The stranding of a whale or dolphin on a beach occurs when cetaceans come ashore and become stuck. Without the support of water, their massive bodies lead to death from dehydration, overheating, or organ collapse. Strandings are caused by a complex interplay of natural occurrences and increasing human influence on the marine environment. Understanding why these highly adapted marine mammals end up on land requires examining their personal health and the forces that disrupt their navigation.
Individual Health and Geographical Constraints
A significant number of single-animal strandings result from poor individual health. Like any mammal, whales are susceptible to disease, which causes weakness and disorientation, making them unable to keep themselves afloat or navigate effectively. Pathological examination often reveals severe bacterial infections, such as bronchopneumonia, or fungal infections like aspergillosis.
Parasitic load also plays a debilitating role; specific lungworms, or nematodes, have been identified in the respiratory tracts of stranded animals, impairing their ability to dive and communicate. Viral pathogens, including Cetacean Morbillivirus (CeMV), can cause immunosuppression, making the animals vulnerable to secondary infections. Furthermore, old age can naturally weaken an individual, making them prone to being swept ashore.
The physical characteristics of certain coastlines also contribute to the danger, even for healthy whales. Gently sloping shorelines, such as tidal flats or expansive sandbanks, are known as stranding hotspots globally. These areas confuse the sophisticated echolocation systems of toothed whales, which rely on sound bouncing off the seafloor to gauge depth and distance. When sound hits a soft, gradual slope, it is quickly absorbed, effectively turning off the whale’s internal sonar and causing navigational errors that lead them into shallow water.
Natural Navigational Confusion
Whales and dolphins use a sense called magnetoreception to navigate the vast ocean, using the Earth’s magnetic field lines as an internal map. These cetaceans may follow underwater magnetic contours that run parallel to the coast during their long migrations. Disruptions to this natural guidance system can cause them to veer off course and become stranded.
A correlation has been observed between whale strandings and solar storms, which are powerful bursts of energy from the sun that can temporarily distort the Earth’s magnetic field. Gray whale strandings, for example, have been found to be over two times more likely on days with high sunspot activity. The mechanism may be the increased radio frequency (RF) noise generated by solar storms, which is hypothesized to scramble the whale’s magnetic sense.
Social cohesion is another factor in mass stranding events, particularly among highly social species like pilot whales and sperm whales. If a single animal, such as a sick or disoriented leader, enters a perilous shallow area, the rest of the pod may follow due to strong social bonds and distress calls. This “follow-me” behavior explains why entire groups of seemingly healthy animals can suddenly become beached together.
Anthropogenic Acoustic Interference
The increasing amount of man-made noise in the ocean is a threat, as sound is the primary sense whales use to navigate, communicate, and hunt. Naval mid-frequency active sonar (MFAS), used by military vessels to detect submarines, has been linked to mass strandings of deep-diving species, especially beaked whales. The sound pulses from MFAS can reach immense volumes and travel for hundreds of kilometers.
Exposure to this intense sound causes a panic response, leading them to swim rapidly away and alter their normal diving patterns. This abrupt change in depth profile prevents necessary physiological decompression, resulting in nitrogen gas bubbles in their blood and tissues. This condition is similar to decompression sickness, or “the bends,” in human divers. Necropsies of stranded whales following sonar exposure have confirmed these gas emboli, indicating biological trauma caused by a behavioral response.
Other sources of acoustic interference include seismic surveys used in oil and gas exploration. These surveys employ air gun arrays that blast high-decibel, impulsive sounds up to 200 dB into the water every few seconds for weeks or months. These loud, persistent noises can injure whales or cause them to abandon feeding grounds and migration routes. General shipping traffic has also raised ambient ocean noise levels, masking the environmental and communication cues whales rely upon for long-distance navigation.
Other Human-Related Impacts
Beyond acoustic trauma, several other human activities weaken whales or drive them toward shore. Chemical pollution poses a long-term threat through the bioaccumulation of toxins in the whale’s fatty blubber. Persistent Organic Pollutants (POPs), such as Polychlorinated biphenyls (PCBs), and heavy metals like mercury and cadmium, build up in the food chain and are stored in high concentrations in cetaceans.
These contaminants suppress the immune system, making whales vulnerable to the diseases and infections that can lead to stranding. In older animals, high concentrations of heavy metals have been found in the brain. Although the direct link to stranding remains inconclusive, the overall toxic stress weakens the animal’s ability to survive.
Entanglement in fishing gear, including nets, lines, and ropes from traps, is another widespread cause of injury and death. Whales can tow this gear for months, leading to severe lacerations, infection, exhaustion, and an inability to feed or swim effectively, resulting in them washing ashore. Physical trauma from vessel strikes can cause immediate death or severe internal injuries that leave the animal stunned or disoriented, driving it into shallow coastal waters where it eventually strands.