Sharks are widely recognized as formidable predators in marine environments, often perceived as being at the pinnacle of the ocean’s food web. Many species, such as the great white, tiger, and bull sharks, function as apex predators, influencing marine health by regulating prey populations. However, the idea that sharks are universally unchallenged is not entirely accurate. Sharks themselves can become prey, presenting a more complex picture of marine food webs.
Marine Mammals and Larger Sharks
Among the most recognized predators of sharks are marine mammals, particularly orcas, also known as killer whales. Orcas are highly intelligent and cooperative hunters, employing sophisticated strategies to target sharks. They have been observed preying on various shark species, including great whites, sevengill sharks, blue sharks, mako sharks, and even bull sharks. A notable hunting technique involves orcas flipping sharks onto their backs to induce a state called tonic immobility, which renders the shark immobile and vulnerable. This allows the orcas to precisely target and consume nutrient-rich organs, particularly the liver, which is a dense source of fats and oils.
The impact of orca predation on shark populations can be substantial. In some areas, such as False Bay, South Africa, the presence of shark-hunting orcas has led to significant changes in shark distribution, with great white sharks abandoning their traditional aggregation sites. While orcas often hunt in groups, there are documented instances of individual orcas successfully taking down large sharks, demonstrating their exceptional proficiency. This inter-species predation highlights that even large, powerful sharks can be outmaneuvered by highly coordinated and specialized marine mammals.
Beyond marine mammals, larger shark species frequently prey on smaller or younger sharks, a phenomenon known as intra-guild predation or cannibalism. Great white and tiger sharks, for example, eat smaller blue sharks. This behavior is common in bull sharks, mako sharks, and some sand tiger sharks, where individuals consume their own kind. This dynamic emphasizes that size often dictates dominance within the shark family. Great hammerhead sharks also actively hunt other sharks, viewing them as high-energy meals despite the inherent risks involved in such predation.
Other Aquatic Predators
Other aquatic predators opportunistically target sharks, especially smaller or more vulnerable individuals. Large bony fish, such as groupers, consume sharks. Goliath groupers, which can grow over 8 feet long and weigh up to 800 pounds, are powerful enough to swallow a blacktip shark in a single bite. These instances often occur when the shark is weakened or caught, such as on a fishing line.
Marine pinnipeds like seals and sea lions can also prey on sharks, particularly juveniles. While their usual diet consists of fish, squid, and crabs, they will attack smaller sharks when the opportunity arises. Observations include Cape fur seals attacking blue sharks and California sea lions preying on blue sharks in Monterey Bay.
In coastal and estuarine environments, large marine reptiles like crocodiles and alligators can pose a threat to sharks. Saltwater crocodiles opportunistically consume sharks, especially smaller bull sharks, which frequent brackish waters. American alligators inhabiting coastal areas have been found with nurse, lemon, and bonnethead sharks in their stomach contents. These interactions often result from predators sharing habitats.
Human Influence
Humans significantly impact shark mortality, primarily through various forms of fishing. Targeted fishing removes sharks for products such as meat, liver oil, and skin. A major driver of shark mortality is the demand for shark fins, leading to “finning,” where fins are removed and the body discarded.
This practice is wasteful and contributes to the endangered status of many shark species. Bycatch, the unintentional capture of sharks in fisheries targeting other species like tuna or swordfish, also accounts for millions of shark deaths annually. Longline fishing, with its extensive lines and numerous baited hooks, is particularly notorious for high shark bycatch rates.
These accidental catches, combined with targeted fishing, have led to a substantial decline in shark populations, with some species experiencing drops of over 70% in recent decades. Sharks are vulnerable to overfishing due to their slow growth rates, late sexual maturity, and low reproductive output. The scale of human impact surpasses natural predation, posing the primary threat to shark populations worldwide.