Scientists think they know why orcas are ramming boats off Spanish coast

Scientists think they know why "killer whales", also known as orcas, have been ramming vessels off the north-western Spanish coast.

The attacks, which began as early as July, have occurred along the same stretch of Spanish and Portuguese coastline from the Strait of Gibraltar to Galicia.

Several boats have phoned in distress calls after being harassed, and one Spanish naval yacht reportedly lost part of a rudder after being rammed.

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Initially stumped by the bizarre behaviour, a group of scientists now claim the attacks appear to have been carried out by two or three younger whales who likely felt threatened by the larger vessels.

Alfredo López, a biology professor at the Coordinator for the Study of Marine Mammals, said the animals were taking "precautionary" steps to protect themselves.

"It's not revenge. They're just acting out as a precautionary measure," he told Spanish newspaper El Pais, adding the attacks did not appear meditated.

"Our interpretation is that they don't have the slightest intention of attacking people."

The attacks prompted Spain's Ministry of Transport, Mobility and Urban Agenda to last week ban vessels over a certain size from sailing between the 100-kilometre stretch of coastline.

"Interactions with killer whales have affected, above all, medium-sized sailboats, with a length equal to or less than 15 metres," the statement read.

"All the encounters with the killer whales took place between two and eight nautical miles from the coast and the sailing speed ranged between five and nine knots, either exclusively under sail or sail and motor."

Despite their common moniker of "killer whale", orcas are actually dolphins who live in tight-knit family groups.

There has been no fatal attack of an orca recorded in the wild, although there have been several deadly attacks by orcas in captivity.