Acoustic backscattering strength analysis for riverbed substrate classification and its relationship with macrozoobenthos of delta mahakam water

Abstract

Riverbed substrate is a one of potential abiotic resources. It is useful as habitat, foraging area, and spawning area for most aquatic organisms (Susanto 2000). Riverbed has a very complex substrate composition from small sized substrate into rocky substrate. Over exploitation makes the quality and composition of bottom waters changed. Research on aquatic habitats are usually based on a sampling basis using a corer or a van veen grabber (Romimohtarto and Juwana 2009). But it is not time efficient, covering only narrow area and can be applied only in limited location. One of method that can help to overcome this deficiency is using hydroacoustic technology. Hydroacoustic technology is a technology which used to detect underwater objects by using sound propagation. Acoustic classification system of bottom substrate can predict the substrate type and its geotechnical properties. Hydroacoustic also has been widely used in various applications including marine geology, civil engineering, military science, and fisheries (Lambert et al. 2002; Richardson et al. 2002). This technology is able to provide information regarding the relief of the bottom, fish density, direction and movement of fish, as well as the type of bottom substrate in a relatively rapid and extensive reach. Sea bottom has characteristic in reflecting and scattering sound waves as well as the waters of the sea surface. The resulting effect is more complex due to the nature of the sea floor that is composed of diverse elements ranging from hard rock to soft clay and layers that have different compositions (Urick 1983). Some initial researches on aquatic habitats in Indonesia have been conducted by Manik et al. (2006) using the measurement of surface backscattering strength (SS) of Quantitative Echo Sounder (QES) and the SS value for identification of fish habitat, Pujiyati (2008) which using hydroacoustic method for relationship analysis between bottom substrate types with fish demersal community, Harahap (2010) in quantification of the value of bottom acoustic backscatter using multibeam echosounder, and Allo (2011) which applying the method of Manik et al. (2006) for the quantification and characterization of acoustic backscattering of bottom habitat in the Seribu island. This study was conducted in two phases. The first phase is collection of field data and the second is data processing and analyzing phase. Acoustic data collected were using hydroacoustics instrument SIMRAD EY60 120 kHz frequency and recorded by ER60 software. Sampling substrate was conducted at 9 stations acoustic data recording. Analysis of acoustic data include the value of SV E1, SV E2, and SS. Analysis of the substrate data include the percentage of the fraction (sand, silt, clay), and macrozoobenthos (density, diversity, dominance). Analysis of the relationship between acoustic values, the fraction of the substrate, and macrozoobenthos are using Principal Component Analysis (PCA). The results showed that the type of substrates that are found on sites are sand, muddy sand, sandy mud, and mud. Based on the value of SS for sand (- 12.97 dB), muddy sand (-13.96 dB), sandy mud (-17.14 dB and -15.64 dB), and mud (-30.87 dB to -19.25 dB). It shows also that sand has a high substrate roughness, hardness, and grain size larger than the type of mud substrate. The acoustic backscattering values of sand were greater than mud. Community structures that inhabit the location of the study are in very poor condition, very low abundance and unstable. Based on the PCA analysis, there is no distinct relationship between the macrozoobenthos community structure with SS value. However, the relationship between acoustic values, substrate type, and macrozoobenthos community structure can be explained by 77.55%, which means there is a strong relationship between the variance-forming components and have been able to explain the condition of the research location.