KEBERADAAN MADIDIHANG (Thunnus albacares) DI SEKITAR RUMPON

Armada penangkapan ikan tuna di Indonesia banyak yang menggunakan rumpon sebagai alat bantu penangkapan. Teknologi alat bantu ini menyebabkan sumberdaya ikan tuna semakin rentan terhadap penangkapan. Hal ini berarti jika perikanan berbasis rumpon tidak dikendalikan, keberlanjutan sumberdaya akan terancam. Pengambilan data dilaksanakan pada tanggal 24 November hingga 3 Desember 2015 di perairan selatan Palabuhanratu. Penelitian ini bertujuan untuk menentukan lokasi dan periode waktu sebagai daerah penangkapan ikan yang layak. Pengamatan mengunakan teknologi hidroakustik portable scientific echosounder SIMRAD EY60, dengan lintasan perekaman mengelilingi rumpon berbentuk bintang (star survey). Hasil penelitian ini menunjukkan dugaan daerah penangkapan ikan tuna layak tangkap di sekitar rumpon berada pada kedalaman 200 hingga 500 meter dengan puncak keberadaan terjadi pada pagi hari.Most of tuna fishing fleet in Indonesia are using FADs as fishing tools. This technology leads tuna resources to be more susceptible to fishing activity. If the fisheries not well managed, the sustainability of fish resources will be threatened. The study was conducted from November 24th until December 3rd 2015 in south of Pelabuhanratu waters. The aims of this study to determine the location and time period as a suitable fishing areas. The studied through underwater acoustic devices portable scientific echosounder SIMRAD EY60 with star survey patterns around the FAD. This study showed that indication of the existence matured tuna based on acoustic observation around FADs occurred within the depth of 200 until 500 metre peak condition mostly found during the early of day light.

Onboard Realtime Processing of GPS-Acoustic Data for Moored Buoy-Based Observation

Realtime observations of vertical/horizontal seafloor movements and sea surface height associated with a huge earthquake are crucial for immediate recognition of its causal fault rupture, so that tsunami early warning can be issued and also the risk of subsequent ruptures can be evaluated. For this purpose, we developed an offshore monitoring system using a moored buoy platform to measure, in realtime, the three observables mentioned above and operated it on a trial basis for a year. While operating the system, GPS-acoustic observation of horizontal movement on the buoy was especially a new challenge. To achieve realtime GPS-acoustic observation under conditions of the limited power supply and narrow bandwidth in satellite communication, we developed special hardware suitable for use on a buoy and software to minimize onboard computational procedures and data transmission. The system functioned properly through the year; 53 regular weekly measurements and 55 on-demand measurements at arbitrary timings. Each measurement consisted of 11 successive acoustic rangings. The buoy tended to drift far from the preferred position for GPS-acoustic measurement, i.e., the center of the seafloor transponder array, due to strong current. The accuracy of the GPS-acoustic positioning achieved ∼46 cm (2σ) even only with “a single ranging” when the buoy was inside the array, while it degraded to ∼1.0 m when the buoy was outside the array. Although the 1.0 m accuracy is a detectable level of possible displacement due to a M8-class earthquake in the source region, further improvement to keep the drifting range smaller despite the current will enhance the utilization of the system.

Acoustic Observation of Zooplankton Using High Frequency Sonar (Observasi Akustik Zooplankton Menggunakan Sonar Frekuensi Tinggi)

Underwater acoustic sampling techniques provide an advantage over traditional net-sampling for zooplankton research. The research presents a methodology for extracting both biological and physical information from high frequency sonar. These methods can easily provide the information that will improve our understanding of the spatial and temporal distribution of zooplankton. Measured acoustic data converted into biological organisms and numerical physics-based scattering models were used in this research. The numerical backscattering process was modeled using the Distorted-Wave Born Approximation (DWBA) to predict the amount of sound scattered by a weakly scattering animal. Both acoustic measurement and DWBA modeled scattering patterns showed that acoustic scattering levels are highly dependent on zooplankton orientation. The acoustic backscattering from zooplankton depends on the material properties (i.e. the sound speed and density of the zooplankton), the shape and size, and the orientation relative to the incident acoustic wave. DWBA model significantly improve the accuracy and precision of zooplankton acoustic surveys. Zooplankton data measurement and DWBA model analysis provide a basis for future acoustical studies.