Authorities in central Japan are urging beachgoers to steer clear of dolphins after a sharp increase in biting incidents, with one expert saying the culprit may be a single overly playful individual.

This year has so far seen 18 swimmers bitten at several beaches in the Fukui region, according to the local Tsuruga coastguard, which logged just a handful of injuries related to dolphins in each of the past two years.

Most of the bites were minor -- some were little more than scratches -- but in one recent incident, an elementary school child wound up requiring 20 to 30 stitches, coastguard official Shoichi Takeuchi told AFP.

At Suishohama Beach -- also known as "Diamond Beach" -- a local tourism association is calling for vigilance, issuing warnings on its website and handing out fliers advising people not to approach or touch the animals.

"Dolphins are usually calm creatures, but they can make you bleed by biting you with sharp teeth, drag you under water and, in the worst-case scenario, threaten your life," the association warns online.

The coastguard said it was unclear whether a single dolphin was behind the incidents or whether there were multiple culprits.

One expert, however, thinks one rogue dolphin may be responsible for the incidents, given identifying characteristics such as its dorsal fin and scars.

"It is probably the work of the same individual," Tadamichi Morisaka, a professor of cetology at Mie University, told broadcaster NHK.

"Instead of trying to harm humans, it might be seeking to interact with humans the way it does with fellow dolphins."