Broad-bandwidth, sound scattering, and absorption from krill (Meganyctiphanes norvegica), mysids (Praunus flexuosus and Neomysis integer), and shrimp (Crangon crangon)

Abstract

Abstract Sound scattering and absorption by Northern krill (Meganyctiphanes norvegica) were measured over the acoustic bandwidth of 30–210 kHz and compared with similar scattering measurements for Antarctic krill (Euphausia superba). The measurements of total target strength (TTS; energy scattered in all directions, averaged over all angles of incidence) match the SDWBA model (stochastic distorted-wave Born approximation) recently developed for Antarctic krill, indicating its validity for other euphausiid species with similar size and shape. However, the TTS of crustaceans with markedly different shapes are not well predicted by SDWBA derived with the generic krill shape and scaled to animal length (L). Therefore, crustacean target strength (TS) may not be estimated accurately by a linear function of log10(L), irrespective of shape, questioning the validity of the current TS relationship used for Antarctic krill derived from data measured from multiple crustaceans. TTS and TS are dependent upon both L and shape, and different crustaceans have significantly different shapes and width-to-length relationships. In contrast, modelled TTS and TS spectra for gravid and non-gravid krill appear to have differing amplitudes, but similar shapes. Additionally, measurements of absorption spectra from decapods indicate that the absorption cross-section increases with the volume of the animal.