Study Group on Target Strength Estimation in the Baltic Sea (SGTSEB)

The Study Group evaluated the results of calculating backscatter properties of a group of herring and a group of sprat collected in October in the Baltic proper using x-rays to obtain the swimbladder shape and KRM (Kirchhoff-ray mode) modelling. Calculated target strength increases monotonically as a function of length for both the herring group (27 specimens) and the sprat group (25 specimens) at 38 kHz. Calculated mean target strengths varied from approximately –42 to –38.5 dB over a length range extending from 75 to 150 mm caudal (i.e. standard length). Mean target strengths of sprat were consistently higher than those of herring of the same size. Mean target strength-length curves differed noticeably in amplitude and pattern between the two species at 120 kHz. Mean target strengths of herring and sprat at 38 and 120 kHz both peak dorsally at approximately 85o relative to the caudal extension of the fish body length axis, but vary over a broad range for both species. This corresponds well to the mean tilt angle of approximately 5° tilt (front part up) relative to the body axis of the swimbladder for both species. Target strengths decrease as tilt angles deviate from horizontal (i.e. head up or head down). Calculated mean backscattering response surfaces showing the variations of the backscattering response with fish length/sound wavelength ratio (L/λ) and tilt angle contain the same general features for both herring and sprat. They peak at an angle of 85 o across the range modelled. Target strength is not consistently greater for one species over the other over the range of frequencies examined (38 – 200 kHz). Sprat target strength exceeds that of herring at low (38 – 90 kHz) and intermediate frequencies (130 – 160 kHz) and vice versa at other frequencies, but the difference does not exceed two decibels at any frequency.