Journal Article Measurement of the volume-backscattering spectrum from an aggregation of Antarctic krill and inference of their length-frequency distribution

12pp.79 - 87 , 2017-06
Antarctic krill, Euphausia superba, were observed using a broadband echosounder and their length-density distribution was inferred from the echo data through a least square inversion. The observation was conducted in the Indian Ocean sector of the Southern Ocean in January 2014. The transmit signal was a 10-ms linear frequency-modulated signal with a frequency sweep of 20–200 kHz. A large aggregation of Antarctic krill was observed at a sampling station over the continental shelf. The volume-backscattering strengths in the frequency range of 85–187 kHz were determined from the observed echoes. In addition, the signal-to-noise ratio at each frequency was estimated and the measured volume-backscattering strengths were evaluated before inversion for accurate inferences. In this inversion, Antarctic krill were modeled as prolate spheroids and the target strengths were predicted by the distorted-wave Born approximation. The acoustically inferred mean length was in good agreement with the mode length determined by a net sampling performed immediately after the echo sampling. Also, the acoustically inferred length-frequency distribution was in reasonable agreement with the determined one from the net samples.
広帯域エコーサウンダーによるナンキョクオキアミ(Euphausia superba)の観測を行い、そのエコーデータから最小二乗法によるインバージョンで体長別の個体数密度を推定した。観測は2014年1月に南大洋インド洋セクターで実施した。送波信号は、パルス幅10 ms、周波数掃引範囲20–200 kHzのリニアFM信号とした。大陸棚上の観測点においてナンキョクオキアミの大きな群れを観測し、収録したエコーから85–187 kHzにおける体積後方散乱強度を得た。さらに,正確な推定値を得るため、インバージョン前に信号対雑音比を推定し、測定した体積後方散乱強度を評価した。インバージョンでは、ナンキョクオキアミの形状を回転楕円体としてモデル化し、そのターゲットストレングスはdistorted-wave Born approximationによって推定した。音響推定した平均体長は、エコーサンプリング直後のネットサンプリングによって把握したモード体長とよく一致していた。また、音響推定した体長組成は、ネットサンプルから得た体長組成と合理的な一致が見られた。

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