Sharks, ancient inhabitants of the world’s oceans, face a complex array of threats that contribute to their mortality. Understanding these factors, which range from natural biological challenges to widespread human impacts, is important for appreciating the pressures on these marine predators. This article explores the various ways sharks die, shedding light on both the inherent vulnerabilities in marine ecosystems and the significant external pressures they endure.
Natural Predators and Biological Vulnerabilities
Sharks, despite their predatory reputation, are not immune to predation by other powerful marine animals. Larger marine mammals, such as orcas (killer whales), are known to hunt and consume sharks, including great white sharks. Orcas often target the liver of sharks, which is rich in oils and nutrients. Some larger shark species also prey on smaller or younger sharks, a phenomenon known as intraguild predation. Larger bull sharks or tiger sharks may consume smaller or juvenile members of other shark species or even their own.
Beyond predation, sharks face mortality from inherent biological vulnerabilities. Diseases and parasites can weaken sharks, making them more susceptible to other threats or directly leading to their demise. Infections can impair their ability to hunt, migrate, or reproduce effectively. Starvation is another natural cause of death, particularly for young sharks or those in areas with depleted prey resources. When food becomes scarce due to natural fluctuations or environmental changes, sharks may not find enough sustenance to survive.
Environmental Stressors
Large-scale environmental changes in the ocean also significantly contribute to shark mortality by altering their habitats and food sources. Ocean warming can disrupt marine ecosystems. Rising water temperatures affect the distribution and abundance of prey species, forcing sharks to search for food in less optimal areas or face starvation. This warming can also influence shark migration patterns and reproductive cycles, reducing their overall fitness and survival rates.
Ocean acidification, caused by the absorption of excess carbon dioxide by seawater, poses another serious threat. This change in ocean chemistry can impact the formation of shells and skeletons in marine organisms that form the base of the food web, affecting shark prey availability. Shifts in ocean currents can further disrupt the movement of prey and the natural pathways sharks use for migration. These environmental alterations weaken shark populations and increase their susceptibility to other pressures.
Human Activities
Human activities represent the most pervasive and impactful threat to shark populations globally. Commercial fishing practices are a primary driver of shark deaths, particularly through bycatch, where sharks are unintentionally caught in nets or on lines meant for other species. Targeted fishing for shark fins and meat also contributes substantially to declines, often involving unsustainable methods.
Illegal, unreported, and unregulated (IUU) fishing further exacerbates these issues, operating outside of management regulations, making true mortality assessment difficult. Pollution also plays a role in shark deaths. Plastic pollution can be ingested by sharks or their prey, leading to internal injuries, blockages, or starvation. Chemical runoff and oil spills introduce toxic substances into marine environments, which can poison sharks or contaminate their food sources.
Habitat destruction, driven by coastal development and destructive fishing methods like bottom trawling, degrades critical shark habitats. Coastal development can reduce the quality and availability of shallow-water habitats essential for juvenile sharks. Destructive fishing practices can damage the seafloor, destroying ecosystems that support shark prey. Additionally, direct shark culling programs intentionally kill sharks, further depleting their numbers.