Sexual dimorphism in body shape of Antarctic krill (Euphausia superba) and its influence on target strength

AbstractSexual dimorphism in the body shape of Antarctic krill (Euphausia superba) was investigated and its influence on target strength (TS) was clarified using a theoretical scattering model. The TS which is used to convert acoustic backscatter to krill density was also presented. Body shape data were obtained from 456 specimens (54 juveniles, 200 males, and 202 females) collected off Adélie Land using a Rectangular Midwater Trawl. The sexual dimorphism manifested as a swollen cephalothorax in female krill with body lengths exceeding 40 mm. The TS of female krill was higher than those of male krill at low frequencies, even when body lengths were the same. This is because of the Rayleigh scattering region and the transition region to the geometric scattering region. The influence of the sexual dimorphism on the TS was small at frequencies exceeding 70 kHz, which are close to the geometric scattering region. The regression curve derived from the predicted TS of 456 specimens was in reasonable agreement with the measured TS in other previous studies, and the regression curve could be applied to the acoustic surveys of Antarctic krill

Diel vertical migration of zooplankton off Adélie Land (East Antarctica) during austral summer, 2010, inferred from echograms

第2回極域科学シンポジウム/第33回極域生物シンポジウム 11月17日（木） 統計数理研究所 3階リフレッシュフロ

Estimation of target strength of Sardina pilchardus and Sardinella aurita by theoretical approach

The target strength (TS) patterns of Sardina pilchardus and Sardinella aurita at 38 and 120 kHz were estimated by a prolate-spheroid model, using measurements of swimbladder length and width. The ratio of swimbladder length to total length (TL) was similar in both species, however the ratio of swimbladder width to TL was smaller and more variable for S. aurita. Assuming a normal distribution of fish swimming orientation angle (θfish) with mean ± standard deviation (SD) of 0 ± 10°, the normalized (by TL) average TS (b20) was estimated to be -64.0 dB (38 kHz) and -65.2 dB (120 kHz) for S. pilchardus, and -66.2 dB (38 kHz) and -67.2 dB (120 kHz) for S. aurita. Compared with currently applied b20 values at 38 kHz, our results under four different θfish assumptions (0 ± 10°, 0 ± 15°, -5 ± 10°, and -5 ± 15°) were 6-9 dB higher for S. pilchardus and 5-7 dB higher for S. aurita. This suggests four- to eightfold overestimation risk for S. pilchardus and three- to fivefold overestimation risk for S. aurita when using the currently applied b20 values

A new seismic survey technology using underwater speaker detected a low-velocity zone near the seafloor: an implication of methane gas accumulation in Tokyo Bay

Abstract Owing to the strict restrictions on the use of air guns in marine seismic surveys due to concerns about their potential impact on the marine ecosystem, there have been several cases where seismic surveys were not permitted. This tendency has been particularly significant in coastal waters where fishing activity is flourishing, which creates blank zones in seismic surveys. The authors, therefore, adopted underwater speakers as environment-friendly seismic sources that can be used under such restrictions. In December 2017, the applicability of underwater speakers as a seismic source was tested in a seismic reflection experiment in the northern part of Tokyo Bay. As a result, shallow subsurface structures were successfully imaged, and a low-velocity zone was detected 7–8 m below the seafloor. In this paper, the concept of environment-friendly seismic survey using underwater speakers is reported. In addition, the potential presence of a methane gas layer that was detected in the low-velocity zone is discussed. If the methane gas is widely distributed near the seafloor in the northern part of Tokyo Bay, a large amount of gas might be released into the water and then into the air when, for example, a large-scale earthquake occurs directly underneath the Tokyo Bay area. Given the high flammability of methane, the features and volume of its distribution must be precisely investigated from the perspective of earthquake occurrences in the metropolitan area