The Annual Variability of Density and Sound-Speed Contrasts and Their Influence on Target Strength of Euphausia pacifica off the Coast of Kushiro, Hokkaido

We measured the density and sound-speed contrasts of Euphausia pacifica off the coast of Kushiro, Hokkaido from 2010 to 2014. These experiments were carried out in May–June (2010) and September–October (between 2011 to 2014). The density contrast was measured using the density bottle method, and the sound-speed contrast was measured by the time-of-flight method using a T-tube in each year. There were no significant differences in the mean values of the measured density and sound-speed contrasts by year, and the mean density and sound-speed contrasts through all years were 1.043 and 1.040, respectively. We also estimated the dorsal aspect mean target strength (TS) using the distorted-wave Born approximation-based deformed-cylinder model (DWBA model), finding that the relationships between total length (TL) mm and the mean TS at each frequency were TS = 60.9 log TL−177.2 at 38 kHz, TS = 57.2 log TL −163.6 at 70 kHz, TS = 52.9 log TL−151.5 at 120 kHz, and TS = 44.2 log TL−135.5 at 200 kHz. Since there was less annual variability in the density and sound-speed contrasts during the period examined in the present study, we recommend that the acoustic survey be conducted in September or October.