Measurements of density contrast and sound-speed contrast for target strength estimation of Neocalanus copepods (Neocalanus cristatus and Neocalanus plumchrus) in the North Pacific Ocean |
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Authors: | Ryuichi Matsukura Hiroki Yasuma Hiroto Murase Shiroh Yonezaki Tetsuichiro Funamoto Satoshi Honda and Kazushi Miyashita |
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Institution: | (1) Laboratory of Marine Ecosystem Change Analysis, Graduate School of Environmental Science, Hokkaido University, 3-1-1 Minato-cho, Hakodate Hokkaido, 041-8611, Japan;(2) Laboratory of Marine Ecosystem Change Analysis, Field Science Center for Northern Biosphere, Hokkaido University, 3-1-1 Minato-cho, Hakodate Hokkaido, 041-8611, Japan;(3) The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan;(4) Hokkaido National Fisheries Research Institute, Fisheries Research Agency, 116 Katsurakoi, Kushiro Hokkaido, 085-0802, Japan;(5) National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa-ku, Yokohama Kanagawa, 236-8648, Japan |
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Abstract: | 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 to 1.009 in N. cristatus and from 0.995 to 1.009 in N. plumchrus. There were no correlations between prosome length (PL) and g. The values of h ranged from 1.006 to 1.021 in N. cristatus and from 1.013 to 1.025 in N. plumchrus and varied with changes in temperature. TS was estimated with the theoretical sound scattering model using the values of
g and h based on the temperature, salinity, and depth of the location where the specimens were collected. Regressions of the tilt-averaged
TS versus 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, ranged from −110.0 to −103.1 dB and from −121.4 to −109.7 dB, respectively.
There was a positive correlation between frequency and averaged TS: the higher the frequency, the higher the value of averaged
TS. The TS at 120 and 38 kHz varied from 14.8 to 16.4 dB in N. cristatus and from 17.9 to 18.7 dB in N. plumchrus, respectively; that at 200 and 120 kHz varied from 2.9 to 5.5 dB in N. cristatus and from 5.3 to 6.5 dB in N. plumchrus, respectively. |
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