Two unusual sea lion attacks in San Francisco Bay at Aquatic Park led authorities to close the popular area to swimmers Friday as officials try to determine the reason for the aggressive behavior.

A sea lion bit a man in the groin area Friday morning as he swam in the waters off San Francisco Maritime National Historical Park, said park spokesman Lynn Cullivan. It was "a very serious bite," and the man was taken to a hospital, he said.

The cove hosts swimming and rowing clubs and is a favorite spot for dedicated swimmers. It's usually a transit area for sea lions heading to Pier 39, where they normally congregate, Cullivan said.

The area just off Ghirardelli Square, a popular square with shops and restaurants, will be closed to swimmers until Monday, he said.

Another swimmer was seriously injured Thursday after a sea lion bit him on the arm. Officers applied a tourniquet and he was transported to a hospital. Officials say he's expected to recover.

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The sea lion got near the swimmer so he splashed water on it, but the animal did not go away, said Matthew Reiter with the San Francisco Police Department's Marine Unit.

"When it didn't work, he yelled at it and then the sea lion came up and bit him on the arm," Reiter said.

A man on a nearby sailboat saw the swimmer in distress, pulled him onto his boat and called police.

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The aggressive behavior is so unusual for sea lions that officials think the same animal attacked both swimmers, Cullivan said.

"It could just be a sea lion being territorial, or it could be an injured or sick mammal," he said.

The swimmers have not been identified, but Cullivan said they were swimming closer to the bay than to the shore.

"People think the bay is their backyard, but it really is the beginning of the wilderness," he said. "There is wilderness out there."

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