A spate of attacks on boats by killer whales along the coast of Portugal and Spain have left ocean scientists baffled. This type of aggression towards vessels is reported to have been rare until a recent surge in distress calls made by sailors under siege by pods of orcas. Most at risk so far has been the sailing boats’ rudders, but there have been reports of some crew members suffering bruising as their boats were rammed from beneath.

The most recent incident happened on Friday, September 11, off the coast of A Coruña, Spain, which involved an 11-meter (36-foot) yacht being taken to the UK. The vessel came under siege as an orca rammed its stern 15 times according to Halcyon Yacht’s managing director Pete Green, reports The Guardian. The incident left crew members perplexed and the vessel without steering, requiring it to be towed back to port for repairs.

It seems the orca attack is far from an isolated incident as it follows a series of distress calls to the coastguard from ships under siege from killer whales. One such attack saw a 14-meter (46-foot) ship turned 180 degrees as nine orcas repeatedly rammed the vessel for over an hour. From the Strait of Gibraltar to Galicia, yachts have been receiving unwanted attention from disgruntled orcas and scientists have no idea why. It’s not uncommon for these highly intelligent and social animals to approach boats and they’ve often been seen passively investigating boats and swimmers, but such attacks until recently were unheard of.

"Orcas are curious by nature, especially the youngsters, and will often swim over to investigate boats and will sometimes investigate swimmers too but never cause any harm," said a representative from Whale and Dolphin Conservation UK in an interview with IFLScience. "To hear that a pod or even just one individual in a group is striking vessels is highly unusual and worrying behavior.

"For many years orcas have been closely associated with the tuna fishing fleet in the Strait of Gibraltar during the summer months and have been observed taking fish off the long-lines. This is an uneasy relationship between orca and fishermen as tuna are highly prized by both. The boats often try to drive away the orca pods. We can only speculate on the alarming behavior of orcas striking sailing boats which may be related to stress as this small, critically endangered population struggles to adjust to the overfishing of its preferred prey, Atlantic bluefin tuna."

It’s not known if the attacks in the last two months are isolated to a single pod of killer whales but Dr Ruth Esteban, who has studied the Gibraltar orcas extensively, told The Guardian that such unusual behavior is more likely to stem from a single pod rather than emerging in multiple groups at the same time. It's possible it's young orcas going through a curious phase, and sharing the same waters has led to contact. 

Nevertheless, Spanish maritime authorities have advised sailors to be vigilant and “keep a distance” to prevent further attacks, but given the orcas appear to be actively pursuing boats it’s unclear if evasion is a substantial enough precaution.

Unusual behavior in whale species such as beaching is often the result of distress or illness, with some linking the use of sonar to a recent spike in beaching and sightings of rare cetacean species. It’s possible the pod’s aggression towards boats could be the result of similar environmental stressors for the animals, but for now the exact cause remains unclear.

[H/T: The Guardian]