Natural Predators
Sharks are not immune to being preyed upon by other marine animals. Larger, more powerful marine mammals occasionally target sharks, particularly orcas (killer whales). Orcas are known to hunt various shark species, including great white sharks, employing coordinated tactics to incapacitate their prey. Orcas sometimes target the shark’s liver due to its high nutritional content.
Beyond marine mammals, other larger shark species also threaten smaller or younger sharks. Cannibalism is observed in many shark populations, where larger adults prey on juveniles or smaller species. For example, bull sharks consume other smaller sharks, and larger great white sharks may prey on smaller individuals or other species in their territory. These interactions are a natural part of marine ecosystems, regulating shark populations.
Environmental and Biological Factors
Sharks face environmental and biological challenges that can lead to mortality. Diseases and parasitic infestations can weaken sharks, making them susceptible to other threats or directly causing death. Bacterial infections, viral diseases, and heavy parasitic loads compromise their immune systems and overall health, impacting their ability to hunt, migrate, or reproduce.
Extreme environmental conditions also threaten shark survival. Sudden drops in water temperature can lead to cold stunning, causing them to become lethargic. Warm water events can stress sharks, affecting their metabolism and increasing vulnerability to disease or starvation due to shifts in prey distribution. Natural changes in ocean currents or severe weather events like hurricanes can disorient sharks or strand them.
Natural habitat degradation impacts their food sources and nursery grounds. Events like coral bleaching or reduced prey availability can lead to starvation or weakened states. Sharks also have biological vulnerabilities, including slow growth rates and late sexual maturity, which means their populations recover slowly from decline. Some species are intolerant to low oxygen levels, which can be lethal in hypoxic zones.
Human Activities
Human activities represent the most significant threats to shark populations globally. Fishing practices are a primary cause of shark mortality, encompassing targeted fishing for fins and meat, and accidental capture, known as bycatch. Sharks are often caught in gear set for other species, such as longlines, gillnets, and trawls, leading to injury or death. Ghost fishing gear, including discarded nets and lines, continues to entangle and kill sharks.
Pollution also endangers sharks and their marine environments. Plastic pollution, from microplastics to larger debris, can be consumed by sharks, leading to internal injuries, blockages, or starvation. Chemical pollutants, such as persistent organic pollutants (POPs) and heavy metals, accumulate in shark tissues, affecting reproductive health, immune system, and survival. Noise pollution from shipping and seismic surveys can disrupt shark communication and navigation.
Human-driven habitat destruction and alteration impact shark survival. Coastal development destroys nursery grounds and feeding areas. Dredging operations disturb the seafloor, altering habitats and reducing prey availability. These activities fragment shark populations and reduce ecosystem carrying capacity.
Climate change is another growing threat, altering the ocean environment. Ocean warming affects shark metabolism and can force species to shift geographic ranges, disrupting food webs. Ocean acidification impacts calcification processes of marine organisms in the shark’s food chain and may affect shark sensory systems. Sea-level rise and increased extreme weather events degrade coastal habitats sharks rely on.