Association of density and sound-speed contrast with maturity stage of Antarctic krill in the eastern Indian sector of the Southern Ocean

The density and sound-speed contrasts of Antarctic krill in the eastern Indian sector of the Southern Ocean were measured to estimate target strength (TS), which is essential for assessing abundance using acoustic methods. The density contrast of gravid females was significantly smaller than that of individuals at other maturity stages (1.039–1.045). Meanwhile, the sound-speed contrast was the highest in gravid females (1.049), followed by mature females (1.044) and juveniles (1.042). The sound-speed contrast tended to increase with total length. Assuming a body length of 38 mm as the average shape for this study, the TS was calculated using the distorted-wave Born approximation (DWBA) model with the density contrasts and the sound-speed contrasts for each maturity stage. Because the trends compensated each other, the maximum TS at 38 and 120 kHz was −79.9 dB and −64.0 dB, respectively, with no significant difference by maturity stage. In the TS pattern, in which body length and shape were estimated for each maturity stage, the main lobe was broader in gravid females, which differed from the TS pattern for other maturity stages. The effect of the expanded cephalothorax shape of the gravid females was particularly significant.