Our understanding of the prevalence of noise in our oceans has changed dramatically in recent years. We now know that the submarine environment is anything but Cousteau's ‘silent deep’. Shipping traffic, naval sonar and exploration for subsea resources, for example, all add to the underwater hubbub, leading to adverse impacts on the ability of marine animals to communicate, altered access to feeding grounds and elevated stress levels. And the largest animal in the world, the blue whale (Balaenoptera musculus), is not exempt from the effects of noise. When exposed to military sonar, they may swim faster, stop feeding and call less frequently; they are known to call louder when they hear shipping. And when airguns are discharged while probing for undersea oil, whales appear to call more frequently. Yet, humans are not responsible for all undersea noise. The waters around New Zealand are alive with deep rumbling of earthquakes as the tectonic plates shift and jar. So, what impact do these thunderous natural sounds have on the sensitive behemoths within earshot?
Dawn Barlow at Oregon State University, USA, and colleagues from her home university and Cornell University, USA, listened to the blue whale undersea chorus recorded by five listening stations in the South Taranaki Bight, New Zealand. They used geological databases to identify individual earthquakes with Richter magnitudes greater than 3 originating within the study area and matched up blue whale calls at hourly intervals in the run up to and following each earthquake. The team kept track of two specific kinds of calls – the ‘D-call’, which is produced by both sexes when feeding and ‘song’, which is produced only by males – measuring how often and estimating how loud the D-calls were and measuring the intensity of song. In addition, the researchers examined the whales’ calls during periods when the earth's crust was silent, to capture seasonal variation in both soundscape and whale behaviour.
To the surprise of Barlow and her colleagues, they found that the whales appeared to be unperturbed by seismic activity. The whales continued producing calls in the periods before and after earthquakes, and there was no difference in the animals’ calling rates. Specifically, no earthquake metrics (magnitude, depth, distance to the epicentre and relative received level) were useful predictors for estimating changes in blue whale calling rate. While the acoustic signature of an earthquake typically lasted less than a minute, these loud and natural occurrences did not seem to bother the blue whales. While the recording set-up did not allow the researchers to calculate the sound level that the whales would have heard, they were able to describe how loud the earthquakes were at the recorders and suspect that they all fell below the noise thresholds thought to disturb the whales’ behaviour.
Scientists are chipping away at understanding how wild animals respond to disturbances by quantifying the responses of many species to specific acoustic disturbances, as individual intrusions can accumulate to impact the health and survival of a population. They are finding increasingly that the existing backdrop of both the animals’ behavioural state and its surrounding environment in which noises are heard matters a lot when it comes to predicting how they react. To contextualise whale responses to anthropogenic sounds, we also need to understand how they respond to natural noises. While the findings in Barlow's study are for a specific baleen whale in a specific region of the world, understanding how other whales respond to simultaneously occurring natural and anthropogenic noises can help to add context to their responses.