S. microcephalus, S. pacificus and S. antarcticus. Taxonomy, distribution, life history, biological characteristics and exploitation by fisheries is discussed. Somniosus antarcticus is caught as by-catch in Australian subantarctic fisheries at Heard and McDonald Islands (Division 58.5.2) and Macquarie Island. Based on present by-catch rates, a risk assessment for this species concluded that there is unlikely to be a serious risk to the stock. A tagging program and continued monitoring of by-catch of this species is ongoing.

The Australian-flagged longliner Eldfisk was approved by CCAMLR to undertake an exploratory fishery for D. mawsoni in Division 58.4.2 in the 2002/03 season. The results of the fishery involving 140 sets between 5 February and 25 March 2003 are summarised. Total catch from the three SSRUs fished was 111.8 tonnes, of which 88.7% overall was D. mawsoni. Principal bycatch species were macrourids (10.2% overall), skates (0.34%) and Muraenolepis sp. (0.27%). The length range of D. mawsoni caught was between 500mm and 1650mm total length, and most fish were in gonad state 1 to 3. Length at first maturity is estimated at 1000mm for males and 1300mm for females.
There were marked differences between the SSRUs in catch composition and D. mawsoni length frequency.

Molecular markers were employed to investigate the fine-scale stock structure of Patagonian toothfish in the west Indian Ocean sector of the Southern Ocean. Four collections of toothfish from the Kerguelen (n=1), Crozet (n=1) and Prince Edward and Marion (n=2) Islands were typed genetically using two fragments of mtDNA and seven nuclear microsatellite loci. MtDNA haplotype diversity was moderate; ranging from 0.331 to 0.343. Allelic variation at the seven microsatellite loci was widespread with observed heterozygosities per locus per collection ranging from 0.000 to 0.900 and allele counts from 1 to 21. There was no evidence for significant mtDNA heterogeneity among the four collections and only weak and inconsistent heterogeneity (based on minor allele frequency differences) at several microsatellite loci among the four collections. Both mtDNA and microsatellite FST values indicated no population sub-structuring among the collections. Genetic variance estimates of both mtDNA (?ST=-0.016) and microsatellites (?ST=-0.005) indicated all variation was seen within the toothfish collections. Comparisons with previously collected genetic data from Heard and McDonald Island collections (n=4) also showed a lack of genetic differentiation among the geographic groups, indicating significant gene flow among west Indian Ocean sector fishing locations.

We measured sink rates of longlines using two different methods: one was string wrapped around bottles and the other electronic time-depth recorders (TDRs). Bottles provided faster sink rates TDRs. Results obtained from the bottle method were more variable, and hence less reliable, among individual deployments than TDRs requiring repeated deployments on a longline. Sink rates measured to 2 m with bottles averaged 0.13 ± 0.02 m.s-1 compared to 0.21 ± 0.07 m.s-1 recorded by TDRs. This difference was statistically significant (t5 = -2.720, P = 0.042). Measuring sink rates to > 10 m proved difficult and unreliable with bottles. Bottles had a higher failure rate (51 – 68 %) than TDRs (> 90 %), for example, with string becoming entangled or bottles vanishing from sight. We recommend while bottles may be an appropriate means for measuring sink rates to shallow depths (> 5 m) that they should not be used for estimating sink rates to greater depths.

The summer diet of two species of icefishes (Channichthyidae) from the South Shetland Islands and Elephant Island, Champsocephalus gunnari and Chaenocephalus aceratus, was investigated from 2001 to 2003. C. gunnari fed almost exclusively on krill (Euphausia superba) in all years. The importance of other taxa (Themisto gaudichaudii, mysids, myctophids) in the diet was negligible. The average feeding rate of C. gunnari inferred from an exponential gastric evacuation model was between 1.0 and 1.5% body weight per day. Most of the stomachs of C. aceratus were empty. Stomachs with food contained mainly krill, mysids and fish. Among the fish taken, locally abundant species formed the bulk of the diet. They were Gobionotothen gibberifrons in 2001, Lepidonotothen larseni and C. gunnari in 2002 and L. larseni in 2003. An ontogenetic shift in feeding preference of C. aceratus was observed: Fish smaller than 30 cm fed on krill and mysids, while larger animals relied primarily on fish.

We studied the food and feeding of five species of icefish in the Elephant Island – South Shetland Islands region in March-early April 2003. C. gunnari took primarily krill. C. aceratus fed on krill and fish with some differences found between Elephant Island and the South Shetland Islands. The bulk of the diet of C. rastrospinosus consisted of krill and fish. Diet intensity, however, was low due to the progressing spawning season. C. antarcticus took almost entirely fish while P. georgianus was a fish feeder at Elephant Island and preyed on krill and fish further south. A large proportion of the stomachs was found to be empty in C. aceratus and C. antarcticus.