Particle motion polarization of offshore fish vocalizations versus ambient and ship noise.

Acoustic particle motion is the primary cue for fish hearing and a vector quantity that contains polarization information (including directionality) relevant to the directional hearing abilities of fishes. Polarization metrics, including ellipse orientation angle, ellipticity angle, and degree of polarization, have been recently applied to describe particle motion polarization in physical acoustical oceanography studies and have yet to be applied to in situ biological signals. This study harnessed data from a compact orthogonal hydrophone array deployed on the seafloor offshore of Florida (part of the Atlantic Deepwater Ecosystem Observatory Network) to investigate particle motion polarization properties of unidentified acoustic fish signals relative to ambient and ship noise. These properties described bivariate particle motion in a vertical plane formed by a source-receiver axis and orthogonal vertical axis. Particle motion of fish signals had more horizontal orientation than ambient noise and ship noise at the closest point of approach, which were more vertically oriented. Fish signals had narrower (small ellipticity) and more temporally stable (high degree of polarization) particle motion ellipses than ship and ambient noise. Applications of this analysis framework to fish bioacoustics studies and relevance of polarization properties to fish directional hearing and sound localization capacity are discussed.