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    Orientation of Antarctic krill in an aquarium

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    Y. Endo (Japan)
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    Acoustic methods appear to be the best way to estimate Antarctic krill abundance directly. Information on body orientation, however, is needed to obtain reliable krill acoustic target strength (TS) values, since TS varies with body orientation. Krill body orientation was measured in an aquarium aboard the R. V. Kaiyo Maru. Average body orientation was 45.6° (SD = 19.6°, N= 67), which was slightly less than that when animals were hovering, 49.7° (SD=7.5°, N=50). Such an acute angle of elevation would tend to reduce the reflected echo intensity when compared to animals swimming horizontally.
    Although mature females with marked swelling of the cephalothorax were not included in the experiment, the center of mass was found to be situated more to the anterior than in males and immature females. The hovering orientation, which is close to average body orientation, of mature males and mature females was estimated on the basis of Kils’ model. Mature females with swollen cephalothorax demonstrated larger tilt angles relative to the horizontal plane than did males, as the center of mass is positioned more to the anterior and the pleopods are less developed in females. It is concluded that the orientation, and therefore TS, may differ according to the maturity stage composition of individual aggregations