A new study published in Biological Letters found that a seismic survey in wide bay at the mouth of the St. Lawrence Seaway caused blue whales feeding and socializing nearby to double or triple their call rates.  The calls were near-range communication signals, rather than the long, loud songs that are heard over hundreds of miles. The research was meant to simply learn more about these social calls, but during the study, their recordings began to pick up the pulses from a seismic survey.  “The whales made more calls on days when the testing was happening. It seems they are having to repeat themselves in order to not lose information,” said lead researcher Lucia Di Lorio.  They also called more on survey days when the sounds were not audible than when they were, and tended to rapidly increase calls when the sounds appeared.

Blue whale at surface. Image: Wikipedia Commons

Blue whales, the world’s largest animal, number just 5-10,000, are solitary for most of the year, making these summer-time feeding gatherings especially important; di Lorio notes that “We don’t yet know a lot about what these calls mean…They come to eat, but also to check out each other, maybe find a mate.”

The results were especially surprising, since the survey in question was using a much lower-power source than many surveys, and at levels much lower than those typically considered likely to cause problems.  “It’s used [here] because it’s thought to have a lower impact on marine life,” di Lorio told the BBC, “But we should definitely reconsider these things, because clearly it’s not only the sound level that’s important; and one thing might be not to do the test when there are lots of whales around.”

The electronic “sparkers” used to create the survey’s sound bursts top out at only 190dB, rather than the 230-240dB of airguns (sparkers are slightly higher frequency as well, but still solidly centered in low-frequency bands of 30-450Hz, primarily 60-250Hz, very similar to airguns, and matching key blue whale communication frequencies). The study notes that “Our results clearly show that blue whales change their calling behaviour in response to a low-medium power technology that is presumed to have minor environmental impact. In fact, the mean sound pressure impinging on the (study) area, and thus probably on the whales present there, was relatively low, 131 dB re 1 mPa (peak to peak, 30 – 500 Hz) with a mean sound exposure level (SEL) of 114 dB re 1 mPa2 s.   The increase in calls was a contrast to some other studies, which have found call rates reduced near seismic surveys, perhaps in part due to animals moving away; in some cases, animals call louder or change the pitch of their calls to cope with noise intrusions.  This is the first study to find a marked increase in call rates, or repeating themselves, which di Lorio suspects may be due to the key role of this area in socializing and feeding (i.e., the communication is time- and place-dependent, and cannot easily be postponed until a quieter time).

The study concludes by noting that the St. Lawrence Estuary “is an important feeding area where blue whales acquire energy and also a place where this wide-roaming, highly dispersed population congregates to engage in social interactions. Reducing an individual’s ability to detect socially relevant signals could therefore affect biologically important processes. This study suggests careful reconsideration of the potential behavioural impacts of even low source level seismic survey sounds on large whales. This is particularly relevant when the species is at high risk of extinction as is the blue whale.”

UPDATE 9/29: A skeptical note received by email from a longtime correspondent: “To me, given a detectable new sound, mammals tend to decrease vocalization if it induces fear, or increase vocalization if it stimulates interest.  How a sparker could block communications rather eludes me, but whales responding to a pulse—be it  noncoherent noise stimulates noise, or actual whale talk directed at the pulse—comes to mind….Sperm whales in the GOM seemed to alter click patterns and perhaps anticipated a pulse once they got the beat.  That was a casual observation by one of the acoustic team – always thought that would have been a great add-on study but we were already at overload working on acoustic tracking for tagging purposes. More at home, my grey parrot, who talks quite a bit, of course becomes completely silent when visitors ask ‘if he talks?’ – his reaction to seeing new people is silence.  Now, if you go into another room and play classical music, the next question is ‘how do you stop him?’  I think there is a complexity to animal behavor and acoustic responses and the reasons blue whales would increase calls could be many.”