Broadbandwidth sound scattering and absorption from krill (Meganyctiphanes norvegica), Mysids (Praunus flexuousus
and Neomysis integer) and shrimp (Crangon crangon)

Sound scattering and absorption measurements were made of Northern krill (Meganyctiphanes norvegica) over the acoustical bandwidth of 30 to 210 kHz and compared to similar scattering measurements made of Antarctic krill (Euphausia superba; Demer and Conti, 2003a). The measurements of total target strength (TTS) match the SDWBA model (Demer and Conti, 2003b) recently developed for Antarctic krill, indicating its validity for other euphausiids species with similar size and shape. However, the total target strengths (TTS) of crustaceans with markedly different shapes (i.e. Mysidacea, a mix of Praunus flexuosus and Neomysis integer; and Decapoda, Crangon crangon), are not well predicted by SDWBA derived with the generic krill shape and scaled to animal length (L). This implies that crustacean target strength (TS) cannot be estimated accurately by a linear function of log10 (L), irrespective of shape, and brings in to question the validity of the current TS relationship used for Antarctic krill because that relationship was derived from data measured from multiple crustaceans including mysids and decapods. TTS and TS are dependent upon both L and shape, and krill, mysids, and decapods have significantly different shapes and girth-to-length relationships. On the other hand, modeled TTS and TS spectra for gravid and non-gravid krill appear to have differing amplitudes, but similar shapes. Additionally, the measurements of absorption spectra from decapods indicate that the absorption cross section increases with the volume of the animal. Collectively, these results provide tools for improving the detection of krill and more accurate estimates of TS, both vital to acoustical surveys. Furthermore, because TTS measurements from both northern and Antarctic krill support the SDWBA model, there are now more opportunities to apply acoustical measurements of one species to the acoustical characterization of the other. Thus, additional measurements of TTS over a range of frequencies, sizes and maturity stages will further improve our ability to identify species acoustically, aid ecosystem studies and assessment and management of both commercially and ecologically important euphausiid species.