The basking shark, Cetorhinus maximus, is the second-largest fish species in the world, surpassed only by the whale shark. This species commonly reaches lengths of 26 feet (7.9 meters) and occasionally exceeds 33 feet (10 meters). The shark is a slow-moving filter feeder, using its immense mouth to strain tiny zooplankton from the water column. Its common name comes from its habit of feeding near the surface, where it appears to be simply resting in the sun. Despite its formidable size, the basking shark is entirely harmless to humans.
Known Natural Predators
The question of natural predation on the basking shark primarily revolves around the Orca, or Killer Whale, which is the only predator known to regularly target large sharks. Orcas are highly intelligent marine mammals with specialized hunting strategies, and different orca populations, or ecotypes, focus on different prey. The ecotypes that target other large marine life are the most likely threat.
Orcas are documented to prey on other large shark species, including the Great White Shark, often targeting the liver due to its high fat and nutrient content. This predation involves a specialized technique where the orca flips the shark upside down to induce tonic immobility, which renders the shark temporarily paralyzed. While attacks on adult basking sharks are rarely observed, a report described an orca removing the liver from a basking shark. Juvenile and smaller basking sharks face a greater risk from other large predatory sharks, such as the Great White Shark, which may opportunistically prey on younger, vulnerable individuals.
Factors Limiting Predation
The sheer scale of an adult basking shark is the most significant factor limiting natural predation. Most adult individuals reach a length and mass that makes them too large and unwieldy for all but the largest orca pods to tackle successfully. A mature basking shark can weigh several tons, presenting a substantial physical challenge to any potential predator.
The shark’s body is also protected by a tough, thick skin that makes penetrating to the shark’s vital organs a difficult task for a predator. Beyond their physical attributes, basking sharks engage in behavior that helps them avoid surface-dwelling predators. During the winter months, they are known to migrate to deep, offshore waters, with some individuals recorded diving to depths exceeding 4,100 feet (1,250 meters). This seasonal migration to the deep ocean places them out of the reach of many surface-hunting marine predators for extended periods. Their movements are often guided by the availability of zooplankton, but their choice of deep-water habitats in the winter contributes to reducing their exposure to the threat of predation.
Threats from Human Activity
Despite having few natural enemies, the basking shark population faces significant challenges primarily stemming from human activities. Historically, the species was heavily targeted by commercial fisheries for its liver, which yields a large volume of oil rich in the compound squalene, and for its valuable fins. This targeted fishing pressure severely depleted populations in the North Atlantic and Pacific.
Modern threats persist even after the cessation of most targeted fisheries. Accidental entanglement, or bycatch, in commercial fishing gear like trawls, surface-set gillnets, and ropes continues to be a major cause of mortality. Because they are slow-moving and feed near the surface, basking sharks are also highly susceptible to vessel strikes from ferries, container ships, and recreational boats, often resulting in serious injury or death.
As filter feeders, they are vulnerable to the indirect effects of a changing ocean environment. Climate change is altering ocean temperatures and circulation patterns, which can affect the distribution and abundance of the zooplankton that form the shark’s entire diet. Changes in this foundational food source could disrupt their migration patterns and reproductive success, posing a long-term challenge to the species’ survival.