Some scientists have theorized that a single whale or dolphin may strand itself due to illness or injury, swimming in close to shore to take refuge in shallow water and getting trapped by the changing tide. Because whales are highly social creatures that travel in communities called pods, some mass strandings may occur when healthy whales refuse to abandon a sick or injured pod member and follow them into shallow water.
Mass strandings of dolphins are far less common than mass strandings of whales. And among whales, deep-water species such as pilot whales and sperm whales are more likely to strand themselves on land than whale species such as orcas (killer whales) that live closer to shore.
Some observers have offered a similar theory about whales pursuing prey or foraging too close to shore and getting caught by the tide, but this seems unlikely as a general explanation given the number of stranded whales that have turned up with empty stomachs or in areas devoid of their usual prey.
Does Navy Sonar Cause Whale Strandings?
One of the most persistent theories about the cause of whale stranding is that something disrupts the whales’ navigation system, causing them to lose their bearings, stray into shallow water, and end up on the beach.
Scientists and government researchers have linked the low-frequency and mid-frequency sonar used by military ships, such as those operated by the U.S. Navy, to several mass strandings as well as other deaths and serious injuries among whales and dolphins. Military sonar sends out intense underwater sonic waves, essentially a very loud sound, that can retain its power across hundreds of miles.
Evidence of how dangerous sonar might be for marine mammals emerged in 2000, when whales of four different species stranded themselves on beaches in the Bahamas after a U.S. Navy battle group used mid-frequency sonar in the area. The Navy initially denied responsibility, but a government investigation concluded that Navy sonar caused the whale strandings.
Many beached whales in strandings associated with sonar also show evidence of physical trauma, including bleeding in their brains, ears and internal tissues. In addition, many whales stranded in areas where sonar is being used have symptoms that in humans would be considered a severe case of decompression sickness, or “the bends,” a condition that afflicts SCUBA divers who resurface too quickly after a deep dive. The implication is that sonar may be affecting the whales’ dive patterns.
Other possible causes put forth for the disruption of whale and dolphin navigation include:
- weather conditions;
- diseases (such as viruses, brain lesions, parasites in the ears or sinuses);
- underwater seismic activity (sometimes called seaquakes);
- magnetic field anomalies; and
- unfamiliar underwater topography.