CCAMLR

During the 1990s and early 2000s, populations of surface-nesting seabirds at Marion Island showed different trends, but for the majority of species numbers decreased. Reduced numbers of gentoo penguins Pygoscelis papua, eastern rockhopper penguins Eudyptes chrysocome filholi, Crozet shags Phalacrocorax [atriceps] melanogenis and probably macaroni penguins E. chrysolophus are most plausibly attributed to an altered availability of food. Decreases in numbers of dark-mantled sooty albatrosses Phoebetria fusca, light-mantled sooty albatrosses P. palpebrata, southern giant petrels Macronectes giganteus and possibly northern giant petrels M. halli may have resulted from mortality of birds in longline fisheries. However, populations of wandering Diomedea exulans and grey-headed Thalassarche chrysostoma albatrosses fluctuated around a stable level. Numbers of Subantarctic skuas Catharacta antarctica and kelp gulls Larus dominicanus breeding at Marion Island also decreased. Kerguelen Sterna virgata and Antarctic S. vittata terns remain scarce at the island. Trends for king penguins Aptenodytes patagonicus were not reliably gauged, but numbers probably remained stable or increased. There were large fluctuations in numbers of king penguin chicks surviving to the end of winter.

Mackerel icefish, Champsocephalus gunnari (Lönnberg), at South Georgia and Shag Rocks in the Southern Ocean have supported a fishery since the 1970s. This study has analysed the length-frequency distribution of C. gunnari from 10 bottom trawl surveys at South Georgia and 9 at Shag Rocks between 1987 and 2002. Most surveys were between December and February (summer), with one in September (Spring). Mean total lengths (TL) of age-classes were determined for each year by examining length frequency plots and tables, and by using CMIX. These methods were only applied where there was distinct modality in the length-frequency distributions. Lengths of age-classes derived by these two methods were not statistically different. Age-class 1 was found for all surveys at South Georgia but was absent for 3 years at Shag Rocks, potentially indicating greater recruitment variability at Shag Rocks. Age-class 4 was found for 8 surveys at South Georgia but for only 1 survey at Shag Rocks, indicating likely differences in mortality between localities. To compensate for variation in the time of surveys, the mean TL of age-classes were standardized to a common day of the year (16 January) based on the Bertalanffy growth curve. The CMIX estimated mean TL of age-classes 1, 2 and 3 were, respectively, 14.5, 23.8 and 30.2 cm at South Georgia, and 18.6, 26.8 and 33.6 cm at Shag Rocks. The mean TL of each age-class of C. gunnari at Shag Rocks was significantly larger than at South Georgia, although the annual growth increment at each locality was similar. This is further evidence that C. gunnari probably hatch earlier at Shag Rocks. A difference in hatching period between the two localities and differences in recruitment and mortality indicates that the mackerel icefish populations at South Georgia and Shag Rocks should be managed as two different stocks. At South Georgia, the mean TL of age-class 1 decreased significantly between 1987 and 1994, and this change was negatively correlated with summer sea surface temperatures during the previous year. Summer maximum sea surface temperatures at South Georgia have increased significantly between 1950 and 2000, and this shift in temperatures is likely to have changed the seasonal timing and level of primary production. The decreased size of C. gunnari may be the result of reduced nearshore food availability linked to climate variability.

Data from single and multi-frequency active acoustic surveys conducted annually in the vicinity of the South Shetland Islands, Antarctica were re-analyzed using updated procedures for delineating volume backscattering due to Antarctic krill, adjusting for signal contamination due to noise, and compensating for diel vertical migration of krill outside of the acoustic observation window. Intra-and inter-seasonal variations in krill biomass density and dispersion were derived from the re-processed data set for surveys conducted in the austral summers of 1991/1992 through 2001/2002. Estimated biomass density ranged from 1 to 60 g m–2, decreasing from mid-range levels in 1991/1992 to a minimum in 1992/1993–1993/1994, increasing to a peak in 1996/1997–1997/1998, and decreasing again through 2000/2001–2001/2002. Although this variability may be attributed to changes in the spatial distribution of krill relative to the survey area, comparisons with the proportion of juvenile krill in simultaneous net samples suggest that the changes in biomass density are consistent with apparent changes in reproductive success. A truncated Fourier series fit to the biomass density time series is dominated by an 8-year cycle and predicts an increase in krill biomass density in 2002/2003 and 2003/2004. This prediction is supported by an apparent association between cycles in the extent of sea ice cover and per-capita krill recruitment over the last 23 years and indications that ice cover in the winter of 2002 is seasonally early and extensive.

This overview reflects the main points of the feasibility of using the Antarctic Shag as a monitor of inshore demersal fish populations, including species of commercial importance. Likewise, the history of the development of this theme in relation to the EMM and FSA Working Groups of CCAMLR since the onset of the 1990s until the recent five year testing period of the Standard Method, is described. Among other conclusions, the analysis of pellets is an adequate method to estimate qualitatively and quantitatively the diet of shags and can reflect differences in fish availability between seasons and areas. Considering the similarities in foraging strategies and reproductive behaviour between Antarctic and sub-Antarctic shags, the methodology proposed could readily be used with other shag species in Antarctica.

Mackerel icefish (Champsocephalus gunnari) has a widespread distribution in the Atlantic and Indian Ocean sectors of the low-Antarctic region. Biological characteristics differ considerably between populations in the southern Scotia Arc and those living further to the north. Fish living in the north mature one year earlier than in the south. They have a much shorter life span and die after they have spawned for 2 – 3 times. The number of eggs produced per gram of body weight is higher in the north. Stocks have declined in most parts of the distributional range due to the impact of fishing and due to natural causes. Increases in populations of Antarctic fur seals at South Georgia and parts of the Indian Ocean appear to have led to increased predation on stocks of icefish. Shifts in hydrological regimes in the northern part of the distributional range have either started to lead or will lead to deteriorating living conditons for C. gunnari in the near future. Fish stock assessment under CCAMLR needs to take these constraints into consideration when providing advice on total allowable catches for fisheries management.