The mass density and sound-speed contrasts against surrounding seawater (g and h, respectively) of Neocalanus copepods (N. cristatus and N. plumchrus) were measured in 2006 and 2007 to compute the theoretical target strength (TS). The values of g ranged from 0.997 - 1.009 in N. cristatus and from 0.995 - 1.009 in N. plumchrus. There were no correlations in prosome length (PL) and g. The values of h ranged from 1.006 - 1.021 in N. cristatus and 1.013 - 1.025 in N. plumchrus and varied by changes in temperature. TS were estimated by theoretical sound scattering model with the values of g and h that computed by temperature, salinity, and depth where the specimens were collected. Regressions of the tilt averaged TS vs. PL were obtained at 38, 120, and 200 kHz. The averaged TS of N. cristatus and N. plumchrus at 120 kHz, which is widely used as a high frequency, were ranged from -110.0 to -103.1 dB and from -121.4 to -109.7 dB. The higher the frequency was, the higher the value of averaged TS. The differences of TS at 120 and 38 kHz varied from 14.8 to 16.4 dB in N. cristatus, from 17.9 to 18.7 dB in N. plumchrus. The differences of TS at 200 and 120 kHz varied from 2.9 to 5.5 dB in N. cristatus, from 5.3 to 6.5 dB in N. plumchrus.
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