The spatial distribution characteristics of ice-fish in the near-bottom layer are analyzed using the acoustic data obtained in the South Georgia area during the Russian bottom trawl surveys 2002. Acoustic and trawl density samples obtained in the near-bottom layer covered by trawl survey are compared. It is shown the possibility to estimate the bottom trawl efficiency. The high heterogeneity of horizontal and vertical fish distribution in the near-bottom layer of the survey area is shown. Influence of this fish distribution heterogeneity on trawl surveys efficiency and reliability is analyzed. In addition, the author discuses the results of bottom trawl surveys carried out in the South Georgia area during season’s 2000 and 2002. It is shown, that observed inter-seasons and inter-vessels differences between fish biomass estimates from bottom surveys may be to a considerable degree stipulated by heterogeneity of fish distribution in the near-bottom layer relative to the fishing gear operation zone (Russian trawl HEK-4M and UK trawl FP120) and the trawl station location rather than by the variability of the stock state.
Abstract:
An anatomically detailed acoustic scattering model has been used to estimate the backscattered target strength (TS) of six mackerel icefish (Champsocephalus gunnari) of total length 26.8–35.0 cm, at 38 kHz. The model used computed tomography (CT) scans of fish to determine the sound speed and density throughout the fish and simulated the complex interaction of the incident sound wave with the acoustic impedance contrasts throughout the fish. A preliminary length to tilt-averaged TS relationship is derived from the model results: 148)(log4.7010-=LTSwhere L is the fish total length in cm and is applicable to fish of length 26–35 cm. Existing in situ TS estimates are for smaller fish (16–26 cm) and hence are not directly comparable, but extrapolation of the in situ and model results reveal a large different in TS (6 dB at 26 cm) between the two datasets.
