SWIMMING BEHAVIOR AND TARGET STRENGTH OF ISADA KRILL (EUPHAUSIA-PACIFICA)

The swimming angle of isada krill (Euphausia pacifica Hansen) was meas ured in a tank and the target strength (TS) values were calculated usi ng a theoretical scattering model. The average swimming angle was 30.4 degrees (s.d.=19.9 degrees), which was about 15 degrees less than tha t reported for Antarctic krill (Euphausia superba). Parameters for the swimming angle distribution were substituted into the straight cylind er 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 d B at 50, 120, and 200 kHz, respectively. These differences will lead t o a 1/6-1/3, 1/30-1/7, and 1/65-1/12 proportional difference, respecti vely, in estimating abundances compared to estimates based on the maxi mum TS. Measurements carried out al higher frequencies are more influe nced by swimming angle than those at lower frequencies. However, the T S is less at lower Frequencies and it is more difficult to filter out noise. Thus, medium frequencies are suggested as the best for krill su rveys. Additionally, when conducting resource surveys by echosounding, it is necessary to assign the appropriate swimming angles for differe nt targets. Swimming angles need to be investigated further under a va riety of conditions.