Sperm whale echolocation clicks heard 50 kilometers away within the Salish Sea?

One of the most intense sounds emitted by animals on Earth is the echolocation click of the sperm whale.  When Yukusam the 13-14 m lone male sperm whale swam through the Salish Sea last month, he was recorded on a suite of hydrophones including the Orcasound Lab live hydrophone in Haro Strait and simultaneously the Lime Kiln hydrophone 5 km away.

Here’s an example of what some of his most intense clicks sounded like as he traveled south along San Juan Island’s west side in the darkness of March 31 (~8-11pm) —

A series of 15 sperm whale clicks, separated by about 4 seconds, as recorded at the Orcasound Lab hydrophone.

We can learn a lot about Yukusam’s acoustic behavior by carefully comparing the available recordings and aggregating as much information as we can about where Yukusam was located — both visually on the surface, and even acoustically underwater when he clicked (more on that in a future post). One intriguing result from observations within the southern Strait of Georgia seems to be that his clicks can sometimes be heard as far away as 50 kilometers!

Here we review some of the evidence that this may be true, along with observations that his clicks are commonly detected at ranges of 5-10 km. We also provide some pointers into the raw data, and offer some back-of-the-envelope bioacoustic analyses of his clicks.

Yukusam’s trip through the Salish Sea

Map of sperm whale locations in WA and BC

The yellow whale symbols are key sightings of Yukusam at the surface. Explore the map by clicking on symbols, or view the map interactively on Google maps.

Basically, Yukusam explored Johnstone Strait from mid-April to mid-March, 2018. Then he took about a week to work his way to the southern Strait of Georgia. About a week later he continued south, made his way through Haro Strait, and presumably returned to the Northeast Pacific via the Strait of Juan de Fuca.

Spreadsheet of Yukusam’s locations in Johnstone Strait and the Salish Sea

More specifics about where Yukusam (and other sperm whales) have been within the inland and coastal waters of WA and BC… The red highlighted periods are times when he may have been recorded by hydrophones…

Strait of Georgia potential clicks

At about the same time on 3/27/2018 that Yukusam was located north of Nanaimo (around noon)by John Ford, Brianna Wright, and Eva Stredulinsky, some potential echolocation clicks were recorded by the southern Strait of Georgia hydrophones maintained Ocean Networks Canada. Here are some of the 5-minute raw data files (spectrogram followed by mp3 file) with the strongest potential click signals (look for the tiny vertical light-blue lines that indicate brief power peaks near 1-3 kHz) during ~12:30-13:15 local time (19:30-20:15 UTC):

While the clicks are relatively weak, they have some of the characteristics of the “slow” clicks Yukusam made a few days later as he swam southward through Haro Strait — regular separation of clicks by ~3.8 seconds and power peaks near 1.5 kHz. The clicks recorded in the Strait of Georgia are often made in a periodic series with an inter-click interval (ICI) of about 4 seconds. For a series of 20 clicks in the 18:47 file I measured a mean +/- s.d. (standard deviation) ICI of 4.0 +/- 2 seconds. The minimum ICI was 3.7 seconds while the max was 4.2 seconds. Most of the spectrograms shown here for the Strait of Georgia data indicate a power peak for most clicks near 1.5 kHz.

If you turn your volume up and wear headphones you should be able to hear the clicks, including some variations in intensity, in this clip from the 19:42 raw data file. Do you think a sperm whale made them?

While the phonation log is a work in progress, at first glance it appears the first faint clicks on this hydrophone (at a depth of 298 meters in the southern Strait of Georgia near the mouth of the Fraser River) were recorded at around 11:22 local time and the last strong clicks were recorded at about 13:12. This makes some sense given the sequence of sightings reported by Ford et al. —  Yukusam was “north of Nanaimo” around noon, passing departure Bay southbound around 14:00, and then going down and up Northumberland Channel from ~15:00-17:00 before heading north past Piper’s Lagoon (N Nanaimo) again around 19:00. (I haven’t listened/looked yet for his clicks at/after 19:00…)

Could Yukusam have been navigating in relatively deep water off the east coast of Vancouver Island, heading generally south/southeast towards Nanaimo when the hydrophone recorded the clicks some 45-50 km away?! The beam of sound emerging from a sperm whale’s head is very focused, so even if he was clicking continuously, the greatest detection ranges would occur only when his head was oriented towards the hydrophone. Could this explain the variable intensity that is evident between many successive clicks received by the hydrophone? Do sperm whales scan with their beam by swinging their head or turning from side to side?

Have a listen to these audio files, or download the lossless raw data from the OND data archive and leave your opinions and interpretations in the comments!

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