Measurement of fish target strength with a split-beam echo sounder

Abstract

Data derived with a 38-kHz split-beam echo sounder have been analyzed to yield target strengths suitable for use with echo integrators. This has required compensation for both thresholding and saturation, since these operations can significantly bias data intended for use with systems, such as echo integrators, whose dynamic ranges are much larger. A nonparametric statistical method is introduced for this purpose. Pure-species acoustic data are extracted in several two-species cases by a method for separating superimposed frequency distributions. Mean in situ target strengths are presented for cod, saithe, Norway pout, herring, redfish, and greater silver smelt. For com- parison with other data, these are expressed through the standard equation ~(TS) =20 log l +b, where ~(TS) is the mean target strength in decibels, and l is the fish length in centimeters. For gadoids of lengths from 10 to over 105 cm, b=–67.5 dB. For herring of lengths from 24 to 34 cm, b=–72.1 dB. The often-ignored problem of obtaining unambiguous biological data by trawl sampling is discussed.