The influence of habitat and time of day on the occurrence of odontocete vocalizations in Onslow Bay, North Carolina

Abstract

To determine whether the occurrence and duration of odontocete vocal events varied by depth or time of day in Onslow Bay, North Carolina, we analyzed acoustic data collected by five underwater recorders. These recorders were deployed in July 2008 at three depths: two in shallow (64–73 m), one in medium (236 m), and two in deep (~366 m) water. We found that habitat influenced the occurrence of odontocete vocalizations, with significantly greater daily vocal activity from delphinids on recorders in deeper waters and sperm whale clicks recorded only on the medium and deep recorders. These findings suggest that a greater diversity and occurrence of animals are located in waters beyond the shelf break in this area, a conclusion supported by visual surveys. We also found an increase in the occurrence of delphinid clicks at night on the shallow and deep recorders, likely reflecting nocturnal foraging activity, and a regular nocturnal occurrence of sperm whale clicks on the medium-depth recorder located near the shelf break, suggesting that one or more sperm whales moved into that area to feed at night. These observations improve our understanding of the occurrence and behavior of odontocetes in this region of the U.S. Atlantic seaboard.