Swimming behaviour and target strength of isada krill (Euphausia pacifica)

Abstract

The swimming angle of isada krill (Euphausia pacifica Hansen) was measured in a tank and the target strength (TS) values were calculated using a theoretical scattering model. The average swimming angle was 30.4° (s.d.=19.9°), which was about 15° less than that reported for Antarctic krill (Euphausia superba). Parameters for the swimming angle distribution were substituted into the straight cylinder model and the corresponding TS values were determined. The mean TS values of 16.4 mm isada krill for hovering animals were lower than the maximum TS; differences were 7.8-5.0 dB, 14.8-8.6 dB, and 18.1-10.8 dB at 50, 120, and 200 kHz, respectively. These differences will lead to a 1/6-1/3, 1/30-1/7, and 1/65-1/12 proportional difference, respectively, in estimating abundances compared to estimates based on the maximum TS. Measurements carried out at higher frequencies are more influenced by swimming angle than those at lower frequencies. However, the TS is less at lower frequencies and it is more difficult to filter out noise. Thus, medium frequencies are suggested as the best for krill surveys. Additionally, when conducting resource surveys by echosounding, it is necessary to assign the appropriate swimming angles for different targets. Swimming angles need to be investigated further under a variety of conditions.