The sonar trap

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Every time there’s a stranding—especially when dozens of whales or dolphins run aground, such as in Mahia last month—we mourn, and wonder what drives the phenomenon.

Scientists can’t answer that with certainty yet: strandings are ethically, logistically and financially tricky to study. But in general, there are a few key theories. Cetaceans can get into trouble while fleeing predators, or hunting in the shallows. Rough seas, dirty water, and loud noises can interfere with their hearing and vision. Sometimes, sick or dying whales are drawn to calm coastlines, and in groups with strong social bonds, entire pods may stick by the sick animals, even at the cost of their own lives.

In a few locations (although not Mahia) it’s thought to be simply the shape of the sea floor that’s the problem. Along with their other senses, toothed whales and dolphins use echolocation to navigate. Bays with long, shallow, gradually falling sandy bottoms (like Farewell Spit in Golden Bay/Mohua, or Mason Bay in Stewart Island/Rakiura)  are thought to be so featureless that the animals’ sonar signals resonate unchecked into the distance, as they would in open water.

“They’re not getting the information they would get in a rocky or steep-slope environment,” says Auckland University marine biologist Rochelle Constantine. “They don’t realise they’re getting into shallow water.” Then, when the tide turns, it turns fast—and the group get stuck.

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