CCAMLR

One hundred and seventy three regurgitated casts of the blue-eyed shag or imperial cormorant Phalacrocorax atriceps were collected in a colony at Half-moon Island, South Shetland Islands, by weekly sampling throughout the breeding season from December 1993 to February 1994. Demersal-benthic fish were the main prey (91% in number) followed by mollusks and polychaetes. Six fish species were represented: Notothenia coriiceps, Nototheniops nudifrons, Trematomus newnesi, Harpagifer antarcticus, Gobionotothen gibberifrons and Pagothenia bernacchii. Notothenia coriiceps was the most frequent (F%=73) and important in mass (79%), followed by N. nudifrons and T. newnesi. The overall composition of the diet was qualitatively and quantitatively similar to that reported for the 1992/93 austral summer at the same colony. However, the pellet analysis throughout the 1993/94 breeding season showed that large N. coriiceps were the main prey during the chick rearing period, whereas when chicks were fledging/post- fledging the diet was more diverse and comprised chiefly prey of lower energetic content like other smaller fish species and non- fish organisms. This may reflect different degrees of energy demand by chicks at different stages of the season.

Data from the acoustic surveys of MV SA "Aghulas" and FRV "Walter Herwig", and the 1981 RRS "John Biscoe" South Georgia acoustic survey were analyzed by geostatistical methods. Estimates of mean density (g m-2) of krill and their variances are compared with published results (Trathan et al., 1993 and Murphy et al., 1991) from statistical analyses based on random sampling theory. A further high-resolution geostatistical analysis of the MV SA "Aghulas" (ping-by-ping) data set of the density of each individual aggregation is also presented. These analyses illustrate the problems of applying geostatistical methods to data from highly aggregated species which can show marked skewness in their histogram of density.

Dual frequency (38 and 120 kHz) acoustic surveys of shelf-break regions in the vicinity of Willis Islands, South Georgia and Coronation Island, South Orkneys, both in the Atlantic sector of the Southern Ocean, were carried out from RRS lames Clark Ross during January 1994. Difference in signal strength between the two frequencies (δMVBS = MVBS 120 kHz - MVBS 38 kHz) was used to partition acoustic targets into three biological categories: Antarctic krill; nekton (larger than krill); and zooplankton (smaller than krill). To counter the absence of krill length-frequency data from Coronation Island a new generic target strength (TS) relationship was derived. This relationship was used in conjunction with appropriate 120 kHz data to calculate krill density estimates for both survey areas. Krill distribution in both regions was extremely patchy, resulting in a highly skewed frequency distribution of density; for example one echo-integration interval contained 104 times the mean krill density. An approximate six-fold difference in mean krill abundance was detected between the two areas, but in comparison with previously published density estimates krill densities in both regions were extremely low (1.7 and 10.7 gm-2 for Willis and Coronation Islands respectively). Gross regional differences in pelagic faunal composition were apparent from inspection of echo-charts, and were quantified in detailed analysis of acoustic signals. The water column in the vicinity of Willis Islands was characterised by a series of diffuse but continuous scattering layers visible only at 120 kHz, and 73% of acoustic targets there were classified as zooplankton. In marked contrast, the Coronation Island region was dominated by dense, discrete patches classified as larger nektonic scatterers, whereas only 12% of targets were classified as zooplankton. Simultaneous predator diet analyses carried out at Bird Island, South Georgia, confirm the profoundly low levels of krill availability, and support the acoustic observation that the Willis Island region contained large numbers of small zooplankton such as the amphipod Themisto gaudichaudii.

Dual frequency (38 and 120 kHz) acoustic surveys were carried out in two regions along the northern coast of South Georgia from RRS lames Clark Ross during January 1994. Difference in signal strength between the two frequencies (δMVBS = MVBS 120 kHz - MVBS 38 kHz) was used to discriminate krill from other acoustic targets. A generic target strength to weight relationship was used to derive krill densities from 120 kHz signals attributed krill. At less than 1 g m-2 , mean weighted krill densities in both regions were extremely low in comparison with previously published density estimates, and provide further evidence of an extreme krill paucity around South Georgia in the 1993/4 season.

Anomalies detected in the firmware of a Simrad EK500 echo-sounder are described so that users are aware of potential problems and how to solve them.

Studied were four primary sources of uncertainty in krill density estimates from acoustic surveys. The variance in system calibration with a standard sphere was evaluated in relation to sphere material and diameter, water temperature, and pulse length (Demer and Hewitt, 1992). Calibration bias was investigated by comparing the theoretical and actual target strength (TS) values of four different standard spheres with the results of a calibration by self-reciprocity (Demer and Hewitt, submitted). Combining the results of these experiments, the accuracy and precision of a system calibration with a standard sphere were estimated as -1.2 dB and ±0.3 dB, respectively. Uncertainty in estimating krill TS was then investigated through in situ measurements (Hewitt and Demer, 1991). The TS data provided corroboration to an empirical model developed from a linear regression. However, TS values have been observed to vary as much as 8 dB, depending on the time of day (unpublished data). Also, Monte Carlo simulations have demonstrated the potential errors in developing empirical models from linear regressions of zooplankton scattering data (Demer and Martin, 1995). Therefore, the accuracy and precision krill TS estimates were conservatively estimated to be 0 dB and ±4.0 dB, respectively. The uncertainty in species delineation was also investigated, through the development of a statistical technique for remote species identification (Demer et al., submitted). The technique was used to apportion the integrated echo energy between two predominant scatterers, Euphausia superba and Salpa thompsoni. These studies indicated that scattering from S. thompsoni contributed to a positive bias in the krill density estimates of 0.6 dB. Finally, uncertainty due to the diel vertical migration (DVM) of Antarctic krill above a down-looking transducer was quantified through time-depth-density analyses (Demer and Hewitt, 1995). A method was developed for compensating acoustic biomass estimates for the effects of DVM. Applying the compensation function to survey data, the resulting biomass estimates were an average of 1.8 dB higher than those calculated disregarding biases due to DVM, with a standard deviation of 0.6 dB. Combining these four sources of uncertainty, the overall bias in krill density estimates was estimated to be 2.4 dB. From a boot-strap simulation the total variance was estimated as ±0.9 dB. Compensating for these components of uncertainty resulted an increase in krill density estimates of74%, with confidence limits of ±55%.

The Scientific Committee has requested advice on the feasibility of conducting a synoptic survey of krill biomass in Statistical Area 48. Example calculations are presented for estimating the amount of survey effort required to achieve a target level of precision using a stratified random sample design and results from the FIBEX and AMLR surveys. These calculations suggest that approximately 1.25 ship-months of survey effort can provide an estimate of krill biomass in Subareas 48.1, 48. 2 and 48.3 with a CV of 20%.

In fisheries acoustics, the standard sphere method of echosounder calibration is most commonly used (Johannesson and Mitson, 1983; Simmonds et al., 1984). To investigate the uncertainty in the method, a series of experiments were conducted in a deep tank (Demer and Hewitt, 1993). Results indicated that variations in pulse length (0.1 and 0.3 ms), water temperature (0.5 to 5.5 C), and choice of standard sphere (tungsten carbide (WC) - 33.2 mm or copper (Cu) - 30.5 mm), could cause corresponding variations in the system gains of 0.3, 0.2 and 1.5 dB, respectively. Additionally, system gain values calibrated with the 30.5 mm Cu sphere were consistently lower than those with the 33.2 mm WC sphere. Prompted by the latter result, direct measurements of target strength (TS) were made of four standard spheres (WC - 33.2 and 38.1 mm, Cu - 23.0 and 30.5 mm) in an anechoic tank. A calibrated hydrophone was used to measure the incident and reflected intensities of the pulse and the measured TS values were compared to their theoretical counterparts. The TS measurements of the 30.5 and 23.0 mm Cu spheres and of the 38.1 mm WC sphere were all about 1.5 dB larger than their theoretical values. However, the 33.2 mm WC sphere exhibited a larger difference of 2.5 dB. A system calibration by the method of self-reciprocity supported the magnitudes of these observed differences. Concluding from the results of these two sets of experiments, system calibration with an optimal standard sphere is estimated to be accurate to -1.2 dB, with a precision of 0.3 dB.

Interrelations between environmental variables, and between those variables and krill CPUE are discussed. Some reliable relationships is revealed. Based on those of krill abundance interannual fluctuations relationship hypothesis is formulated.