SAN FRANCISCO — Two unusual sea lion attacks in a San Francisco Bay cove 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 as he swam in the waters off San Francisco Maritime National Historic Park, park spokesman Lynn Cullivan said. 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 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 an arm. Officers applied a tourniquet and he was transported to a hospital, where he had a least two surgeries.

Christian Einfeldt said the animal trailed him as he swam from the beach to the mouth of the cove. When he turned to swim back, the massive sea lion was right there, he told TV station KGO.