ON BOARD A BOAT IN THE STRAIT OF GIBRALTAR - From the surface, the azure waters seem calm and inviting in this narrow stretch where the Atlantic Ocean meets the Mediterranean Sea.
The barren coast of Spain looms in one direction; the tip of Africa in the other, less than 10 miles away. From time to time, schools of small fish leap out of the water in unison, as if in symphony.
But with binoculars trained on the horizon, the boat's captain keeps his eyes peeled for something possibly even more terrifying: orcas, also known as killer whales, which have been colliding with boats with disturbing regularity in recent years.
According to the Atlantic Orca Working Group-GTOA, a collaboration of Spanish and Portuguese scientists that monitors the Iberian killer whale population, about 700 orca encounters have been recorded in the past five years. At least half a dozen yachts, fishing boats and sailboats have sunk.
For Spanish fishermen who hunt for tuna, marlin and swordfish at night in the Strait of Gibraltar, the possibility of an encounter with an orca adds an extra element of fear to their already dangerous work.
"Of course they can sink you," fisherman Manuel Merianda told NBC News earlier this month as he plucked stray stingrays from his fishing net after reeling in the night's catch. "They break your rudder and water and waves start coming into your boat, and once the water comes in, there's nothing you can do."
On one of his recent fishing trips, Merianda's boat was followed by a pod of killer whales, an experience he described as terrifying. But when asked if that risk made the killer whales his enemy, Merianda shook his head.
"We are the ones who are in their habitat," he said in Spanish. "We are the ones who don't need to be there."
However, the encounters have become so frequent that Spanish authorities have issued a warning, urging sailors to stay close to the shoreline, as the orcas are not likely to stray there, especially in the summer months.
Often the creatures ram into the rudders, rendering the vessels unusable, and sometimes they even bite off chunks of the boats.
Why Iberian killer whales attack ships in the Strait of Gibraltar, one of the world's busiest waterways, has quickly become one of the sea's most terrifying mysteries. The highly intelligent animals are known for being generally peaceful, especially toward humans, and prior to 2020, such interactions were virtually unheard of.
Not everyone believes that these events, while frightening and dangerous, are literally "attacks."
"Attack implies something aggressive toward humans," said Janek Andre, whose organization WeWhale tries to protect orcas. "These orcas are just playing. So ultimately we call it - and everyone should call it - an interaction."
Almost every day during the summer months, Andre and his teammates board a small boat off the Spanish coast and sail into the strait to track the movements of the orcas. When they spot them, they radio their location to sailors in the area and encourage them to stay away.
Marine biologists have a range of competing theories about what the orcas are doing, from the "play" theory that André advocates to the idea that the orcas are taking revenge, either for the damage that boats have done to orcas in the past or for the human-caused pollution of the waters they depend on.
One theory is that it's just a fad: that the orcas, like teenagers seeking popularity, have simply adopted a strange learned behavior that has, for whatever reason, become the trend of the moment.
However, new research offers a competing theory that is gaining growing support from researchers studying killer whale habitats.
Scientists at the Bottlenose Dolphin Research Institute, located off the northwest coast of Spain, have noted that the once-dwindling population of Atlantic bluefin tuna has rebounded in recent years, becoming the mainstay of the killer whales' diet. Atlantic bluefin tuna are tough prey, swimming faster than killer whales and growing to 4 meters in length.
In a paper published this month in the scientific journal Ocean and Coastal Management, the scientists argue that what people see as attacks are actually older orcas training younger orcas in hunting techniques needed to successfully take down their prey.
"They have to ram, they have to hit, they have to bite, to isolate these big tuna. And it has to be done collaboratively," Bruno Diaz Lopez, the institute's founder and chief biologist, said in an interview. "So how do they reinforce this technique? By practicing."
Orcas, or "killer whales," can grow up to 27 feet long and weigh up to six tons. They are known as the ocean's apex predators and are extremely intelligent, with their own language of clicks and whistles that varies by region.
The subspecies that attacks boats is called the Iberian killer whale, and its future is far from certain. The creatures are on the International Union for Conservation of Nature's "Red List" of critically endangered species. There are believed to be only 35 of them left, and their reproductive rates have fallen to alarming levels.
Faced with sharing the water with the giant mammals, sailors in the strait have tried various methods to deter orcas when they surface. Some make noise to scare them away, throw sand into the water, or rev their engines and try to leave the area. Others advocate turning off the engine, staying still, and playing dead.
As he sailed through the lapping waters back to the coast of Spain, André, the founder of WeWhale, said he believed people would never be able to know for sure why so many interactions with orcas occur.
"We're not orcas. So you can do a lot of science and studies," Andre said. "But at the end of the day, it's such an unknown world to us, what's going on underwater and how these animals actually interact with each other."
This article was originally published on NBCNews.com
