CCAMLR

A major biological/physical survey of the waters off East Antarctica was carried out between January and April 1996. The focus was on the distribution and abundance of Antarctic krill in CCAMLR Statistical Area 58.4.1 but in addition, measurements were also made on a whole suite of oceanographic and biological variables over the entire survey area. This paper summarises the range of variables measured and provides an overview of some preliminary results.

In January-March 1996 a hydroacoustic survey for Antarctic krill was conducted in CCAMLR Division 58.4.1 for the purposes of estimating overall biomass (B0). The krill biomass in the area surveyed (873,000 km2) was estimated to be 6.67 million tonnes with a CV of 27%. Krill were more abundant in the west of the survey area 800-1200E than in the 120°-150°E region. The majority of the krill detected were found in the top 80 metres of the water column.

The CCAMLR Ecosystem Monitoring Programme has two aims: to detect changes in critical components of the antarctic ecosystem, and to distinguish between changes due to the harvesting of commercial species and changes due to environmental variability. Data from Seal Island are used to construct a multivariate model which relates chinstrap penguin breeding success, krill abundance and sea ice conditions, which effectively predicts chinstrap success given sea-ice data with an R2 of 0.914. This model is then used to propose a method of distinguishing between effects of environmental variation and harvesting on chinstrap breeding success. Other Antarctic Peninsula predator parameters are also briefly analysed. The results demonstrate that this methodology could be applied to other CEMP sites, even with the relatively short time series now available, and provides a way forward in the analysis and interpretation of the CEMP indices.

Analysis of seasonal sea ice cover of a 1,250,000 km2 area off the northwestern side of the Antarctic Peninsula indicates that four "ice events" have occurred during the last 1 7 years. The most recent event was first apparent in 1994 and continues through the latest data available. Variability between ice events in seasonal timing, areal extent, seasonal duration and persistence over multiple years is apparent. Annual curves of sea ice cover were integrated over time to produce an annual index of sea ice cover in units of 106 km2-months.

Results from two acoustic and net surveys conducted in the vicinity of Elephant Island during the 1996 austral summer indicate very good krill recruitment from spawning in 1994/5. Areas of high krill density were mapped north of King George and Elephant Islands where water depth was greater than 200m. One-year old juvenile krill numerically dominated catches during the first survey in January and were widely distributed throughout the southern portion of the survey area. Large, sexually mature adult krill were caught during both surveys north of the islands, but were numerically dominate only during the second survey in February-March. Intermediate size krill were caught in very low numbers reflecting poor recruitment from spawning in 1992/93, and 1993/94. Biomass and abundance estimates were the highest since 1992 when the effect of good recruitment from spawning in 1991 was observed. The abundance of salps was similar to 1995 and two to three orders of magnitude less than observed in 1993 and 1994. Following three winters of relatively low sea ice cover off the western side of the Antarctic Peninsula, the winter of 1994 marked the beginning of a period of relatively extensive ice coverage continuing through 1995. These observations support the hypothesized relationships between winter sea ice conditions, the lack of a spring-time salp bloom, the timing of spawning by adult krill, and the success of krill recruitment proposed by Loeb and Siegel (1994a) and Siegel and Loeb (1995).

Four indices of prey availability are calculated for prey surveys conducted in the vicinity of the Seal Island CEMP site during the austral summers of 1990-96. The indices are measures of average prey density, depth, distance from Seal Island, and persistence over time. Acoustic data from two AMLR surveys each year were sub-sampled to include the foraging range of predators breeding at Seal Island. The average depth of the prey field and its average distance from Seal Island were positively correlated; no other relationships between the indices were apparent. Indices of prey availability were compared with indices of predator performance at Seal Island. The depth of the prey field and its distance from Seal Island appear to have a positive effect on the duration of chinstrap foraging trips, but not on breeding success. The distance of the prey field from Seal Island appears to be negatively correlated with both the duration of fur seal foraging trips and pup growth rate.

There is a growing body of evidence that the climate of the Antarctic Peninsula region has been warming over the past 40 years with an associated decreased frequency of winters with extensive sea-ice development. These trends potentially will have a major impact on the structure and function of the Antarctic marine ecosystem. In the Antarctic Peninsula region winter sea-ice coverage is a major factor regulating recruitment and population size of Antarctic krill (Euphausia superba) and population dynamics of salps (Salpa thompsoni). Strong krill recruitment success there follows years of extensive winter sea-ice development and large summertime salp blooms follow winters with relatively little sea-ice. An order of magnitude decrease of krill population size, increased incidence of massive salp blooms, and decreased abundance of krill-dependent Adelie penguins (Pygoscelis adeliae) in the past 15 years suggests that the food web may be affected by climate change. The 1994/95 austral summer season followed the first prolonged winter sea-ice season in three years and provides a strong contrast to the previous years which had little or no winter sea-ice development. The hypothesized relationships between krill and salp population dynamics and winter sea-ice conditions are confirmed and the relative importance of krill and salps within the Antarctic food web are assessed here.