Orcas Are Shouting Over Boat Noise – And It Might Be Making Them Hungry
Picture yourself at a noisy bar. You realize that you have been shouting at your date all night in order to be heard. Well, orcas in Puget Sound are in kind of the same situation.
Marla Holt, a research biologist with NOAA's Northwest Fisheries Science Center, has found that loud boat noise forces endangered orcas to raise the volume of their calls.
But the question, Holt says, is "so what? What are the biological consequences of them doing this?”
To answer that question, Holt and her NOAA colleague, Dawn Noren, a research fishery biologist, studied captive bottlenose dolphins.
They had the dolphin swim into a floating plastic helmet device and whistle at a normal level for two minutes. Then they rewarded the dolphin with fish. The device measured how much oxygen the dolphin used to accomplish that task.
Here's the dolphin whistling at a normal level (audio).
Then by knowing oxygen consumption, the scientists calculated the metabolic rate of the dolphin while making calls at a comfortable level.
Next, they had the dolphin whistle more loudly for two minutes (audio).
Holt and Noren found that when the dolphin was whistling harder and louder its metabolic rate rose by up to 80 percent above normal resting levels.
Just like with humans, when marine mammals' metabolism goes up, they burn more calories. Dawn Noren calculates that a dolphin making the louder call for two minutes would burn the same amount of calories it would get from eating half of a small fish.
"If you add it up, if you have multiple incidences where you’re increasing your vocals to compensate for a noisy environment," Noren explained. "You could have the need for them to increase their food consumption quite a bit.
The volume changes the bottlenose dolphins performed for the experiment were comparable to volume changes observed in wild orcas when they encounter vessels.
Holt says that as the region considers proposals to expand coal and oil shipments and naval training activities, this research could be used to calculate specific impacts on marine mammals like endangered orcas. There are just 81 of them left and their preferred food supply, chinook salmon, is also on the endangered species list.
"The concern," Holt said, "is that for animals that are maybe just getting by or not really getting by, we could say, this is how much more fish it would cost an animal if it was disturbed that much more."
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