Abstract
The dorsal aspect target strength functions of ten species of fish collected by bottom trawl were measured at two frequencies. The average TS were calculated with respect to fish tilt angle distribution, which is assumed to have a mean value of -5 degrees and a standard deviation of 15 degrees. Both maximum TS and average TS were normalized with the fish length (L[cm]) squared and the 2/3 power of body weight (W[g]), namely, TS=20logL+A, TS=20/3log W+B.The results obtained were as follows:(1) The target strength is generally higher at low frequency (25kHz) than at high frequency (100kHz). But for two species (Chub mackerel and Kandari), neither maximum nor average TS differ between low and high frequencies.(2) The target strength of Silver pomfret, a bladderless fish, is clearly lower than that of bladder fish at two frequencies.(3) The normalized TS (A or B) were calculated for the maximum TS (Amax or Bmax) and average TS (Aavg or Bavg). The results are: Amax = -63 - -59 dB, Aavg = -68 - -64 dB at low frequency; Amax = -65 - -59 dB, Aavg = -72 - -65 dB at high frequency, and Bmax = -52 - -45 dB, Bavg = -57 - -50 dB at low frequency; Bmax = -54 - -45 dB, Bavg = -60 - -52 dB at high frequency.
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