Characteristics and seasons of blue and finback whale sounds along the U.S. west coast as recorded at SOSUS stations

Abstract

Blue whale sounds were doublets, each member lasting about 19 s. ‘‘A’’ sounds mostly occurred at 19.7 Hz; ‘‘B’’ at 22.2 with a short tail at 19.8. A few individual whales appeared to call for hours or perhaps days within hydrophone range with a constant background din of like signals. Sounds occurred in June–December, peaking as a high-level chorus in September–October and increasing ambient noise up to 20 dB. Interseasonal occurrence patterns at the same stations were remarkably similar. There was no clear evidence of longitudinal migration in the broad area, Mexico–Canada. These sounds, compared with subsequent blue whale recordings elsewhere, strongly suggest a dialect. Finback sounds mostly were doublets, each lasting about 1 s. The first member shifted downward (25.5–15.6 Hz) as did the second (34.6–17.5 Hz). Intervals between signals of a series varied from 9–21 s. Finback sounds were rare in summer, but numerous in the form of winter and spring choruses after nearly all blue whale sounds ceased. Localizations showed that the finbacks swam about 6 kn, and they had a source level of 183 dB re: 1 μPa, 1 m. Assuming even rough correlation between numbers of whales and sounds, SOSUS was able to reveal the seasonal occurrence and abundance of blue and finback whales over the entire western U.S. coastal region.