Why do dolphins and whales strand themselves? Beaching could be down to MAN-MADE noise
DOLPHIN and whale strandings could be caused by man-made noise at sea, suggests new research.
The study suggests they may get tired trying to flee the perceived threat and end up beaching themselves.
Researchers say that surviving in an environment that actively impedes their progress can leave them vulnerable when escape is necessary.
Coupled with the need to conserve limited oxygen reserves, the researchers said the magnitude of the challenges faced by many whales and dolphins when threatened becomes clear.
Professor Terrie Williams, of the University of California Santa Cruz, is fascinated by how marine mammals balance their energy demands with their finite oxygen supply.
She said: "Amazingly, there has been only a handful of studies that have actually measured the energetic cost of a dive for dolphins or whales."
Explaining that fleeing dolphins beat their fins continually when swimming full-out, while adopting a more leisurely burst-and-glide style during a routine dive, Professor Williams wondered just how much energy each swimming style uses.
Professor Williams and her colleagues found that beaked whales fleeing from low frequency sonar use 30.5 per cent more energy than unstartled animals.
They spent more than six months working with a team of experts training six bottlenose dolphins that had previously worked with the US Navy to participate in swimming tests that would allow them to measure the metabolic costs of the different swimming styles.
In the first test, the dolphins learned to swim at their most comfortable speed while pushing against a force plate in the wall of the pool as the researchers filmed the number of fin beats.
In contrast, the second test required the animals to dive down 10 metres wearing a fin-beat tracker and swim through a series of hoops before returning to the surface.
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Professor Williams was able to directly measure the metabolic cost of each dive by training the animals to surface in an air dome where she could record how much oxygen the animals inhaled as they recharged the oxygen stores they had consumed while swimming.
And when the researchers included killer whales in the metabolic measurements, Professor Williams had to build an outsized respiration dome to accommodate the larger animals.
After months of patience, the team was eventually able to calculate that bottlenose dolphins consume 3.3J/(kg stroke) during routine swimming, but the energy consumption almost doubles to 6.4J/(kg stroke) when swimming their hardest.
And when the researchers added the killer whales' fin-beat cost to a plot including the swimming costs of bottlenose dolphins, harbour dolphins and belugas, they finally had a tool that they could use to estimate the diving costs of any cetacean.
Loud man-made noise pollution is thought to be responsible for some mass strandings, so Prof Williams contacted Brandon Southall, who had recorded how a Cuvier's beaked whale reacted to 20 minutes of loud sonar.
With the recording showing that the whale's fin-beat pattern increased significantly from around 13.6 to about 17strokes/min, she calculated that the startled animals would use 30.5 per cent more energy as their metabolic rate rocketed to power the fleeing animals' fin beats.
And the whale did not recover swiftly, continuing to use the most costly fin beats for almost two hours after the noise stopped.
Professor Williams added: "Not all strokes are the same in terms of energy expenditure for swimming dolphins, and this has enormous implications for the cost of flight from aversive stimuli by wild cetaceans.
"In view of the number of cetacean mass strandings across the globe and the increase in human presence in the oceans, such data are critical. The animals in our care provided that opportunity."
The findings were published in the Journal of Experimental Biology.