We monitored the underwater movements of Ganges River dolphins using stationed stereo acoustic data loggers. We estimated these movements using changes in the relative angle of the sound source direction (trajectory). Of the total acoustic recordings (66 h), 26.2% contained trajectories of dolphins, and 78.6% of these trajectories involved single animals, suggesting that dolphins tended to swim alone and were localized near the monitoring station. The observed trajectories were categorized as follows: staying type characterized by small changes in the sound source direction, moving type A (moving in the same direction), and moving type B (moving up and down the stream during recording). The average interpulse intervals of sounds in moving types A and B were significantly shorter than that of the staying type, suggesting that dolphins produce the former types of trajectories to echolocate across shorter distances during movement. The frequency of occurrence of moving type A increased during the night, whereas that of type B increased in the late afternoon and that of the staying type increased during the daytime. These results indicate that dolphins moving at night tended to use short‐range echolocation, whereas during the day, they remained in relatively small areas and used long‐range sonar.

In this paper, a sperm whale click analysis scheme is proposed in order to calculate the position of individual sperm whales in a group using data received at two arrays deployed near the surface. The proposed method mainly consists of two parts: short baseline (SBL) with classification and long baseline (LBL) with class matching. In SBL with classification, a click is automatically detected, and its direction of arrival is calculated. The clicks are then classified based on their direction vectors. The class data are then sent together with direction data and matched to the other array's class data. LBL with class matching is used for localization. The classification algorithm can be used to estimate the number of whales clicking and to list potential candidates for LBL matching. As a result, the proposed method is able to localize the positions of the whales in a group. The performance of the proposed method is evaluated using data recorded off Ogasawara islands with two arrays near the surface. The three-dimensional underwater trajectories of six sperm whales are extracted to demonstrate the capability of the proposed method.

the Autonomous Underwater Vehicle "r2D4", constructed in 2003, is a cruising type AUV weighing 1,600 kgf with a length of 4.4 m. It is fitted with a side scanning sonar, an interferometry sonar, a CTDO, and an in-site chemical analyzer "GAMOS" for measurement of manganese concentration. The AUV "r2D4" has been deployed off Sado-ga-shima Island along a fault line in the Sea of Japan (2003), over Kuroshima Knoll (2003), North West Rota 1 Underwater Volcano (2004) where it took video images of plumes, and Myojin-sho Underwater Crater (2005), and demonstrated its high performance in autonomy and reliability in hostile environments. During NH06-4 Leg 3 cruise in December 2006, we deployed "r2D4" in the rift valley of Segments 15 and 16 of the Central Indian Mid-Ocean Ridge System. The first area "r2D4" surveyed is "Roger Plateau (tentative name)" at 19 34S, 65_51E. The AUV "r2D4" performed three separate dives and found some anomalies in the concentration of manganese, but we could not identify the location of the origin of these anomalies. The second area is a narrow gorge in the middle of segment 16. The AUV "r2D4" descended to the east peak of the rift valley, with a depth of 2,400 m, near the center of the gorge. After calibration of its magnetometer, the AUV jumped off the cliff to the bottom of 2,700 m deep gorge. The AUV "r2D4" cruised over the bottom of the gorge for about 6 hours at 3.3 knots and took side scan images of the bottom surface covering 25 km2.The images show that there is a flat wide lava plain, which is 26 km in length and has an average width of 2.7 km, i.e. 70 km2 in area. At the north part of the plain, there is a small rock mound. When "r2D4" passed over the rocks, it found significant anomalies in the concentration of manganese and turbidity. Based on the results of the following Tow-yo survey, we came up with a conclusion that there is hydro-thermal activity at this location. It is concluded that based on a carefully-designed navigation plan the cruising type AUV can be a powerful tool for survey of mid-ocean ridge systems with complicated bottom topography.

No abstract