CCAMLR

The oceanography of the South Georgia region is principally that of the Southern Ocean rather than the South Atlantic. A combination of factors, including advection, local bathymetry and high local productivity, leads to relatively large concentrations of krill (Euphausia superba) in the region and this is a food source for large populations of penguins and seals that breed at South Georgia. A history of over-exploitation of the marine resources of the region has led to the development of ecosystem approaches to management. The current system for managing krill harvests includes precautionary features that account for interannual variability in krill populations and the food requirements of natural predators. However, in future it may be possible to use information from the natural krill predators to set reasonable levels of krill exploitation.

The study tested the hypothesis that the distribution of critical habitat for foraging by female Antarctic fur seals breeding at the island of South Georgia was influenced by physical gradients in the oceans and also by the need to avoid local competition for food. It also tested the hypothesis that fur seal predation was capable of causing local depletion of prey species. When foraging in support of dependent offspring, fur seals travelled down physical gradients defined by the bathymetric features of the continental shelf around the island of South Georgia. Fur seals foraging from different sites followed the same pattern of travel. There was no detectable difference in this behavior among years when there were different amounts of prey available. Females were constrained to forage mainly within 100 km of the location at which the offspring was being raised. When this constraint was removed at the end of lactation, females foraged to much greater ranges and there was evidence that they dispersed to specific regions associated with the continental shelf east of Patagonia (>1000 km) and to the northern edge of the Antarctic pack ice (500 km). A model of the spatial distribution of foraging by lactating female fur seals predicted spatial distributions that were consistent with past observations from ship-based surveys. The model also allowed estimation of the spatial impact of fur seals on krill. This suggested that, in extreme cases and assuming that krill influx is limited, female fur seals could eat most of the krill present in some regions where they forage intensively. However, mean consumption was about one-tenth of the mean density of krill.

1. This study examined the changing status of the marine ecosystem at the island of South Georgia (Southern Ocean) using up to 27 variables measured over 22 years from 3 upper trophic level predators that specialise in foraging upon krill (Eupuasia superba Dana). These variables included population size, breeding performance, offspring growth rate, foraging behaviour and diet. A method was developed for reducing these multivariate time-series to a single vector, called a combined standardised index (CSI).
2. Sensitivity analyses showed that missing values had a large effect upon the accuracy of the CSI but this effect was reduced if the individual variables were highly correlated. The level of correlation and proportion of missing values within the empirical data set were within the acceptable range. Individual variables had widely varying influence upon the CSI but, in general, those with longer time series had the greatest influence.
3. Principal components analysis showed that variables representing offspring growth tended to explain the greatest proportion of the variability in the CSI and this was followed by variables representing diet.
4. There were 3 years in which the CSI showed extreme and significantly low values. There was a significant non-linear functional response (similar to the Holling Type II functional response) between the overall CSI and krill biomass and a similar relationship existed when the CSI was calculated for each species individually.
6. Separate analysis of variables that were likely to be representative of changing population size showed the presence of a significant decline between 1977 and 1998. There was no trend in the CSI from variables representative of foraging conditions during the summer breeding season. The study has shown that the marine ecosystem at South Georgia shows acute but transient variability that is amplified in the response of upper-trophic level predators. There is less certainty that trends in populations are a consequence of shifts in the degree to which the ecosystem can support krill-feeding seals and penguins.

This paper describes current research plans to estimate the size of land-based marine predator populations breeding at South Georgia with a view to providing critical data about interactions with commercial fisheries. The research will also develop methodologies that reliably assess trends in the abundance of these populations. The research will focus upon predator species that depend upon marine resources that are also harvested by commercial fisheries, principally Antarctic krill. It will concentrate upon those species where there is a high probability of interaction with fisheries and significant potential for resource competition under existing fisheries management policies. The research will also develop new methods that reliably estimate predator population sizes whilst minimising the level of disturbance associated with data collection.

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.