Beaked whales are among the most elusive of all cetaceans (whales, dolphins and porpoises) and are incredibly hard to study. But new research using passive acoustic monitoring is shining a light on the foraging behaviour of these rarely seen and little-studied animals.
“Beaked whales are extreme deep divers that spend most of their lives at depth and surface only briefly, making them particularly challenging to study using traditional visual surveys or animal-borne tagging approaches,” says lead author Héloïse Frouin-Mouy, a bioacoustics expert at the University of Miami.
To learn more about these enigmatic animals – without needing to see or tag them – the researchers eavesdropped on the whales to pinpoint where they are. "To conduct acoustic tracking, we deployed specialised underwater listening systems on the seafloor off the coast of Louisiana at approximately 1,100m depth,” says Frouin-Mouy.
Sensors recorded the echolocation clicks, which enabled the scientists to make 3D reconstructions of the whales’ movements. "By measuring small differences in the arrival times of clicks across individual sensors, we estimated the direction of sound sources,” she says.
They recorded three different species of beaked whale: goose-beaked, Gervais’ and Blainville’s. It’s possible to differentiate each species by their clicks – in the same way that you might use birdsong to identify different species – so the researchers could map out the behaviour of each of them based on their vocalisations.
This is the first time the deep-diving behaviour of a Gervais’ beaked whale has been described in detail.
Goose-beaked whales seemed to spend longer on foraging dives (around 20 minutes) compared with Blainville’s beaked whales (nearly 14 minutes) and Gervais’ beaked whales (just under 13 minutes).
Goose-beaked whales also plummeted almost or all the way down way to the seafloor to forage – perhaps unsurprising given that this species, also known as Cuvier’s beaked whale, is the deepest-diving mammal on the planet. The findings are published in the journal PLOS One.
The data revealed that the different species seemed to be foraging at different depths, which suggests they feed on different prey items.
"This interpretation is consistent with stomach content analyses showing that goose-beaked whales prey on larger cephalopods, which are typically found at greater depths than the prey of other beaked whale species,” says Frouin-Mouy.
Even when we don’t see these whales interacting with us in the water, humans can have a huge impact. Researchers believe that the Deepwater Horizon oil spill in 2010 caused beaked whale populations to decline by as much as 83 percent. This striking statistic “underscore[es] the urgent need to better understand these elusive species and the threats they face,” she says.
Studies like this that help us understand where and how deep these whales dive can help researchers to estimate how abundant they are and keep track of how they are faring against human stressors, such as noise and chemical pollution, overfishing and habitat loss.
Top image: ocean (not the study area). Credit: Getty
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