1Graduate School of Environmental Science, Hokkaido University, Hakodate, Hokkaido 041-8611, 2Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
The density and sound speed contrasts between Euphausia pacifica and sea water change with the season. In this study, we examined the effect on target strength (TS) and lipid content which is a cause of these changes. Density contrasts increased from spring to summer, but sound speed contrasts decreased. There was a negative correlation between density contrasts and wax esters, which are one of the lipid contents, and a positive correlation between sound speed contrasts and total lipid. The range of variation estimated by the DWBA model using both ratios was narrow. The mean TS of E. pacifica (TL 15 mm) ranged from −105.9 to −105.2 dB at 38 kHz, from −91.3 to −90.5 dB at 120 kHz and from −86.3 to −85.4 dB at 200 kHz.