Multiyear study of blue whale calls in the north Pacific

Abstract

Multiyear acoustic recordings provide new insight on blue whale behavior in the Northern Pacific. We analyze six years of blue whale acoustic recordings made in the North Pacific as part of the ATOC (Acoustic Thermometry of Ocean Climate) and NPAL (North Pacific Acoustic Laboratory) projects. Blue whale calls are recorded as a component of ambient sound data, preserved as spectra with 500 Hz Nyquist frequency in 1 Hz bins, averaged over 170 s and recorded at 5 min intervals at each of 13 sites from 1994 to the present. These data verify a separation of blue whale calls into a Western Pacific version with two primary frequencies near 20 Hz and weak harmonic components, and an Eastern Pacific version with a primary near 17 Hz, a strong harmonic component near 50 Hz and a strong overtone near 90 Hz. The frequency of the Eastern Pacific blue whale call has shifted downward throughout the last 40 years, and the present data, combined with published sources, show the frequency shift to be secular and nearly linear with time. [Work supported by ONR.]