CCAMLR

At-sea metabolism (C02 production) and water turnover of six breeding Grey-headed Albatrosses Diomedea chrysostoma were measured, using the doubly labelled water method, at Bird Island, South Georgia. Mean food consumption (estimated from a water influx rate of 1.01 l d-1 and data on dietary composition) was 1200 g d-1 or 50.4 W. At-sea metabolism (derived from a rate of CO2 production of 3.98 l h-1) was 27.7 W, 2.5 times the estimated basal metabolic rate (BMR). On average the birds ingested nearly twice as much food energy as they expended to obtain it. The metabolic rate during flight (estimated from at-sea metabolism and activity budget data) was 36.3 W (range 34.7-39.0 W) or 3.2 (range 3.0-3.4) times the predicted BMR. This is the lowest cost of flight yet measured, but consistent with the highly developed adaptations for economic flight shown by albatrosses. These results are briefly compared with data for other polar vertebrates (penguins, fur seals) exploiting similar prey.

Encaged aggregations of swimming krill have been measured at 38 and 120 kHz. The results indicate that the target strength values are substantially lower than previously assumed.

The vertical trajectory of phytoplankton in the Antarctic ocean is modelled as a random walk with a diffusivity derived from a simple Ekman layer model and turbulence theory. The surface mixing layer is shallow, but diffusion is large enough to effect the vertical distribution of phytoplankton in the upper ocean.

Acoustic data obtained on the 4-5 January 1987 aboard the R/V Professor Siedlecki were used in three descriptors of krill spatial aggregation: power spectra for krill biomass fluctuations in space, semivariogram (spatial autocorrelation of krill biomass) and the frequency distribution of krill biomass estimate. The wavenumber spectrum resembles a white noise at scales of 2-20 km, although at scales smaller than 1 km the spectrum appears to lose its power significantly. The semivariance of biomass does not vary significantly over most distances between points except for the distances smaller than 1 km. The computed frequency distribution of krill biomass is bimodal and appears to be the mixture of two lognormal distributions. These two distributions may correspond to the between and within patch biomass. These results of data analysis suggest that krill patch size or rather a basic swarm size should be smaller that 200 m, finest resolution of our data analyzed, and the real spatial distribution of krill should be the manifestation of the balance between the dispersion of the basic swarm units and long-range density-dependent attraction of the units. Simple dynamical and kinematical models can interpret the observed result.

The heavily exploited fish stocks of South Georgia have been declining since the early 1970's. Use of a non-parametric method revealed a significant change in this community from the 1986/87 survey to the 1987/88 survey. This change was the result of reductions in the abundance of Champsocephalus gunnari and Notothenia squamifrons, and the increased presence of Pseudochaenichthys georgianus. Fishing mortality was the most likely mechanism affecting these changes in C. gunnari and N. squamifrons, but reasons for the increase of Ps. georgianus are unclear. More surveys are needed to provide the data necessary to understand the dynamics of this system.

Commercial fish stocks around South Georgia have been declining since the early 1970's. A survey to monitor these stocks was carried out from December 1987 to January 1988. It complements earlier surveys in the same area and showed that most of the stocks are continuing to decline. Notothenia rossii is showing no signs of recovery and Champsocephalus gunnari is likely to continue declining in abundance.

Two primary issues are at question for hydroacoustic assessments of krill, The first is the methods applied to establish biomass in a survey area and the second is the improvement in accuracy of target-strength measurements. In the case of statistical methods, there are no clear guidelines for deciding what method is most appropriate, this is made even more difficult by the fact that most survey methods assume the population is fixed in space, relative to the sampling interval. There remain several unsatisfied needs for improvements in sampling design and tests for systematic trends in survey data collected from non-stationary populations, which have not been well addressed by present techniques. However, this does not invalidate the use of available methods to conduct surveys and analyze results. In the case of target-strength accuracy, even if the present values were very accurate, the issue of interest would seem to be not the absolute amount of biomass present in an area, but rather how it is distributed. The issue of patchy years vs more even distribution would seem to have more impact on ecosystem management than absolute accuracy of biomass estimates.