We found out why some whales stop eating in response to sonar sounds
In September 2002, a number of beaked whales were beached and killed during a NATO naval exercise in the Canary Islands. It was the first time we began to really understand the negative effects of sonar noise on cetaceans, which include cetaceans, dolphins and porpoises.
But why did the sound of the sonar particularly affect beaked whales and not other whale species?
In our new research, we 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) began studying the effects 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 exhibited dramatically more severe changes in their feeding habits (swimming away quickly and silently while increasing their diving and non-feeding periods) than blue whales.
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But until now, the reasons for this differential response between species have been unclear. So we decided to investigate whether they responded to human-made sounds in a similar way to 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 tagging the animals with suction cup recorders – 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 longfin pilot) and one baleen whale species (humpback whale). ).
We measured their reduction in feeding time when exposed to marine sonar – varying from one to four kilohertz – and compared it to their response to recordings of sounds made by predatory killer whales.
Links to Predator Threat
We found that both marine sonar and predator sounds caused a significant reduction in feeding time in the four whale species. In contrast, feeding activity was unaffected when we exposed it 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 around 75%). . Sperm whales had the least response, reducing the time they spent feeding by about 50% to both sounds.
What is clear is 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.
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For example, narwhal behavior and distribution is influenced by arctic killer whales, and as you might expect, they are sensitive to human-made sounds such as airgun bursts 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.