Sea Trials with Modulation Sector Scanning Sonar

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Abstract
Sea trials have been carried out to assess the potential application of a high resolution modulation sector scanning sonar to fisheries problems. The equipment had a range resolution of 8 cm and bearing resolution in the horizontal plane of 0·33°. Small shoals 3–5 m in diameter, believed to be pilchards, were detected up to the maximum range available of 183 m, which suggests a longer range potential for shoals of equivalent or greater target strength. Working from an anchored ship the sector scanner was used to track fish shoals in the open sea and to observe certain aspects of shoal structure and behaviour. Counts were made of the number of discrete targets in one shoal at different ranges. A midwater trawl was examined in detail with the sector scanner, and the behaviour of fish was recorded both ahead of and close to the trawl, and within the net. Fish were observed to keep pace with the doors and bridles and to escape from the path of the trawl. A small bottom trawl was detected by the sector scanner at a depth of 55 m.

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