2005). Acoustic methods are the most efficient for the mapping and monitoring of large benthic areas (Anderson et al. 2008), and a low-cost alternative to direct sampling for mollusc reefs (DeAlteris, 1988, Wildish et al., 1998, Allen et al., 2005, Grizzle et al., 2005, Hutin et al., 2005, Lindenbaum et al., 2008, Snellen et al., 2008, JiangPing et al., 2009 and Raineault et al., 2011). However, no similar method has been developed for infaunal mollusc populations such as razor clams. Atlantic razor clams inhabit intertidal and subtidal sandy bottoms because oxygen can diffuse
though them, which is not the case with muddy bottoms. These solenids can dig down to depths of 60 cm. A habitat preference CAL 101 for sandy bottoms with finer granulometry has been observed, although this has been related to larval settlement
(Holme, 1954 and Darriba Couñago and Fernández Tajes, 2011), and thus does not affect their distribution in seeded beds. Furthermore, as razor clams are not sensitive to sand composition or grain shape, their presence has to be detected independently of the different acoustic responses caused by the different types of sediments. The acoustic response from the ocean bottom has two components: scattering from the rough water-sediment interface and volume backscattering. The former is caused by the impedance contrast between sediment and water, whereas the latter originates from sediment grains, shell debris and infaunal species. Both contributions are so mixed that it is difficult to characterise Raf inhibitor the sediment structure using this
information (Diaz et al., 2004 and Anderson et al., 2008). It is generally assumed that for high-frequency echosounders (i.e. f ≥ 100 kHz) the backscattered energy originates mostly in the water-sediment interface Wilson disease protein (because of the high attenuation of the compressional waves in the sediment). However, when shell hash is present in the volume, its scattering may dominate above the critical (grazing) angle for frequencies just above 60 kHz ( Lyons 2005). The acoustic signal returning to an echosounder contains not only power but also phase information from the wavefront. Measurement of phase differences at different parts of the transducer allows point-like scatterers to be located: the phase difference is related to the angle formed by the scatterer’s line of sight and the acoustic beam axis. This is actually the principle behind split-beam echosounders (Foote, 1986, Bodholt et al., 1989 and Simmonds and MacLennan, 2005). The first commercial split-beam echosounder was introduced in 1984 and took advantage of new electronic technologies and developments in acoustic signal processing (Foote et al. 1984). The transducer of a split-beam echosounder is usually divided into four quadrants, which allow the measurement of angles in the athwartship and alongship directions.