Evaluation of threshold-induced bias in the integration of single-fish echoes

• Published in: ICES Journal of Marine Science
• Main Author: Reynisson, Páll
• Format: Text
• Language: English
• Published: Oxford University Press 1996
• Subjects: Regular Articles; Sebastes mentella
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/53/2/345; https://doi.org/10.1006/jmsc.1996.0047

In fisheries acoustics the bias introduced by a threshold in the integration of echoes from fish and other marine organisms has long been recognized. This bias is especially evident when the fish are observed in scattered concentrations. The method proposed here for evaluating the threshold-induced bias in the integration of single-fish echoes requires that the target strength distribution of the fish and the directivity pattern of the transducer be known, as well as the integration threshold in terms of the scattering cross-section of the fish. It is shown that for transducers usually employed in fisheries acoustics, the threshold effect is for all practical purposes independent of the transducer in question, thus generalizing the effect for a specific distribution. Results from direct measurements of the threshold-induced bias in the echo integration of oceanic redfish ( Sebastes mentella ) at 38 kHz are presented and compared with results obtained by the method proposed. The validity of the assumptions made in the numerical calculations and the effect of possible deviations from the true situation are discussed.