A 59-year-old man died Saturday morning after being pulled out of a Fitchburg lake the day before by a police officer, authorities said. According to Fitchburg’s deputy fire chief, Kevin Curran, a woman called 911 Friday afternoon to report that a man was behaving strangely by swimming in Mirror Lake in Coggshall Park. Curran said locals usually just walk around the lake, which is “not user-friendly for swimming whatsoever.” When police Officer Derek Sullivan arrived, Curran said, people began yelling that the man had disappeared beneath the water. Sullivan and a young man pulled the man to shore, and he was then taken to the hospital. A spokesman for the Worcester County district attorney’s office, Tim Connolly, said the man was pronounced dead at UMass Memorial Medical Center in Worcester early Saturday. The victim’s identity has not been disclosed, and Connolly said the medical examiner hasn’t determined the exact cause of death.