In recent years, there have been reports of groups of killer whales exhibiting "terrifying" behavior by ramming hundreds of small boats off the coast of Spain, which has left scientists perplexed.

"They directly attacked the rudder, not swimming around the boat, not playing with anything," Friedrich Sommer told Agence France-Presse (AFP) as he recalled how his sailboat Muffet was damaged earlier this year by "three or four" killer whales.

"They rammed at full speed against the rudder," the German sailor told AFP as he waited for his vessel to be fixed in Barbate, a coastal town in Spain's southern Cadiz province.

Rafael Pecci, manager of the shipyard where it is being repaired, is already busy repairing another sailboat that was attacked by killer whales, also called orcas.

"It has completely lost the rudder," he said, pointing to the damaged part of the vessel.

On Barbate's main beach, the mast of a sailboat that sank after it was rammed by killed whales in early May juts out of the water, a reminder of the danger ships face.

The attacks began in 2020 and they have taken place mainly between Cadiz and the port of Tanger in northern Morocco, near the Strait of Gibraltar.

Killer whales are drawn to these waters by the presence of their favorite prey - bluefin tuna that migrate through the Strait of Gibraltar to breed in the warmer Mediterranean.

So far this year, Spain's coast guard has recorded 28 "interactions" between orcas and sailboats.

Between 2020 and 2022, there were nearly 500 such encounters, according to data released by the Atlantic Orca Working Group, known by its Spanish abbreviation GTOA, which researches orcas in the region.

'Fury'

Jose Luis Garcia Varas, who heads the oceans program at WWF Spain, told AFP that "very little is known about the causes of these interactions."

One theory is that a killer whale, christened Gladis, had a bad run-in with a boat in the past and is now teaching other killer whales to attack vessels in turn.

Gladis is the matriarch of a pod of killer whales that have been blamed for several boat attacks.

Stories and videos of the attacks have been widely shared on social media, turning the orca into a meme.

Killer whales "are group-oriented, they are very intelligent and are able to transmit knowledge orally", Garcia Varas said.

The species live, hunt and move in very closely connected family pods.

These tightly knit, matriarch-led groups have in some populations been shown to have their own pod-specific dialects.

Maria Dolores Iglesias, head of the environmental group Agrupacion Voluntarios de Trafalgar, believes Gladis has died and her descendants have continued their attacks out of anger.

One of Gladis's granddaughters has shown "fury" in her encounters with boats, Iglesias added.

'Quite terrifying'

But Renaud de Stephanis, a biologist with the Spanish conservation group CIRCE who has been investigating the boat encounters, said they could be a form of "play."

"We don't have a final conclusion," he said.

De Stephanis is putting tracking devices on killer whales that will allow the authorities to locate them and hopefully minimize their encounters with boats.

Killer whales can measure up to nine meters (30 feet) and weigh up to six tonnes as adults.

While not all their collisions with boats lead to damage, when it does often it is a vital part that is destroyed like the rudder or mast.

"It is quite terrifying," April Boyes wrote on her Instagram account after her boat was attacked by killer whales while crossing from Portugal's Azores Islands to Gibraltar.

"We turned off the engine and waited, they started to bump into the rudder continuously for over an hour," she said, also posting a video showing crew members removing water from the boat to prevent it from sinking.