Orcas may be teaching others to attack boats following a spate of strikes on sailboats off the coast of Europe, some observers say.
Sailors have reported a series of "coordinated" attacks by a group of orcas, including a May 22 strike on a 26-foot vessel sailing off the coast of Cape Spartel, near the Strait of Gibraltar.
"[Six] orcas arrived, 2 adults very big, 4 smaller ones," sailor JP Derunes wrote in Orca Attack Reports, a Facebook group dedicated to flagging orca activity. "Both rudders destroyed and blocked ... Boat to be hauled off later this week."
That attack followed a nighttime strike on May 4, when a Swiss yacht named Champagne, which was also sailing through the Strait of Gibraltar, was attacked by three orcas. They struck its rudder, eventually sinking it, reported Yacht, a German boating news outlet.
At least 15 human-orca incidents were recorded in 2020, the year in which the aggressive encounters are believed to have begun, according to a study published in the journal Marine Mammal Science. Many of those attacks included orcas biting or striking the rudders of sailboats.
No casualties appear to have been reported in the attacks.
Scientists said spikes in aggression may have been started by female orca whom scientists have named "White Gladis."
White Gladis is believed to have suffered a "critical moment of agony" such as a boat collision, which inflicted trauma on the orca, triggering a behavioural switch that other killer whales have learned to imitate.
The majority of orca-sailor encounters have been harmless.
"In more than 500 interaction events recorded since 2020 there are three sunken ships. We estimate that killer whales only touch one ship our of every hundred that sail through a location," Alfredo López Fernandez, a biologist at the University of Aviero, told Live Science.
According to a study in Biological Conservation, a peer-reviewed journal, "sophisticated learning abilities" have been found to exist in orcas, with imitation found to be particularly significant.