The orca, or killer whale (Orcinus orca), is recognized globally as the ocean’s apex predator, possessing no natural predators. Sitting at the top of the marine food web, the species faces a complex array of threats that dictate its mortality. The most significant dangers to the orca’s survival stem from chronic and acute impacts originating from human activity in the marine environment, rather than from natural causes.
Natural Mortality Factors
Even without a natural predator, orcas still succumb to aging, disease, and injury. Biological aging is a significant factor, with life expectancy differing between the sexes. While females can live into their 80s or 90s, the average male lifespan is much shorter, often around 30 years, likely due to the physiological toll of competition and growth.
Debilitating diseases also contribute to mortality, including viral, bacterial, and fungal infections. Necropsies reveal that orcas can suffer from ailments such as stomach ulcers, heart disease, tumors, and various respiratory disorders. Parasites like roundworms and flukes are common, but they typically only become fatal when the animal is already weakened by illness or injury.
Trauma from internal conflicts or hunting accidents represents another avenue for natural death. Though orcas are highly cooperative hunters, failed attacks on large prey, such as large whales or seals, can result in severe injuries that lead to a slow decline. Conflicts within or between different pods can also result in blunt force trauma, though many traumatic deaths are now linked to human interactions.
Pollution and Chronic Toxic Exposure
The most pervasive threat to orcas globally comes from chronic exposure to Persistent Organic Pollutants (POPs). These chemicals, primarily Polychlorinated Biphenyls (PCBs) and the pesticide DDT, are fat-soluble and were widely used in industrial applications before being banned decades ago. Because they do not break down easily, they remain in the environment, leaching into the marine food web.
These POPs undergo biomagnification, where their concentration increases exponentially at each successive level of the food chain. As an apex predator, the orca consumes large volumes of contaminated prey and accumulates the highest concentration of these toxins in its blubber. For example, an orca that stranded in 2002 contained PCB concentrations that were among the highest ever recorded in a marine mammal.
The stored toxins directly compromise the orca’s immune system, causing immunosuppression and making the animals highly susceptible to infectious diseases. Exposure to PCBs increases the risk of tumors and severely impairs the function of the endocrine system. The most devastating effect, however, is on reproductive success, particularly in populations that already face other stressors.
Female orcas offload a large portion of their lifetime accumulation of POPs to their offspring through their milk, poisoning the firstborn calf. This transfer can result in a high rate of calf mortality, with some populations experiencing reproductive failure in nearly 70% of pregnancies. For highly contaminated pods, this toxic burden has led to no successful births for decades, placing these small populations at risk of collapse.
Physical Interference and Habitat Disruption
Direct human activities in the ocean introduce physical and acoustic dangers that disrupt the orca’s habitat and foraging behavior. One direct cause of mortality is physical trauma from vessel strikes, which involve collisions with large ships. Necropsy data indicate that vessel strikes are a significant source of death, responsible for a substantial portion of trauma-related fatalities across all age classes.
Another major physical threat is entanglement, where orcas become trapped in discarded or active fishing gear, often referred to as ghost nets or lines. This includes non-pelagic trawl gear used for bottom fishing, which has been documented killing multiple orcas in a single year in some regions. Entanglement can lead to immediate drowning or a slow death from starvation, exhaustion, or severe injury.
Noise pollution from shipping traffic, seismic surveys, and naval sonar further disrupts the orca’s ability to navigate and hunt. Orcas rely heavily on sound for communication and echolocation to find prey, and excessive background noise can mask these acoustic signals. Studies show that vessel noise causes whales to spend less time in successful deep-water foraging dives, especially for females burdened by the need to feed their young.
Prey depletion, driven by commercial overfishing, disrupts the orca’s food source, compounding the stress from noise and contaminants. For specific populations, such as the Southern Residents who rely heavily on Chinook salmon, a reduction in prey forces them to expend more energy to find food. This weakens their overall body condition and makes them more vulnerable to disease and the effects of chronic toxic exposure.
Acute Environmental Hazards
Sudden, non-anthropogenic events can also result in acute mortality, though these are generally less frequent than chronic threats. Stranding is one such hazard, where an orca becomes beached on shore, often due to severe illness, injury, or disorientation. While the causes of mass strandings are not fully understood, individual strandings can be triggered by rapidly receding tides that trap a whale in shallow water.
Severe starvation can occur suddenly when a pod’s primary food source experiences a rapid shift in migration patterns. If a pod cannot quickly locate alternative prey, the resulting nutritional stress can lead to death, a condition sometimes observed in geographically constrained animals. This starvation risk is exacerbated when an animal is already immunocompromised by pollutants.
In polar regions, orcas face the risk of entrapment in sea ice, a sporadic but recurrent natural phenomenon. Rapidly forming ice sheets can seal off open water, trapping a pod and preventing them from reaching the surface to breathe. While some pods manage to escape, many succumb to exhaustion or freeze, with the lethality of the event depending on the stability of the ice formation.