The attacks have increased in the recent years - from one in 2022 to 18 so far in 2024.
At some beaches in Japan, elaborate security arrangements are made to watch out for fins. But it's not sharks the authorities and lifeguards are worried about - it's dolphins. And not the entire pod, just a single lonely, sexually frustrated dolphin. According to the BBC, this mammal is responsible for a spike in the attacks on swimmers in Wakasa Bay, about 300 kilometres from Tokyo. As many as 18 people have been injured in these attacks, including a primary school child whose fingers needed 20 stitches.
The attacks have increased in the recent years - one in 2022, six last year and 18 so far in 2024.
And local authorities are sure that it is a single dolphin that is attacking people.
"It is reasonable to assume that it is the same individual, as the wounds on the tail fin are similar to those of the dolphins seen off the coast last year, and it is rare for dolphins, which normally move in groups, to be alone for such a long time," Tadamichi Morisaka, a cetology professor at Japan's Mie University, told Japanese outlet NHK.
The dorsal fin is like a dolphin's fingerprint, as each has distinctive notches, ridges and pigmentation.
Other experts have also weighed in, suggesting why one dolphin may be behind all these attacks.
"Bottlenose dolphins are highly social animals and this sociality can be expressed in very physical ways. Just as in humans and other social animals, hormonal fluctuations, sexual frustration or the desire to dominate might drive the dolphin to injuring the people it interacts with. Since they are such powerful animals, this can lead to serious injury in humans," Dr Simon Allen, a biologist and principal investigator with the Shark Bay Dolphin Research project, told the BBC.
The biologist theorised that the dolphin may have been ostracised from its own community and seeking companionship.
"Most of the time, in my experience, this is more a defensive behaviour when humans get too close to these dolphins and do not know how to conduct themselves," said Dr Matthias Hoffmann-Kuhnt, a marine mammal expert at the National University of Singapore.
The dolphins are considered very friendly towards the humans, but their attacks can be fatal. One widely reported incident came from Brazil, where a dolphin, nicknamed Tiao, injured 22 people in 1994.
Authorities in the coastal town of Mihama have installed signs warning people that not only can the mammals "bite you with their sharp teeth and cause you to bleed", but they can also "drag you into the sea, which could be life-threatening".