A cove known as Aquatic Park has been closed to swimmers for a few days by the National Park Service because of three attacks in just a week. Such attacks are rare but not unheard of.
A man named Rick Mulvihill suffered a severe bite Friday morning and was hospitalized, the San Francisco Chronicle reported, quoting the fire department.
Mulvihill belongs to a club whose members take early morning swims in waters with an average temperature in December of about 53 degrees Fahrenheit (12 degrees Celsius).
"It hit him right here, close to the family jewels," said fellow club member Bob Roper, 79, pointing at the inside of his upper thigh.
All the attacks might be by the same animal, which could be a sea lion or a harbor seal, said Claire Simeone, a veterinarian.
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