Experiences using the ES-400, split-beam echo sounder, with special reference to the single-fish recognition criterion
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Realistic estimates of in situ target strength can be made when the fish concentrations are acoustically resolved into single individuals. In the split-beam echo sounder, as in all other systems available for in situ measurement of target strength, the pulse length of the received echoes is used for single-fish recognition. Measurements made in a clean layer of 0 and 1-group herring showed that the ES-400 echo sounder could produce unrealistically high target strengths when the fish density increased slightly above the level where all fish were resolved as individual targets. Results from comparing mean target strengths at several density levels indicate a too wide acceptance window in pulse length when measuring small fish.