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Characterization of the seabed surface sediments off the western Istrian coast (Northern Adriatic) using a quantitative echosounder

During the annual acoustic survey on small pelagic fish made during 2009 within PELMON Project, acoustic data on seabed were simultaneously collected. Though often omitted, such data could be useful in ecological and fishery studies. Variety of seabed substrate along the other biotic and abiotic environmental factors can influence fish distribution and behaviour. In studies aimed at investigating seabed surface and subsurface sediment, seafloor penetrating low frequency (e.g. 3.5 kHz) sound is usually used. In this study we analyzed echo signals from seabed collected at the frequency of 38 kHz, which is commonly used in fishery acoustic assessment surveys, in order to test their usefulness in terms of general seabed discrimination/classification. Investigated area is part of the northern Adriatic shelf (Fig. 1), greatly affected by modern and palaeo-Po River sediment supply, in which, generally, two main sedimentary regions could be distinguished (VDOVIĆ & JURAČIĆ, 1993 ; MATIJEVIĆ et al., 2008). In its W-NW part fine-grained (muddy) sediments prevail and belong to the modern Po muddy prodelta. Distribution of this mud is controlled by northward currents, creating tongue-shaped zone (PIGORINI, 1968). In the E-SE part relict coarse-grained (sandy) sediments dominate, reflecting terrestrial sedimentation on the alluvial plain during Pleistocene (PIGORINI, 1968). The area between these two sedimentation zones is covered with mixtures of two sediment types. In the eastern part (Istrian coastal zone) minimal recent sedimentation occurs, where strong karst relief influences the sediment distribution pattern, allowing very coarse and very fine sediments to be found near each other (MEISCHNER, 1973 ; FÜTTERER & PAUL, 1976). Acoustic data were collected aboard the R/V Bios Dva using SIMRAD EK60 split-beam scientific echosounder with calibrated hull-mounted SIMRAD ES38B transducer, along 6 transversal and 5 longitudinal transects off the western Istrian coast (Fig. 1). Collected data were post-processed with Echoview® (Ver. 4.80) software, used for data calibration, integration and extraction. Elementary distance sampling unit (ESDU) of 0.25 nmi (463 m) has been used to average acoustics data collected within cells bordered by sounder-detected sea bottom line and corresponding sea bottom -0.5 m linear offset line. For each cell mean volume backscattering coefficient (SV mean) was calculated. Obtained DGPS positioned SV values were imported in ODV (SCHLITZER, 2010) to generate graphical description (Fig. 2). In addition, 17 sediment grab samples collected along acoustic transects prospected during 2005, 2006, 2007 and 2008 were analyzed in order to obtain additional information on actual ground truth, and were used for seabed echoes classification. Acoustic data have indicated differences in the investigated area regarding sediment characteristics. Mud-dominated shallow western part revealed weaker backscattering strength (-17 to -26 dB), while deeper sand-dominated eastern part had greater SV values (-8 to -11 dB), as expected (MANIK et al., 2006). Transitional zone between these two areas coincide with the zone of mixed sediment, which was confirmed by dual nature (muddy sand, sandy mud) of analyzed sediment samples collected in this area. Analyzed acoustic data are in accordance with literature information and ground truth sediment data of the investigated area. Therefore, according to obtained results, we consider the acoustic data collected by scientific echosounder at 38 kHz as a useful tool for seabed sediment characterization, in both ecological and fishery studies or for establishing routine procedure for the preliminary geological analyses.

acoustic survey; Northern Adriatic shelf; seabed; sediment types

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