CCAMLR

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).

In the third year of standardised recording of oil, fishing gear and marine debris at Bird Island, South Georgia, one wandering albatross and two snow petrels contaminated with oil were reported. Ingested and regurgitated plastic debris were reported for wandering albatrosses (12 items) and grey-headed albatrosses (two items). One gentoo penguin chick was released from entanglement in a packaging band; another (uncut) band was found at a nest of a northern giant petrel. Parts of fishing gear were reported in association with grey-headed albatrosses (one squid jig decoy), black-browed albatrosses (one bird impaled with a long-line hook, another hook found at a nest) and wandering albatrosses (two squid jigs; six hooks swallowed and impaled (four in adults, two in chicks), the remainder being hooks and lines regurgitated beside nests). Levels of plastic debris in fishing gear associated with seabirds at South Georgia have returned to the levels recorded in 1993194, relatively few items having been reported in 1994195. The reduction in 1994195 probably related mainly to changes in the long-line fishing season. The evidence of discarding of plastic waste and the loss of long-line fishing gear, especially hooks, is a continuing cause for concern.