Cetacean stranding occurs when whales, dolphins, or porpoises become stuck on land or in water too shallow to escape. The phenomenon is broadly categorized into two types: single strandings, which involve one animal or a mother-calf pair, and mass strandings, which involve three or more individuals simultaneously. Single strandings are frequently linked to individual sickness or injury, while mass events often involve highly social species. Understanding why these animals end up helpless on shore is complicated, as the causes are often multi-factorial, involving a combination of natural vulnerabilities and human impacts.
Natural Biological and Navigational Errors
Many stranding events are a consequence of natural factors that compromise an individual whale’s ability to navigate or survive in the open ocean. A whale suffering from severe illness, a high parasite load, or the disorientation that comes with old age may become too weak to swim effectively or make sound decisions about its location. These animals may intentionally seek the shallower, calmer waters near the coast as a final attempt to rest or recover, only to become trapped as the tide recedes.
Navigational confusion is another natural factor, especially in areas with certain coastal geography. Whales and dolphins use echolocation to map their underwater surroundings. However, the sounds emitted can be poorly reflected by gently sloping coastlines or large, soft sandbars. This poor reflection can make the shallow water appear to be a continuation of the deep ocean, leading them toward the shore.
The Earth’s magnetic field may also play a role, as many species of whales are believed to use these magnetic lines as an internal compass for long-distance migration. Scientists hypothesize that sudden, localized disruptions in this geomagnetic field, perhaps caused by solar storms or unusual magnetic anomalies, can throw off a whale’s sense of direction. A whale whose magnetic map is temporarily distorted may follow an incorrect path that leads them directly into shallow, hazardous coastal waters.
Impact of Anthropogenic Noise and Pollution
Human activity introduces stressors that interfere with a whale’s sensory world and overall health. The most direct human-related cause is the use of mid-frequency active sonar (MFAS) by naval vessels, which has been strongly linked to mass strandings of deep-diving beaked whales. The intense, sudden sound from sonar appears to trigger an extreme panic or flight response, causing the whale to ascend rapidly from deep water.
This rapid ascent overrides the deep-diving physiology of the whale, leading to a condition similar to decompression sickness, or “the bends.” Necropsies on beaked whales stranded following naval exercises have revealed lesions and nitrogen gas bubbles in their blood and tissues, confirming trauma consistent with this rapid pressure change. The pain and incapacitation from this acute acoustic trauma can cause the whale to beach itself.
In addition to acute noise, chronic noise pollution from commercial shipping and oil exploration can mask the calls whales use to communicate, navigate, and locate prey over long distances, causing chronic stress. Furthermore, chemical contamination from accumulated toxins like polychlorinated biphenyls (PCBs) and heavy metals can weaken a whale’s immune system. A whale with a compromised immune system is more susceptible to disease and neurological impairment, making it more prone to navigational errors that lead to stranding.
Social Structure and Mass Stranding Events
Mass strandings are most common among highly social toothed whales, such as pilot whales, false killer whales, and sperm whales. These species live in pods with strong social cohesion, often led by a single matriarch or dominant individual. If the leader or a key member of the pod becomes sick, injured, or disoriented due to natural or human-related causes, the entire group may follow that distressed individual into shallow water.
This herd mentality means the animals prioritize social bonds over self-preservation, remaining near their struggling pod mate even when they are physically healthy. The distress calls of the stranded whale can prompt others to approach the beach, leading to a disastrous collective event. Even when human rescuers manage to refloat and push healthy animals back out to sea, they often attempt to re-strand themselves to remain with the rest of their group. This powerful social drive is why mass stranding events are a recurring tragedy for these specific species.
The Immediate Physical Consequences of Beaching
Once a whale is stranded, its body is immediately subjected to the force of gravity. On land, the weight of the animal begins to crush its internal organs, including the lungs and blood vessels. This immense pressure on the body cavity constricts blood flow and leads to organ damage and circulatory collapse.
A stranded whale is also at high risk of rapid overheating, or hyperthermia. Their thick layer of blubber insulates them, but on land, this insulation quickly causes their internal temperature to spike. The whale is unable to dissipate this heat without the surrounding water, leading to fatal overheating and dehydration. The whale’s weight also restricts the movement of its chest wall, which impairs its ability to properly inflate its lungs, often leading to slow suffocation.