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Development and testing of two towed volumetric hydrophone array prototypes to improve localization accuracy during shipboard line-transect cetacean surveys
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  • Alternative Title:
    Recovery records of green turtles observed or originally tagged at French Frigate Shoals in the northwestern Hawaiian Islands, 1973-2013
  • Description:
    "Passive acoustic monitoring (PAM) of cetaceans using towed hydrophone arrays is a principle component of shipboard cetacean population surveys conducted by the National Marine Fisheries Service (NMFS). The traditional array design used by the NMFS includes a linear configuration of hydrophones housed inside a castor oil-filled polyurethane tube that is towed behind the ship while traveling at approximately 10 knots. Linear arrays are excellent tools to detect vocal cetacean groups and estimate their location during real-time operations. Conical bearing estimates are obtained using the time difference of arrival (TDOA) between individual cetacean sounds on two hydrophones in a linear array. Once bearings to the animals converge relative to the ship, a perpendicular distance estimate from the transect line to the animals can be determined (Rankin et al., 2008). Depending on the circumstances, the acousticians may then direct the ship to turn in order to guide the visual observers to the location of the animals. However, acoustic tracking can be challenging due to the limitations of the bearing information from the linear array. The convergence of conical bearing angles cannot discriminate between signals originating from left, right, up or down. For dolphins that primarily vocalize in surface waters, the up/down ambiguity is not a large concern, but the left/right ambiguity often requires more ship time to determine the correct location of the animals. For deep-diving species like beaked whales, the ambiguity in declination angle can result in a biased estimate of their perpendicular distance from the transect line (when projected to the surface). A more efficient and precise localization method would offer better results and more accurate data for several cetacean species"--Introduction. [doi:10.7289/V5TM784H (http://dx.doi.org/10.7289/V5TM784H)]
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