Why whales stop eating in response to the sound of son…
But why did the sound of the sonar particularly affect beaked whales and not other whale species?
In our new researchwe discovered that each species’ response to predators could explain why some whales and dolphins are more sensitive to these human-made sounds.
It was in the early 2000s that we (along with other researchers around the world) started doing the effect of sonar on free-roaming whales. These new “behavioural response studies” exposed different whale species to gradually increasing sonar levels – with careful monitoring to keep the animals safe from harm. We were then able to identify the level of sonar noise at which behavioral changes occurred.
From this early research we knew that feeding is often compromised when marine mammals are disturbed by sonar, and some species are significantly more sensitive to this exposure than others. For example, Cuvier’s beaked whales showed dramatically more severe changes in their feeding habits (swimming away quickly and silently while increasing their dive and non-feeding times) than blue whales.
But so far the reasons for this differential response between species were unclear. So we decided to investigate if they responded to human-made noises in them in a similar way on their response to predators, as some theories suggest.
Exposing whales to noise
Most whales are themselves the prey of another whale, the killer whale. Some species, including beluga and beaked whales, have few defense mechanisms. But others are safer because of their large body size, like sperm whales, or large social groups, like pilot whales.
This means that different species react differently to the presence of killer whales. We wanted to find out if four whale species respond to marine sonar in the same way as they do to the sounds of predatory killer whales – and whether the differences between the species are related to their natural risk exposure to these fearsome predators.
By marking the animals with Recording devices with suction cup – which capture the timing of sounds and movements – we were able to monitor the feeding and movement of 43 tagged whales off the coast of Norway: three toothed whale species (northern bottlenose dolphin, sperm whale and long-finned pilot whale) and one baleen whale species (humpback).
We measured their reduction in feeding time when exposed to these marine sonar – varying from one to four kilohertz – and compared them to their response to recordings of sounds made by predatory killer whales.
Links to Predator Threat
We both thought so marine sonar and the predator sounds resulted in a significant reduction in feeding time in the four whale species. In contrast, feeding activity was unaffected when we exposed them to the sounds of seagoing vessels without sonar or other control sounds.
Strikingly, each species responded similarly to sonar and predation sounds: northern bottlenose whales responded the most and stopped foraging completely (100% loss of feeding time), followed by humpback and long-finned pilot whales (both about 75%). . Sperm whales had the least response, reducing the time they spent feeding by about 50% to both sounds.
It is clear that the difference in hearing sensitivity between species is not enough to explain the observed difference – because the humpback whales, which hear best in the sonar frequency band, were not the most sensitive.
Instead, our results suggest that orca risk plays a role in controlling responses and that adaptations to their predators may explain whales’ sensitivity to human-made noise.
The northern bottlenose whales, which rely on crypsis (staying hidden) and flight to reduce their risk of death from killer whales, were very wary and abandoned feeding when they heard sounds of potential threats – but the species were less vulnerable to predation , were also less responsive to sounds from killer whales and sonar.
Effects on arctic whales
Our findings can help predict which whales are likely to be extremely sensitive to human-caused ocean noise — and help us set appropriate conservation priorities.
The results are particularly relevant for cetacean species in the Arctic, as they are at the highest risk of predation.
For example, the behavior and distribution of narwhals is affected Arctic killer whales and as we would expect, they are sensitive to man-made noises such as airgun impulses and ship noise.
As sea ice is rapidly declining, cetaceans and cetaceans in the Arctic are facing a double whammy – increasing predation from more orca movements into ice-free zones, and increasing noise from human activities such as seismic exploration, the military and shipping.
In addition to the risk of immediate injury or death, it is important to consider the impact of human disturbances on their diet and other behaviors. DM/ML
This story was first published in The conversation
Patrick Miller is Professor of Biology at the University of St Andrews. Charlotte Cure is a bioacoustics researcher at UMRAE, France. Saana Isojunno is a Research Fellow at the University of St Andrews.