Sharks have navigated marine environments for hundreds of millions of years, predating dinosaurs. These ancient predators embody a long evolutionary history and play a foundational role in ocean health. However, they now face unprecedented threats, leading to a dramatic global population decline.
Human Fishing Practices
Unsustainable fishing practices represent a significant pressure on shark populations worldwide. Targeted fishing, driven by demand for shark fins, meat, and liver oil, employs methods such as longlines, gillnets, and trawls.
A more widespread problem is incidental bycatch, where sharks are unintentionally caught in fishing gear aimed at other species. Pelagic longline fisheries, for instance, frequently catch sharks, with over 20 million pelagic sharks estimated killed annually as bycatch. Many sharks caught as bycatch die either during capture or after release.
Sharks possess biological traits that make them particularly susceptible to overexploitation. Many species exhibit slow growth rates, reach sexual maturity at a late age, and produce relatively few offspring. This means populations struggle to rebound quickly after depletion. Since 1970, the global abundance of oceanic sharks and rays has declined by over 70%, primarily due to increased fishing pressure. This decline coincides with a doubling of global fishing pressure and a tripling of shark and ray catches during the same period.
Habitat Alteration and Pollution
Physical destruction and degradation of marine habitats pose a substantial threat to shark populations. Coastal development, including residential construction and shipping, directly impacts crucial shark habitats such as coral reefs, mangrove forests, and seagrass beds. These areas serve as vital nursery grounds, feeding sites, and refuges for many shark species, particularly juveniles. Dredging, often associated with coastal development, can also negatively affect young sharks by altering their environment.
Various forms of pollution further compromise shark health. Plastic waste impacts sharks through entanglement and ingestion; discarded fishing gear (“ghost nets”) is a major source of entanglement. Ingested plastics can block digestive tracts or leach chemical pollutants.
Filter-feeding sharks, such as whale sharks, are especially vulnerable to ingesting microplastics. Chemical pollutants like heavy metals and pesticides accumulate in shark tissues through bioaccumulation and biomagnification. As apex predators, sharks are prone to higher concentrations of these toxins, which can transfer to developing pups. Noise pollution from human activities, like shipping, can also disrupt shark behavior and migration patterns.
Climate-Driven Environmental Shifts
Global climate change introduces environmental shifts that stress shark populations. Ocean warming directly affects shark physiology, metabolism, and the distribution of their prey. Many shark species are highly migratory, and rising ocean temperatures are altering their traditional migration patterns, causing them to shift geographic ranges poleward. These shifts can lead to sharks moving outside protected areas, increasing their vulnerability to fishing. Changes in ocean temperature also influence prey distribution and availability, forcing sharks to adapt or face food scarcity.
Ocean acidification, caused by the absorption of increased atmospheric carbon dioxide into seawater, can damage shark physiology by altering their blood chemistry and potentially affecting their neurological functions. Increased acidity can impair sharks’ ability to track prey by odor, a crucial sensory mechanism for hunting. There is also evidence suggesting that ocean acidification can corrode shark denticles, potentially impacting their swimming efficiency. While some research suggests sharks might adapt their teeth to resist acidification, the overall impact on their feeding and survival remains a concern.
Rising sea levels further alter coastal habitats. These changes can degrade or inundate critical shallow-water areas that many shark species rely on for nurseries and feeding grounds. This reduces the availability of safe and productive spaces for young sharks to grow.
Consequences for Marine Ecosystems
Sharks, as apex predators, play a balancing role in marine ecosystems. They occupy the top of the food chain, helping to regulate other marine animal populations. By preying on weak or diseased individuals, sharks contribute to the health of prey populations. Their presence also influences the behavior and distribution of prey species, preventing overgrazing in important habitats like seagrass meadows and coral reefs.
The decline of shark populations can trigger trophic cascades, which are ripple effects through the food web. For example, fewer sharks can lead to an increase in their prey, such as certain fish species. These increased prey populations might then consume more of their own food sources, potentially leading to imbalances at lower trophic levels. On coral reefs, shark removal can alter the behavior of smaller predatory fish, shifting their diets and potentially leading to outbreaks of invertebrates like crown-of-thorns starfish, which can damage corals. The loss of sharks can therefore degrade the health of coral reefs and seagrass beds, impacting their biodiversity and functioning.