The application of multibeam sonar technology for quantitative estimates of fish density in shallow water acoustic surveys

The paper describes the main drawbacks in the application of conventional acoustics in shallow waters, and reviews the advantages and limitations that existing multibeam sonar present in these ecosystems. New techniques and methods for adapting multibeam sonar to shallow waters are proposed and discussed. A method for analysing acoustic data from shallow waters through image analysis process is presented and some examples are considered. The results show that scattered fish can be observed individually and counted, and that schools are described in their morphology and behaviour. From these results an 'ideal' acoustic device is defined: a sonar operating at more than 300 kHz with a coverage of at least 120 degrees in one direction and, depending on the needs of the user, 15 degrees or 1 degrees (which can be modified easily) in the perpendicular plane. The beam opening-angle is 0.5 degrees in the centre beam, increasing to 1.0 degrees at the 60 degrees steer-angle, giving a total of 240 beams. Multibeam sonar data could be used for several purposes in shallow waters, in particular to estimate fish density and biomass, and study spatial and temporal behaviour of fish. (C) 2000 Ifremer/CNRS/INRA/IRD/Cemagref/Editions scientifiques et medicales Elsevier SAS.