Monday 25 January 2016

Why do sperm whales wash up on beaches?

Five sperm whales have washed up in England. They are thought to belong to the same all-male pod as 12 others that were found dead around the Netherlands and Germany last week. But what caused this mass death, asks Claire Bates.

Sperm whales are deep-sea mammals that tend to live and hunt at a depth of around 3,000m. While females and their young stick to the tropics, males range further afield and are regularly spotted around Iceland, Norway and Shetland.

However, they quickly run into trouble if they enter the North Sea. This is because it lies on the European continental shelf, where the seafloor is only 200m down at its deepest point.

Sperm whales rely on sonar to navigate. They send out sound pulses, which bounce back off distant surfaces, helping them to form a clear picture of their underwater world. This doesn't work on a shallow sandy seabed, like Britain's, and whales quickly become disorientated.

The answer

Sperm whales become disorientated in the shallow waters of the North Sea and become stuck on sandbanks.

"The pod probably followed a shoal of squid into the North Sea around the New Year and then got stuck," says Peter Evans, director of The SeaWatch Foundation.

"As they head south the water becomes shallower. Once they end up on a sand bank it's all over for the whale. It causes cardiovascular collapse and their organs start failing."

Andrew Brownlow, from the Scottish Marine Animal Stranding Scheme, went across to perform an autopsy on one of the whales stranded off the Netherlands.

"The whales were in good condition and so hadn't died from dehydration, which can happen as they get all their water from their food," he says. "We were also able to rule out a boat strike or entanglement, which is a common cause of strandings."

"But we weren't able to look at the brains so we can't rule out various illnesses, or a sudden noise that scared them."

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