Basking sharks are the second-largest fish in the ocean, reaching lengths of 12 meters (about 40 feet), and that size means very few animals can threaten them. Adult basking sharks have no confirmed regular predators. But they are not invincible. Killer whales, large sharks, and blood-feeding parasites all take their toll, and humans have historically been the most significant threat of all.
Killer Whales: The Top Natural Threat
Orcas are the only predator reliably documented hunting sharks of this size. While direct observations of orcas attacking basking sharks specifically remain rare, closely related behavior has been recorded in detail with whale sharks, another giant filter-feeding species of similar build and size. Between 2018 and 2024, scientists in the southern Gulf of California documented four separate orca predation events on juvenile whale sharks around 6 meters long. The hunting technique is collaborative and precise: pod members work together to strike the shark and flip it upside down, inducing a state called tonic immobility where the shark can no longer move voluntarily or dive to escape.
Once the shark is immobilized, the orcas target the pelvic and belly area, where there is less muscle and cartilage protecting major blood vessels. The goal is the liver, an enormous, oil-rich organ that in basking sharks can account for roughly 25% of the animal’s body weight. In the whale shark events, a single adult male orca nicknamed Moctezuma participated in three of the four hunts, suggesting certain pods or individuals may specialize in hunting large sharks. This pattern of liver extraction matches orca predation behavior documented with great white sharks off South Africa, where specific orcas have been linked to repeated kills targeting the same organ.
Other Large Sharks
Newborn basking sharks measure between 1.5 and 1.7 meters (about 5 to 5.5 feet). That’s large enough to escape most marine predators, but not all of them. Great white sharks are the most likely candidate for preying on young basking sharks, given their overlapping habitat in temperate coastal waters and their documented ability to take large prey. However, no confirmed kill of a basking shark by a great white has ever been recorded. The Canadian government’s species assessment notes that young individuals are “most likely vulnerable to other large shark species,” but treats this as a reasonable inference rather than an observed fact. Once basking sharks grow past a few meters in length, the list of animals physically capable of killing them narrows to essentially just orcas.
Sea Lampreys and Other Parasites
Basking sharks don’t just face threats from apex predators. Sea lampreys, eel-shaped parasitic fish, attach to basking sharks and feed on their blood for extended periods. Lampreys spend one to two years as blood-feeding parasites in the ocean before returning to freshwater to breed, and basking sharks are a known host species. A study published in Hydrobiologia examining basking sharks in the Northeast Atlantic found that 41.8% of observed individuals had lampreys attached to their bodies. That’s a strikingly high rate of parasitism.
Lampreys latch on with a sucker-like mouth lined with teeth, rasping through the skin to access blood vessels. While a single lamprey is unlikely to kill a healthy adult basking shark, heavy parasite loads can cause tissue damage, blood loss, and increased vulnerability to infection. Various species of copepods, small crustaceans, also parasitize basking sharks by attaching to their skin, gills, and fins. These parasites feed on mucus, skin cells, and blood, and are a constant presence rather than a lethal threat.
Humans Have Been the Biggest Predator
For centuries, the most dangerous thing that ate basking sharks was the commercial fishing industry. Basking shark liver oil was a prized commodity, particularly among Scandinavian fishermen who used it to treat wounds, heart disease, and other ailments. The liver is extraordinarily rich in a compound called squalene, and a single basking shark’s liver could yield hundreds of liters of oil. Targeted fisheries in Norway, Ireland, Scotland, and Canada killed tens of thousands of basking sharks through the 20th century. Their fins also entered the shark fin trade, and their meat and cartilage found various regional markets.
Today, shark liver oil supplements remain commercially available and are marketed for immune support. The basking shark is listed as Endangered on the IUCN Red List, with a decreasing population trend. The primary human-driven threats now include bycatch in fishing gear, vessel strikes in coastal waters, pollution, and climate change affecting the plankton blooms they depend on for food. Most countries with significant basking shark populations have banned targeted fishing, but recovery has been slow. These sharks grow slowly, reproduce late, and likely have long intervals between litters, making their populations especially fragile once depleted.
Why So Few Predators?
Basking sharks survive largely through sheer size. An adult weighing several tons and stretching 8 to 12 meters simply doesn’t fit into many predators’ capabilities. They also spend much of their time in open water or at moderate depths, reducing encounters with coastal ambush predators. Their skin is covered in a layer of tough, tooth-like scales called dermal denticles, adding a physical barrier. Unlike faster-swimming sharks, basking sharks are slow and docile, which makes them vulnerable when a predator is large and coordinated enough to exploit that. But in the ocean, only orcas consistently fit that description.