Acoustic target strength of Japanese common squid, Todarodes pacificus, and important parameters influencing its TS:: Swimming angle and material properties

The squids, one of the cephalopods, is important species in fisheries and ecology. If acoustic properties of squid can be well examined, especially target strength (TS), acoustic method can be provided a good tool for squid survey. The purpose of the study is to investigate the TS property of the Japanese common squid, Todarodes pacificus, and parameters influencing TS: swimming angle and material properties, density (g) and sound speed contrast (h). In the swimming angle measurement, the mostly head part of the squid stands below the horizontal axis, and averaged swimming angle was -17.7 degrees (s.d.: +/- 12.7 degrees). Based on the tethered TS measurements, the mean standardized TS values (b(20)) were found to be highly correlated with the tilt angle, and the resultant fitted equations for b20 were expressed as: b(20) = -73.3 + 0.48.theta + 0.0122.theta(2) + 0.00016.theta(3) for 38 kHz and b(20) = -72.6 + 0.53.theta + 0.0134.theta(2) + 0.00014.theta(3) for 120 kHz, where theta is the negative tilt angle in degrees. The material properties, g ranged between 1.0209 and 1.0396 (mean 1.0288), and mean h for individual, multiple and tilted squid were 1.041, 1.042, and 1.039, respectively. In the model estimation, the estimated TS data from the measured h (1.04) were higher than those of previous studied h (1.007) and were close to maximum TS data that were obtained from live squid. The results suggest that further work is needed to conduct the TS measurement for various sized squid, and to decide suitable acoustical parameter for model estimation. These data can help in making acoustics a quantitative survey tool for biomass estimation of the squids.