Studies of acoustic target strength of squid, 5: Effect of swimming on target strength of squid
1990
Arnaya, I.N. (Hokkaido Univ., Hakodate (Japan). Faculty of Fisheries) | Sano, N.
The effect of swimming movement on dorsal aspect target strength of squid was investigated through controlled experiments in a water tank with special reference to surume ika (Todarodes pacificus). These measurements were conducted by moving the squid in the sound beam of a transducer (Beam angle of 36 degrees, ensonifying frequency of 50 kHZ and pulse repetition rate of 150 pings/min). The different speeds of the movement were based on the results of the swimming speed measured in the aquarium. The average swimming speed was 0.28 m/s, and for comparison other swimming speeds of 0.12 m/s (slow swimming) and 0.40 m/s (fast swimming) were also employed. Results of the target strength measurements are expressed through the probability density function and mean target strengths. It is found that, contrary to bladder fish, swimming movement significantly effects squid target strength. The two major factors which influence swimming speed, body volume (or shape) and body tilt angle, have a significant effect on dorsal aspect target strength. This is because squid swim mainly by jet-propulsion: when swimming quickly the external diameter of their mantle changes by about 30%, but when swimming slowly it changes by only 10%. Furthermore, as swimming speed increases, tilt angle decreases. The average tilt angle typically decreases from about -30 degrees when hovering to near -3 degrees at high speed. For the present study in which the average swimming speed was 0.28 m/s and the normal speeds was not higher than 0.40 m/s, the average tilt angle was not more than -10 degrees
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