Dolphins attacked four swimmers in the ocean near the shores from Japan the day before (July 16), leaving one person in their 60s injured with multiple fractured ribs.

It’s the second year in the same sequence that dolphins have injured people on beaches in the Fukui prefecture on the Sea of Japan coast. In the summer of 2011, at least six people were wounded by a single animal, which is believed likely to be two or three Indo-Pacific bottlenose dolphins ( Tursiops aduncus) -with the most severe case that required 14 stitches to heal the gash on the bottom on the thumb according to Japanese news website the Mainichi.

This year and last year, dolphins walked to the water’s edge. The man whose ribs split was swimming at a distance of 16 inches (5 meters) from the shore of Suishohama Beach when a dolphin attacked the man, according to local police and the Asahi Shimbun, the news website in Japan. Another man suffered bites to his left arm the same day, and BBC also reported that two additional beachgoers were bitten later in the day, but there is no information on the incidents.

Another dolphin was caught threatening people in this location the day before on Tuesday (July 17). A video of the incident shared by BreakingOne shows people hurriedly leaving the water after seeing a large dolphin swimming in the shallows and trying to catch an individual wearing a ring made of pink rubber.

Then, it appears to nip at the woman’s legs and rock the inflatable. After a short time, the dolphin disappears from the shore and towards two people aboard an inflatable dinghy that is towing another person wearing inflatable rubber rings.

Recent incidents have brought the number of incidents involving dolphins within Fukui to six for the year to date, authorities declared. Still, it’s unclear whether the animal caused them similarly. Authorities have advised swimmers to avoid touching or approving the animals.

Wild dolphins aren’t known to assault humans, but there was the exception of one case of an attack that killed a person in 1994 in the waters off Sao Paulo in Brazil. However, marine mammals have been recognized to bite or drag individuals underwater if they feel harassed, threatened, or get used to receiving food, as per National Oceanic and Atmospheric Administration (NOAA) Fisheries. According to the agency, if they’ve come to depend on or expect handouts, dolphins could be aggressive and pushy when they aren’t getting them.

Dolphins are powerful and wild creatures, Giovanni Bearzi, Zoologist and the president of the non-profit group Dolphin Biology Conservation in Italy, said to Live Science in an email. “A part of the problem may be that some humans tend to see them as pets, or as invariably ‘nice’ animals, sometimes overlooking the risks involved in dealing with wildlife,” Bearzi added, noting to the effect that “our unaware or overly ‘friendly’ behavior may trigger aggression.”

Dolphins could also strike if they believe that swimmers in areas that are important to their reproduction or foraging activities are not heeding the animal’s signaling signals to warn them, Bearzi said. “With human swimmers, the dolphins may send acoustic and other warnings that the swimmers fail to detect or interpret correctly. In other words, the dolphins may first tell the swimmers to ‘go away,’ and because the swimmers don’t do that, an attack is triggered.”

The incidents could be triggered by one dolphin with an aggressive nature. However, they were likely cautious, Bearzi said. “An adult bottlenose dolphin could easily kill a swimmer if [it] really intended to do so,” Bearzi explained. “These attacks seem to be warnings rather than true attempts to do serious harm.”