CCAMLR

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.

The hydroacoustic survey found a low krill abundance in most areas covered by last years survey. The total biomass in the vicinity of Elephant Island was estimated from 120 kHz data to be 260 k tons and that in the Bransfield Strait South of King George Island was 39 k tons for a total of 299 k tons in the combined areas. The estimated 200 kHz survey data were higher, giving 715 k tons near Elephant Island and 83 k tons in the Bransfield Strait. The survey results apply to 7,453 nm2 near Elephant Island and 2,894 nm2 in the Bransfield Strait. The full survey found (120 kHz data) 385k tons (in 7,787 nm2) in the Bransfield Strait and the area North of King George Island and 309k tons (in 8,836 nm2) in the expanded area around Elephant Island.

A general framework is presented to develop, test and integrate component models of the distribution and dynamics of Antarctic krill population at various spatial and temporal scales. We suggest that models of increasing complexity be developed iteratively for variability and patchiness of krill abundance. Incremental models should then be compared to statistical descriptions of the observed distribution patterns at various scales of observation to ascertain the plausibility of the model and identify critical processes to be added. An analysis of spatial distribution of krill in the Bransfield Strait area reveals that purely physical models of turbulent redistribution are not sufficient to explain krill distribution at small scales. We therefore propose to develop a modified diffusion-reaction model incorporating spatially variable growth rates of krill, krill loss rates due to predators, density-dependent attraction of krill to account for the small scale aggregations.