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The victim, who has not been identified, was surfing Saturday morning when he was bitten, according to ABC News station KITV.
Honolulu EMS responded to the scene, according to the Honolulu Star-Advertiser, and EMS treated the surfer for a shark bite on his lower left leg.
Honolulu Ocean safety is patrolling the area of the attack, and has posted warning signs, according to KITV.
Lifeguards at an adjacent beach, White Plains, were notified.
The beach where the attack happened does not have lifeguards.
The reported shark bite comes after a spate of animal attacks over the last few weeks.
A woman in Florida last week was killed by an alligator after going on a walk with her two dogs.
Earlier this month in Yellowstone National Park, three people were attacked over a four-day period: a 51-year-old woman was attacked by a mother elk protecting her calf; a 53-year-old woman was also attacked by an elk -- also protecting her calf; and a woman was gored by a bison bull that charged a group of people.
Separately, people were recently attacked by monkeys and a decapitated rattlesnake.