The attack started when a person fishing off the Manhattan Beach Pier hooked the shark and spent over 30 minutes trying to reel it in, which Flores said, made the shark grow agitated.
The victim was in a group of long-distance swimmers about 300 yards off the beach, where Flores said the shark bit the man.
The fisherman cut the line and a surfer put the injured swimmer on his board, taking the victim ashore with the help of Los Angeles County lifeguards. Paramedics began treating the man.
The victim's identity was not released, but Flores described him as a middle-aged man.
The shark remained in the area for the next 20 minutes and then disappeared into the murky water, Flores said.
The beaches remained open, but a mile-long stretch was temporarily off limits to swimmers, Flores said, as lifeguards patrolled in boats to make sure the water was clear.