The grey reef shark, a predator in tropical waters, is common across coral reef ecosystems of the Indo-Pacific and Indian Oceans. They help maintain the balance of their marine environment. Often found patrolling reef edges, they are a classic reef-dwelling shark, moving through sunlit shallows and deeper drop-offs.
Physical Characteristics and Habitat
Grey reef sharks have a stocky, torpedo-shaped body, bronze-grey above and pale white underneath. A black margin marks their tail fin’s trailing edge, with dark tips on other fins. Some populations in the Western Indian Ocean and Red Sea may have a white-edged first dorsal fin. They generally reach 1.88 meters, with larger individuals up to 2.55 meters.
They inhabit warm, clear waters around coral reefs, atolls, and lagoons across the Indo-Pacific and Indian Oceans. They prefer reef drop-offs and channels with strong currents, found from the surface to 280 meters deep. While usually near the bottom, they may ascend to the surface, especially when investigating food.
Social Behavior and Diet
Grey reef sharks are social, often forming groups of a few to over one hundred individuals during daylight hours. These groups occur near reef passes and shallow lagoons; sharks disperse at night to hunt individually. They spend time with the same associates daily, suggesting they recognize other sharks.
Their diet includes small bony reef fish like cowfish, butterflyfish, and surgeonfish. They also prey on cephalopods (squid and octopus) and crustaceans (shrimp and lobsters). Hunting is often heightened at night.
A unique behavior is their distinct threat display, used to warn off danger. When cornered or threatened, the shark arches its back, lowers its pectoral fins, and swims with an exaggerated, stiff side-to-side motion, sometimes in spirals or figure-eight loops. This display, which can also include raising its snout, signals agitation and a potential defensive strike if the threat persists.
Relationship with Humans
Grey reef sharks are inquisitive and may approach divers, especially upon initial entry. While not aggressive towards humans, their behavior can change with food present or if provoked or cornered. They are popular for dive tourism due to their consistent presence at certain reef sites.
Bites on humans have occurred, often during spearfishing when a shark strikes a speared fish near a diver. These incidents are defensive, as the shark attempts to deter intrusion into its personal space rather than actively hunting a person. Recognizing and respecting the shark’s threat display, which signals discomfort, is important for divers to avoid escalating interactions.
Conservation Concerns
The grey reef shark is assessed as Endangered by the International Union for Conservation of Nature (IUCN). Population reductions are estimated at 59.2% over 44 years, reflecting significant declines across its range.
Threats include targeted and bycatch fishing, driven by demand for their fins, meat, liver, and skin. Their association with coral reefs also makes them vulnerable to habitat degradation from coral reef destruction, poor water quality, and climate change impacts like rising sea temperatures and ocean acidification. The species’ slow reproductive rate, with females giving birth to one to six pups every two years after a 9 to 14-month gestation, further exacerbates their vulnerability to overfishing and environmental pressures.