CCAMLR

Acoustic surveys designed to assess krill abundance are costly in time and money, so the opportunity was taken in 1992 to produce a krill biomass estimate as a by-product of a fish stock-assessment surveyin CCAMLR Subarea 48.3. Acoustic transects were run between trawl stations using a sounder operating at 38 kHz and 120 kHz. The results have been analysed for all straight sections of track when the ship's speed exceeded 7 knots and when the ship was over the shelf around the main island of South Georgia. The appearances of echoes on echo charts were used to decide which echoes to include in krill estimates. The results were partitioned by depth to remove deep echoes that were thought to be mostly due to other scatterers. A threshold at one frequency was used to remove noise and any echoes too weak to be separated from the background. The remaining fraction was further subdivided using the ratio of backscattering strength at the two frequencies into a) echoes from krill-sized scatterers and smaller and b) echoes from larger scatterers. The data in the krill-size subset has been converted to density estimates, these are presented for day and for night sections of survey and at locations mapped around the island. An overall mean for daytime transects was 95 g m-2.

Horizontal and vertical velocities of fish schools were measured using an acoustic Doppler current profiler (ADCP). To determine three orthogonal velocity vectors (east, north, and vertical), it was required that the four ADCP beams simultaneously insonified a fish school, in the same depth bin. Velocity vectors which satisfied these conditions were extracted from individual ping velocity estimates and ensemble averaged to determine the average speeds and directions of fish aggregations. The results suggests that the ADCP can be a useful tool for observing fish behavior in certain situations. Some applications may include the quantification of horizontal and vertical migration patterns of large scattering layers and possibly vessel avoidance reaction. The method can be enhanced by utilizing the radial velocity components from each beam and correcting for platform motion.

Calibration of echo sounders for fish stock assessment are commonly performed using the standard sphere method (Johannesson and Mitson, 1983; Simmonds et al., 1984). To determine the accuracy of the method, direct measurements of target strength (TS) were made of three standard spheres (Copper (Cu) - 23.0 mm and tungsten carbide (WC) - 33.0 mm and 38.1 mm). At the best case range of 5 m, the TS measurements of the spheres differed from the theoretical values (derived by integrated intensities), by -0.1, 0.3, and 0.1 dB with standard deviations of 0.0, 0.3, and 0.2 dB, respectively. The operative measure (derived by peak intensities), differed from the theoretical values by -0.2, O.4, and 0.2 dB with standard deviations of 0.1, 0.3, and 0.2 dB, respectively. To characterize the precision of the method for a fixed pulse length (0.3 ms) and water temperature (18.9°C), a Simrad EK500 echosounder was used to measure sphere TS versus time. Over two 15 hour periods, the measured TS ranged 1.2 dB for a 23.0 mm Cu sphere and 1.4 dB for a 38.1 mm WC sphere. Vector admittance measurements were made of an ES120 transducer versus water temperature (0.06-16.8 °C). Although the measurements were not free-field and consequentially noisy, the trends versus increasing water temperature, indicated decreasing admittance at the operating frequency (119.047 kHz), decreasing resonance frequency, and increasing motional resistance. Judging from these experiments, system calibration at 120 kHz, at a fixed water temperature, using an optimal standard sphere, and a 0.3 ms pulse length, is estimated to be accurate to to.3 dB, and precise to ±0.2 dB for measuring TS; while accurate to ±0.2 dB, and precise to ±0.2 dB for echo integration. Additionally, more pronounced imprecision may be contributed by instabilities in the echosounder electronics. Furthermore, when operating under conditions of varying water temperature, associated changes in transducer performance may cause significant increases in calibration uncertainty. The temperature effects on system gain are consistent with predictions (Blue, 1984), and prior experimental results (Demer and Hewitt, 1993).