Blue whale vocalizations and the seasonal variability of ambient noise levels at the Aloha Observatory.

Abstract

In February, 2007 the Aloha Observatory added a cabled, bottom mounted hydrophone, which supplied a 20‐month continuous recording of ambient noise from 0.02 Hz to 11 kHz. This observatory is situated at a depth of 4700 m, 100 km north of Oahu. This site is known to be close to (possibly within) one of the primary habitats of NE Pacific and NW Pacific blues, and other whales during the winter. Numerous previously reported measurements throughout the NE Pacific Ocean have demonstrated that the fundamental frequencies of NE blues vocalizations have systematically decreased from about 22.5 Hz in 1958 to about 16.5 Hz in 2007 (the cause of the frequency shift is unknown). Limited measurements suggest that the fundamental frequencies of NW blues vocalizations have also decreased with time and were about 18 Hz in 2007. Ambient noise levels from the Aloha site were analyzed in 1‐Hz bands for 1 year to permit observation of the seasonal dependence of ambient noise levels as a function of frequency. The data reveal distinct peaks in noise levels at 16.5 and 18.5 Hz between November and March, consistent with observations of the fundamental frequencies of NE and NW blue whale vocalizations. Since blues and other whales repertoire of vocalizations cover a broad frequency range, the contributions of their vocalizations to noise levels at other frequencies will be considered.