Balaenoptera acutorostrata

Photo courtesy of NOAA

In general, all "calls" attributed to minke whales consist of very rapid sequences (lasting about a minute altogether) of short frequency-modulated (FM) pulses, in the frequency range of 30 to 450 Hz, with durations of 100-200 msec each. Two types of pulse sequences are most common:

•  In one type of "call", the pulse rate starts fast and ends slower, as in Figure 1.

•  In the second type, the pulse rate starts slow and ends faster, as in Figure 2.

Pulse sequences occur over a wide frequency range, and there are many variations of the pulse rate:

•  Figure 3 depicts a pulse sequence in which the pulse rate is relatively constant.

•  Figure 4 depicts a pulse sequence in which pulse rate varies during the "call".

Pulse sequences often occur in pairs with sequence pairs separated by several minutes. "Calls" (both paired or unpaired) with nearly identical features are usually repeated regularly every 6-12 minutes.

The four illustrations below are sound spectrograms of minke whale recordings, three of which were made by Navy ocean-bottom surveillance microphones in the western North Atlantic, and one (Figure 2) during winter 1994 research in the West Indies.

To hear each recording, click on the spectrogram. In each case, the sound you will hear is speeded up ten times faster than normal speed, which also raises the pitch of the whale sounds by a factor of ten (slightly more than three octaves). The curly-looking shapes in Fig 1and the dark vertical streaks around 300 hz in Fig 3 are parts of song from humpback whales. The humpback song is audible against this background as a series of high-pitched chirps and whistles.

Figure 1

Figure 2

Figure 3

Figure 4

Still Some Questions: Since very few simultaneous visual sightings and acoustic recordings of minke whales have been obtained, there is still some question as to whether the sources of the pulse sequences, as described above, are produced by minke whales, and only minke whales. In the late winter of 1994, an attempt was made to verify the source during a 10-day research cruise in an area 50-75 miles north of Puerto Rico and the Virgin Islands. Humpback songs and "minke" sounds were detected everyday, but no humpbacks or minke whales were ever seen.

Photograph by Adam Frankel.