GEOFF BENNETT: A small group of orcas is causing a lot of damage to boats off the Iberian Peninsula, raising questions about why the orcas are doing it.

Stephanie Sy looked into it and has this report.

STEPHANIE SY: In the middle of the night last October, Artur Napoleao was sailing miles off the coast of Portugal to deliver a boat to a client.

As a professional skipper and sail instructor, it's a journey he's taken many times before.

ARTUR NAPOLEAO, Professional Skipper and Sail Instructor: At 4:00 a.m., it was my turn to go to sleep.

So, I went down.

I start taking my nap, and, 30 minutes later, boom.

STEPHANIE SY: He ran up to the deck to find the boat surrounded by a pod of orcas.

ARTUR NAPOLEAO: They mess around for five minutes, and then they went to another boat more far away from me, like one, two miles.

STEPHANIE SY: But the pod returned, not once, but three times, the visits spanning over several hours, well after sunrise.

ARTUR NAPOLEAO: I got really scared, until I realized, until I saw the orcas and see their moments and how gentle they were when I stopped the boat.

STEPHANIE SY: In the past three years, hundreds of boat and orca interactions have been reported off the coasts of Portugal, Spain, and Morocco.

According to the Spanish research group GTOA, in 2022, there were 207 reported interactions in the Strait of Gibraltar alone.

The strait is a narrow waterway that separates Spain and Morocco.

And the meetings are not always gentle.

Just last month, killer whales surrounding a boat sailing through the Strait of Gibraltar were met with loud noises, an attempt by the crew to scare them off.

The pod rammed the boat continuously for over an hour, managing to remove the rudder.

With seawater gushing into the boat, the experienced crew of four issued a mayday call, and were eventually towed to shore unharmed.

In another recent instance off the coast of Spain, three orcas repeatedly struck a yacht, causing it to sink entirely after Spanish Coast Guards rescued the crew on board.

RUI ALVES, Creator, Orcas.pt: We are having one incident every day, on average, OK?

There are days we have two or three.

STEPHANIE SY: To better track these events, Rui Alves started this Web site last year to log orca encounters.

It connects a network of sailors who share geographic locations of sightings and attacks along the Spanish and Portuguese coasts.

This map shows incidents reported just this month.

RUI ALVES: So, for me, it was an idea.

If you spread the word about something that happens in an area, people will avoid that area.

Maybe they will keep the boat and say, I don't sail today because there are many orcas at that point.

I will sail tomorrow, or maybe I will go in another direction.

STEPHANIE SY: Orcas living near the Iberian Peninsula are a distinct subpopulation and a critically endangered one.

But why these orcas are repeatedly damaging boats is something experts are trying to understand.

One theory making the rounds?

Revenge.

Some scientists believe an orca named White Gladis suffered a traumatic injury from a boat, and may be teaching other orcas how to attack similar vessels.

But Monika Wieland Shields, director of the Orca Behavior Institute, says that kind of activity would be uncharacteristic of the animal she studies.

Play is a more likely explanation, she says.

MONIKA WIELAND SHIELDS, President, Orca Behavior Institute: It's just not something that we have seen, for whatever reason.

They're the top predator in the ocean.

They're capable of attacking and killing all kinds of species that are around them.

But they have never targeted that talent or that predator-like behavior towards humans.

STEPHANIE SY: Despite the killer whale nickname, orcas are the largest species in the dolphin family, and no humans have yet to report major injuries from the boat confrontations.

For the "PBS NewsHour," I'm Stephanie Sy.