CCAMLR

One of the longest continuing data sets involving a marine organism in the Antarctic is that of annual estimates of breeding population size of Adélie penguins Pygoscelis adeliae at colonies on Ross Island, Ross Sea, 1959 to 1997. The sizes of these colonies have displayed significant interannual variability during the 29-yr period. We hypothesized that changes are related to natural environmental factors; and used path analysis to analyze annual variation in population growth in relation to physical environmental factors during that part of the record with comparable sea-ice satellite imagery from 1973 to 1997. The Ross Sea sector of the Southern Ocean lying north of Ross Island, from 150° E to 130°W, comprised our study area. Annual population growth measured during summer was explained best, and inversely, by the extent of sea-ice in the study area 5 winters earlier, and in some way related to the Southern Oscillation. Analysis of a subset of the sea-ice data from 1979 to 1997 indicated strong correlations to ice conditions in the eastern portion of the study area (174 to 130°W), and virtually no correlations to the western half (150° E to 175° W). This result supported other indirect evidence that the Ross Island penguins winter in the eastern Ross Sea/western Amundsen Sea. A demographic model indicated that variation in survival of juveniles and subadults might account for the observed population variation, and would also explain the 5-yr lag as 5 yr is the average age of recruitment to the summer breeding population. Extensive sea-ice during winter appears to reduce subadult survival, expressed subsequently when these cohorts reach maturation. We hypothesize that extensive (more northerly) sea-ice limits access of penguins to productive waters known to occur south of the southern boundary of the Antarctic Circumpolar Current, with starvation or increased predation disproportionately affecting less-experienced birds. The observed patterns of penguin population change, including those preceding the satellite era, imply that sea-ice extent has changed significantly over recent decades.

The Antarctic fur seal and macaroni penguin are sympatric top predators that occur in the Southern Ocean around South Georgia where they are, respectively, the main mammal and bird consumers of Antarctic krill. In recent years the population of fur seals has increased whereas that of macaroni penguins has declined. Both species feed on krill of similar size ranges, dive to similar depths and are restricted in their foraging range at least while provisioning their offspring. In this study we test the hypothesis that the expanded fur seal population at South Georgia may have resulted in greater competition for the prey of macaroni penguins leading to the decline in their population. We use a) satellite-tracking data to investigate the spatial separation of these two species whilst at sea during the breeding seasons of 1999 and 2000; b) diet data to assess potential changes in their trophic niches between 1989 and 2000. Foraging ranges of the two species showed considerable overlap in both years but the concentrations of foraging activity were significantly spatially segregated. Over the last 12 years the prevalence of krill in the diet of the two predators differed with less krill in the diet of macaroni penguins than Antarctic fur seals in recent years. Krill in the diet of macaroni penguins was significantly correlated with estimated krill abundance between 1994 and 2000. We found little evidence for direct competition between fur seals and macaroni penguins as, although very similar in trophic niches, they showed significant segregation in their spatial resource use.

In the Antarctic Peninsula region current, long-term changes in the physical environment have significant potential to affect populations of Antarctic krill Euphausia superba, a keystone foodweb species. To investigate this we analysed data on krill-eating predators from 1980-2000 at South Georgia. Indices of population size and reproductive performance showed declines in all species and an increase in the frequency of years of low reproductive output. Changes in the population structure of krill, and its relationship with reproductive performance, suggest that the biomass of krill within the largest size class was sufficient to support predator demand in the 1980's but not in the 1990's. We suggest that the effects of underlying changes in the system on krill population structure has been amplified by predator-induced mortality, resulting in breeding predators now regularly operating close to the limit of krill availability. Understanding how krill demography is affected by changes in physical environmental factors and by predator consumption and how, in turn, this influences predator performance and survival is one of the keys to predicting future change in Antarctic marine ecosystems.

The growth rates of Antarctic fur seal Arctocephalus gazella pups estimated from weighing cross-sections of the population were compared with measured/inferred changes in the the availability of their main prey species, Antarctic krill Euphausia superba from 1989-2000. There was no relationship between growth rate and mass at weaning and there were counterintuitive indications of higher growth rates in years of low krill availability. Biases reflecting changes in the component of the population available for sampling appear to invalidate the widely held assumption that inter-annual differences in growth rate can reliably be derived from differences in the slope of a linear relationship based on cross-sectional population samples. A new index was developed, based on the deviation of pup mass at age in each year compared to the multi-year mean, that was not dependent on assumptions of linearity. The indices of growth deviates produced a more logical relationship with other indices of pup development and related more appropriately to variations in prey availability. The potential impact of methodological biases on the interpretation of growth rate suggests that comparisons of growth rates should not rely on assumptions regarding the underlying growth pattern.

In marine ecosystems, characterisation of the foraging areas and habitats of predators is a key factor in interpreting their ecological role. We studied the foraging areas of macaroni penguins at Bird Island, South Georgia, throughout the breeding seasons of 1999-2001 using satellite tracking. We investigated differences in foraging ranges and characteristics between different stages of the breeding season, between sexes, between years and between individuals. During incubation, on foraging trips of 10-26 days, both sexes travelled long distances from Bird Island (male average = 572 km; female average = 376 km) in a north-westerly direction towards the Maurice Ewing bank; some individuals, particularly males, travelled across the Polar Front to forage in the Polar Frontal Zone. In contrast, during the chick-rearing period, both sexes mainly foraged relatively close (average 62 km) to South Georgia over the continental shelf. Foraging trip characteristics differed between males and females during chick rearing: females traveled further on average and on more direct trips. During chick rearing, males and females on longer foraging trips covered longer distances and travelled further from Bird Island. There were no interannual differences in characteristics of foraging trips although sex differences in some parameters varied between years. The bearings of chick-rearing foraging trips were non-random and most were in a north-westerly direction. Variation, both intra- and inter-individual, in bearings of foraging trips was high. Travel speeds were slower during foraging trips in the chick-rearing period than during incubation, probably relating to the differences in distances travelled. The stage of the breeding season, the constraints on the penguins during different stages, and sex were important in determining variation in foraging range and characteristics in macaroni penguins, but year and individual effects were relatively unimportant.

Antarctic krill Euphausia superba has a central role in the ecosystem of the Southern Ocean and knowledge of its growth rate is central to determining the factors influencing population dynamics. The length of Antarctic krill in the diet of Antarctic fur seals Arctocephalus gazella at South Georgia revealed a consistent increase in size between c 42 - c 54 mm over the period October-March, indicating growth rates much higher than predicted by existing models. Geographical variation in growth rate may result in 2 year-old krill at South Georgia attaining the same size as 3 year old krill in the Antarctic Peninsula region. The effect of geographical variation in growth rate on the population structure of krill has important implication for comparing the fate of individual cohorts over large scales and in the interpretation of krill life cycles.

The WG-EMM sub-group on methods is potentially in a position to change its emphasis from one that considers methods for collecting data as part of CEMP to one that considers and develops methods for analysing and using those data to provide management advice. Within this expanded role there are 4 key areas; 1.to maintain the process of validation and checking of the existing CEMP database, 2. to evaluate appropriate methods for analysis of existing parameters, 3. to develop methods to combine parameters at appropriate spatial and temporal scales and 4. to develop methods to take CEMP data forward as management advice.

Standardisation of methods to measure krill and estimate maturity and feeding status is essential if studies involving inter-comparison of datasets are planned. The methods used by workers from different institutes are outlined in the hope that standardisation by WG-EMM can be established and thus improve the quality of data from research surveys as well as through the CCAMLR International Observer Programme.

Three separate acoustic surveys of Antarctic krill (Euphausia superba) were conducted around South Georgia in the 2000/2001 season: in October 2000 (early-season); during late December / early January 2000/2001 (mid-season), and in March 2001 (late-season). The surveys were the first in a newly-planned five-year series of observations designed to complement and extend an existing time series maintained by the British Antarctic Survey regularly since 1996 (and on a more ad hoc basis since the early1980s). We hoped that conducting several surveys in one season would provide information on short-term variability that could be used to set data from more restricted “snap-shot” cruises in a broader context. The early- and late-season surveys were associated with logistic support voyages to South Georgia and were restricted to four transects within a box to the north-west of South Georgia. The dedicated mid-season survey covered that box in more detail (twice as many transects) and, in addition, examined boxes to the north, north-east and south-west of the island. Together these surveys provided temporally and spatially extensive coverage around South Georgia. Krill density in the western box in the early-season survey was very low (3.5 g m-2 ) but rose significantly (P = 0.048) by mid-season (to 34.7 g m-2). In a pattern that is consistent with observations from previous years, krill density in the western box mid-season was less than that in the eastern box (80.4 g m-2). Analysis of mid-season western survey box transect data revealed no significant difference between the mean krill density derived from only those 4 transects surveyed early- and late-season or from the full 8 transects. Our first occupation of a survey box off the central north coast of South Georgia mid-season revealed a krill density of 47.2 g m-2 that was intermediate between the eastern and western areas. The size structure of the krill in the central region also reflected a mix of those to the east (generally small) and west (generally large). Krill density to the south-west of South Georgia was 32.1 g m-2 mid-season. By March, krill density in the western survey area had fallen significantly (P = 0.04) from the mid-season high to 7.8 g m-2. Our multiple surveys at South Georgia have revealed major intra-annual changes in krill density at the island and have shown that the timing of the acoustic survey can significantly effect the estimate of krill density. The multiple estimates of krill density will allow indices of reproductive performance of top level predators to be compared to prey availability at time scales more appropriate than have previous single “snap-shot” acoustic survey data. This is a crucial step in the elucidation of response functions of dependent species to changes in krill abundance, and could be a useful contribution to ecosystem management.

The autonomous underwater vehicle (AUV) Autosub-2 was deployed on eight missions ahead of RRS James Clark Ross in the northern Weddell Sea and in the Bransfield Strait to assess avoidance of the research vessel by Antarctic krill Euphausia superba. The AUV was equipped with the same type of scientific echosounder as the research vessel (Simrad EK500 operating at 38 kHz and 120 kHz) and measured the density of krill along transect acoustically (g m-2 wet mass) prior to the ship’s arrival. We hypothesised that if krill avoided the ship, perhaps in response to radiated noise, then the ship should detect less krill than the AUV (which is very quiet). We were unable to detect any significant difference between the amount of krill detected by the ship or the AUV, either at the transect level or at finer scales within transects. We conclude, therefore, that avoidance by krill of RRS James Clark Ross will not significantly bias krill abundance estimates by this vessel.