The scalloped hammerhead shark (Sphyrna lewini) is a distinctive marine species recognized by its uniquely shaped head, which features a central indentation and prominent grooves, giving it a ‘scalloped’ appearance. Found globally in warm, coastal, and open ocean waters, these sharks often form large schools. The International Union for Conservation of Nature (IUCN) has classified the scalloped hammerhead as Critically Endangered, indicating a severe risk of extinction. This status is primarily due to their biological traits, intense fishing, and habitat degradation.
Biological Factors Increasing Vulnerability
Scalloped hammerhead sharks possess biological characteristics that make them susceptible to population decline. They exhibit a slow growth rate, with females reaching sexual maturity between 15 to 17 years and males around 6 years. This delayed maturity means individuals must survive for a long period before they can reproduce.
Once mature, their reproductive output is low. Females give birth to 12 to 41 pups after an 8 to 12-month gestation, but they do not reproduce annually, often breeding every other year. Their schooling behavior, while serving various ecological purposes, inadvertently makes them easy targets for large-scale fishing operations, as many individuals can be caught simultaneously. This combination of slow growth, late maturity, and low reproductive output leaves scalloped hammerheads with a low capacity for population recovery.
Intense Fishing Pressure
Intense fishing pressure worldwide is the most significant threat to scalloped hammerhead populations. These sharks are targeted for their fins, highly valued in the international shark fin trade, particularly for use in shark fin soup. Estimates suggest that between 1.3 and 2.7 million scalloped and smooth hammerheads are exploited for this trade annually. This demand often drives illegal, unreported, and unregulated (IUU) fishing practices, which further exacerbate population declines.
Beyond targeted fishing, scalloped hammerheads are frequently caught as bycatch in fisheries aiming for other species, such as tuna, swordfish, and shrimp. Fishing gear like longlines, gillnets, and trawls capture these sharks. When caught on longlines, scalloped hammerheads, which rely on constant movement for oxygen intake (ram ventilation), often suffocate, leading to high at-vessel mortality rates, sometimes exceeding 90%. This means that even if released, many do not survive.
Population declines of 83% to 98% in some Atlantic regions and up to 99% in parts of the Mediterranean Sea reflect these threats. To regulate international trade and promote conservation, the scalloped hammerhead shark is listed under Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
Habitat Degradation
Habitat degradation also contributes to the decline of scalloped hammerhead sharks, impacting their essential living spaces. Coastal development and pollution from various sources, including chemical runoff, agricultural waste, and plastics, directly harm their marine environments. These pollutants can lead to bioaccumulation in the sharks and their prey, affecting their health and reproductive success.
Climate change contributes to habitat degradation through rising ocean temperatures and ocean acidification. Warmer waters can alter the distribution of their prey, forcing sharks to shift foraging grounds, and increased temperatures can also affect their metabolism. Ocean acidification can damage the sharks’ denticles, which are tooth-like scales covering their skin, potentially impairing their ability to swim effectively. Additionally, more frequent and intense storms can devastate critical nursery areas in shallow coastal bays and mangroves. The degradation of these vital ecosystems, which serve as safe havens and food sources for juvenile sharks, indirectly impacts the species’ survival.