CCAMLR

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 However, TS values have been observed to vary as much as 8 dB, depending on the time of day (unpublished data).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.

During the Italian Antarctic Expeditions to the Ross Sea, in the years 1989 - 1994, using the 38 and 120 kHz BioSonics dual-beam hydroacoustic system, numerous swarms of krill Euphausia superba Dana were registered . Analysis of their physical parameters was carried out. It is known that insects appear to disperse according to a reaction diffusion model with a constant diffusion coefficient (Okubo, 1980, 1986). It was assumed that the above mentioned model can also be applicable to krill swarms. Using physical parameters of the krills' swarm (density, length), the above assumption was verified.
It was found that the vertical density pattern of krills' swarm, with a mean density of up to 1000 individuals/m3, is consistent with the insect model predictions. That is why cohesive forms of krill aggregations are usually termed "swarms" rather than "schools", which is normally used for fish groupings.

Historical hydrographic data from Bransfield Strait and the region west of the Antarctic Peninsula were analyzed to provide a description of water mass distributions and circulation patterns. Circumpolar Deep Water (CDW), which is characterized by temperatures above 1.0°C, salinities of 34.6 to 34.73 and oxygen values below 4.5 ml 1-1, is the most prominent water mass in this region, is found between 200 and 700 m, and is present in all seasons throughout the region examined. Below 200 m this water mass floods the continental shelf west of the Antarctic Peninsula. CDW is also found in Bransfield Strait, but the distribution is limited to the northern side of the Strait near the South Shetland Islands. Mixing of CDW results in reduction of the oxygen content of the overlying Antarctic Surface Water by 25 to 45%, which suggests an average annual entrainment rate for the west Antarctic Peninsula of 0.7 to 1.43 X 10-6 m s-1. The freshwater input needed to balance the salinity input from CDW is on the order of 0.63 m y-1, which can be supplied by local precipitation and advection of ice into the region from the Bellingshausen Sea, which then melts. The annual heat flux associated with CDW is 12 W m-2 , which is sufficient to melt this amount of ice. A second prominent water mass, Bransfield Strait Water (