A really interesting new study uses anti-predator responses to create a relative index of disturbance levels to assess the disturbance to sperm whales caused by sonar.
One of the big questions when it comes to disturbance to marine mammals is: “is the disturbance biologically significant?”. If a whale responds to man-made noise by changing behaviour or vocalisations or movement… what does this mean biologically for the animal? This new study has created a relative index of disturbance levels by looking at a natural disturbance response: the anti-predator response.
Killer whales predate on sperm whales. The presence of a predator can be considered a natural high-level disturbance stimulus which animals have evolved responses to. For example: animals can reduce their risk of predation by changing their behaviour ie: changing from foraging, resting or socialising (all fitness enhancing activities) to fleeing from a predator. Sperm whales respond to killer whale calls by changing their movement and avoiding the area, stopping feeding activities and by producing more social sounds and grouping together. This study looked at the responses of tagged sperm whales to different sonar types and compared these responses to the anti-predator response.
Sperm whales responded to the different sonar types in a similar way to the killer whale calls. In response to the 1−2 kHz upsweep naval sonar the sperm whales showed changes in movement, avoided the area, changed or stopped feeding and resting and produced more social sounds. The sperm whales also responded to 6−7 kHz upsweep naval sonar but to a lesser extent which suggests this sonar type causes less of a disturbance effect on this species – at least in these experiments. The only real difference between their responses to killer whale calls and to sonar was that the animals grouped together in response to killer whale calls which they didn’t do in response to sonar. This is therefore considered to be a response specific to predators.
Conclusion: sperm whales are able to change their behavioural response to different types of disturbance stimuli. Previous studies have shown that sperm whales respond to seismic airguns by changing movements and reducing buzzing rates but did not stop feeding or show avoidance behaviour, which is a lower response on the relative index of the disturbance levels than that cause by navy sonar.
One thing that is important to keep in mind is an animals previous exposure history to a sound source. For example, animals that have previously been exposed to high levels of navy sonar may well react differently to those who haven’t been previously exposed. It would be interesting to know how/if the response level changes on the relative index of the disturbance levels after long-term exposures!
We’re trying to understand how these different kinds of disturbance from man-made and natural noises impacts marine mammals as part of our work on PCoD. You can check that out here.