Why do killer whales attack boats?

Since May 2020, an extraordinary and worrying phenomenon has been making headlines in the maritime press: killer whales are attacking, or rather actively interacting with, sailing boats off the Iberian Peninsula. More than 700 incidents have been recorded to date, mainly along the Spanish and Portuguese coasts, occasionally as far north as Brittany. Several sailing boats have sunk. The phenomenon is without precedent in the modern history of interactions between humans and cetaceans, and biologists are still debating its causes.

A very specific pattern of interaction

The word “attack” is probably excessive. What biologists observe is a coherent and repeatable behavioural pattern. The killer whales target the rudder blade almost exclusively: the moving part of the steering gear beneath the hull. They favour sailing boats of less than 15 metres, and the interaction can last 30 minutes to 2 hours. Damage to the steering gear is frequent, and the boat becomes impossible to steer. Actual sinkings remain rare, but they do occur: several sailing boats have gone down after their rudder was completely destroyed.

No human injury has been recorded, despite the considerable mass of the animals (up to 5 tonnes). They make no attempt to come aboard, do not go for the occupants, and generally keep their distance from people. Everything suggests that the target is the boat itself, and more precisely the part that protrudes below the hull.

The play and culture hypothesis

The hypothesis favoured by most researchers today is that of cultural play behaviour. The International Whaling Commission has analysed a large number of videos and concluded that the behaviour observed resembles playful exploration: the killer whales appear to be playing with a new and interesting piece of equipment, much as a cat does with an unfamiliar object.

More precisely, the behaviour is thought to have been started by a single individual, named Gladis Blanca, and to have spread culturally within the small resident population of the Strait of Gibraltar, some fifty animals. Young killer whales, which are particularly curious and playful, are thought to have taken up the behaviour through social learning, as has been documented for other cultural innovations among cetaceans.

This hypothesis is supported by several observations: the concentration of the phenomenon within a restricted population, its sudden appearance in May 2020 (rather than a gradual emergence over time), and its transmission between individuals identifiable by photo-identification. The profile matches exactly that of a cultural “fashion” in a cetacean population, of the kind documented in humpback whales (lobtail feeding) or the dolphins of Shark Bay (sponging).

The trauma hypothesis

A competing hypothesis, defended by some researchers, proposes a traumatic trigger. Gladis Blanca is thought to have been injured by a sailing boat, in a collision with a propeller or an impact with a rudder, and to have developed an avoidance or aggression response towards these objects. Her behaviour would then have been copied by other killer whales, either out of social solidarity or simply through observational learning.

This hypothesis is not mutually exclusive with the play hypothesis: an initial traumatic event could have triggered the interest in rudders, which then turned into a playful practice over the generations.

The Iberian killer whale population: already fragile

The population involved is that of the resident killer whales of the Strait of Gibraltar. It is a small population of about 40 to 50 individuals, listed as Critically Endangered by the IUCN since 2019. It is specialised in hunting Mediterranean bluefin tuna, exploiting the annual migration of these fish through the strait. The population is under several pressures: the collapse of tuna stocks, high chemical contamination (PCBs), heavy shipping traffic and growing acoustic disturbance.

The sailing boat interactions have a worrying side effect: several killer whales have been seen with injuries probably caused by boat propellers in reaction to (or in anticipation of) the encounters. For a population already on the edge of extinction, every loss is critical.

What to do during an interaction

The Spanish and Portuguese maritime authorities have issued several recommendations. Slow down or stop the engine completely as soon as killer whales appear. Do not try to flee: killer whales swim faster than any sailing boat. Do not slap the water, shout or try to drive the animals away: these reactions appear, on the contrary, to attract them. Report the interaction immediately to the nearest maritime centre, and use the OrcaIberica application set up specifically to map the incidents.

Killer whales attacking boats is a reminder of several essential things. First, the extraordinary capacity for cultural innovation of cetaceans. Second, the fragility of their populations in the face of our activities. And finally, that a fine-grained understanding of behavioural ecology is needed to manage these interactions without panic and without stigmatising animals whose very survival is at stake.

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Further reading