Killer whales, also known as orcas, are powerful and intelligent apex predators that dominate marine environments across the globe. These highly social mammals exhibit complex hunting strategies and sophisticated communication, making them formidable inhabitants of the ocean. Despite their commanding presence and adaptability, orcas are not immune to the forces of mortality, facing various threats that can lead to their demise.
Environmental and Accidental Causes
Natural environmental factors contribute to killer whale mortality. Natural strandings occur when orcas become beached, often due to navigational errors or while pursuing prey into shallow waters. While severe weather events are rarely a direct cause of death, extreme conditions can indirectly affect orcas by disrupting their prey’s distribution or disorienting the whales themselves.
Food scarcity, often from natural ecosystem shifts, also poses a threat. For instance, Southern Resident killer whales, relying on Chinook salmon, face challenges when salmon populations decline due to natural fluctuations or environmental changes. Injuries from interactions with large, dangerous prey, such as whales or sharks, can also lead to severe trauma, though successful attacks are infrequent.
Health and Disease
Killer whales are susceptible to various health issues and diseases. Natural decline due to old age is a factor, as bodies gradually lose resilience. Bacterial infections are a significant concern, with pathogens like Salmonella, Staphylococcus, and Pseudomonas found in respiratory systems, potentially causing pneumonia or systemic illness, especially in individuals with compromised immune systems.
Viral infections, such as morbillivirus, are a potential threat, having caused mortality in other cetacean species and posing a risk if transmitted to orca populations. Fungal infections, though less studied, are increasingly reported in wild and captive cetaceans, contributing to respiratory and skin issues. Parasitic infestations, particularly from worms like Contracaecum or Anisakis, can weaken orcas, especially young or malnourished individuals, by penetrating stomach linings and leading to secondary bacterial infections.
Human-Caused Mortalities
Human activities pose pervasive threats to killer whale survival. Chemical pollution, including persistent organic pollutants (POPs) like PCBs and DDT, accumulates in orca tissues through their diet. As apex predators, orcas experience biomagnification, leading to high concentrations of these fat-soluble toxins that can impair their immune and reproductive systems. Heavy metals like mercury, cadmium, and lead also accumulate in orcas and their prey, causing toxicity and impacting health.
Plastic pollution poses a double threat through ingestion and entanglement. Orcas can consume plastic debris, leading to internal injuries or blockages, while entanglement in discarded fishing gear or plastic waste can cause severe injuries, starvation, or drowning. Habitat degradation further compounds these issues, particularly through prey depletion from overfishing and habitat alteration affecting food sources like Chinook salmon.
Noise pollution from shipping and sonar disrupts orcas’ echolocation, which they rely on for hunting, navigation, and communication, making it harder to find food and coordinate pod activities. Vessel strikes, or collisions with ships, are a significant cause of traumatic injury and death for orcas, with necropsy studies revealing blunt force trauma in some mortalities. While less common today, historical whaling practices severely impacted orca populations in the past.