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Mar. 10, 2015 - Updated 04:16 UTC



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Island Occupied by Sea Lions

Makoto Kawamura

Apr. 4, 2017

The Sea of Okhotsk between northern Japan and Russia is known for its bountiful marine life.

It's also home to the Steller Sea lion. This winter, a record number of them migrated to Northern Japan.

The uninhabited island of Benten is 1km off the coast of Hokkaido's Cape Soya.

A large population of Steller sea lions occupies the island.

Female sea lions weigh around 300 kilograms and males can reach one ton.

As our boat approaches, they jump into the water and make an alarming sound.

Under water, the sea lions swim in a graceful manner. They display a calmer nature than on land.

The animals are known for their cautiousness. But after finding out we intend no harm, some of them approach us. They're actually very curious and intelligent.

One of them is blowing bubbles into the water. It is believed to be mimicking the divers.

The number of Steller sea lions living on Benten Island started rising sharply last year.

Marine biologist Yoko Goto has been conducting research on the island for 5 years. She says such large numbers are unprecedented.

"Usually, we have up to 300 or 400 of them. Today, we have more than 1,000. I went over the records and documents from last year and I didn't see anything like this," she says.

Goto collects their droppings and analyzes them to see what they are eating. She says one reason the sea lions gather on the island is for food. They can get a steady supply of prey like North Pacific Giant Octopus.

"One thing is the food. And when seas get rough, sea lions tend to leave. But that doesn't happen around here. It's a comfortable place for them," Goto explains.

But sea lions can make life difficult for local fishermen. They take fish from fishermen's nets.

In Hokkaido, the damage to the fishing industry amounted to more than $16 million last year.

"They eat up octopuses. They just love them. They bite and ruin the fishing net. I'm afraid of these sea lions but they're lovely. I'm not sure what I should feel," says a fisherman.

The Steller sea lion is a protected species internationally and scientists worldwide are collecting data to determine behavioral patterns and migration ranges.

Some of the sea lions we saw had numbers on their backs. These were marked by Russian researchers in a breeding ground in their country.

Scientists are trying to find out the reason for the sudden migration of Steller sea lions from Russia. They think global warming could be the explanation. The Sea of Okhotsk used to be covered with an ice floe in the winter, blocking the movement of the animals.

But the ice has been shrinking in the last decade, making the ocean easier to navigate.

Japanese researchers are now putting tracking devices on them to find out more about their behavior in local waters.

"If we understand when and how deep the sea lions hunt and eat fish, we can determine where they find it easy to congregate and where the damage to the fishing business is likely to occur. Maybe we can come up with ways their behavior won't overlap with fisheries," says a researcher.

Authorities are starting to take urgent measures. Fisheries officials are planning to equip fishing boats with devices that make sounds to drive the animals away.

It's likely that competition with local fishermen will intensify.