Acoustic detection and tracking of abyssopelagic animals: description of an autonomous split-beam acoustic array

Importance of pelagic animals in transport of organic matter at abyssal depths has been suggested based on vertical distribution and gut content analysis. We developed an autonomous acoustic instrument to detect individual pelagic animals, measure their target strength, and track their movements across specific depth boundaries in the deep sea. This instrument consists of a split-beam line array with a beam pattern narrow in the vertical and omnidirectional in the horizontal. Animals (acoustic targets) ⩾2 cm in length can be detected in an insonified radius of 100 m around the array with a position resolution of ca 42 cm. The velocity of a target animal can be resolved using a closely spaced ping sequence. The line array is deployed as a bottom-moored free vehicle with controller electronics, preamplifier, and a battery source for deployments up to several weeks at depths to 6 km. A deployment in the central North Pacific at 100 m altitude above the bottom (5762 m total depth) for 25 h recorded four targets moving through the acoustic field. Estimated size of the targets, based on initial target strength analyses, suggests that two are crustaceans and one is a grenadier fish. The fourth target is indicative of an animal larger than any we have observed in video camera deployments or caugh with baited traps and hooks at this station. Our initial measurements show that abyssopelagic fauna can be detected and their target strengths and movements measured with the acoustic array.

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