Improved mapping of schooling fish near the surface: comparison of abundance estimates obtained by sonar and echo integration

A method is described for recording fish schools near the surface by means of a computerized sonar system, a conventional echo integration unit, and sampling by a near-surface trawl. The sonar system consists of a 95 kHz SIMRAD SA950 multi-beam sonar connected to a work station with software for automatic detection and measurement of schools, and logging of the sonar data. The technique for near-surface trawling is based on re-rigging a pelagic trawl by extending the upper bridles, removing the weights, and attaching two large buoys to each wing. Pelagic fish stocks were mapped in the north-eastern Atlantic and off Namibia. The abundance estimates obtained by the near-surface recording method were compared with that obtained by conventional echo integration alone for four different surveys. It is concluded that the near-surface recording method can provide more precise mapping and abundance estimates of pelagic fish stocks in schools near the surface.