The Theory Behind Mass Cetacean Strandings

In February, a pod of nine orcas stranded on the shores of New Zealand’s South Island.

The images that appeared online were horrific, depicting what were once intelligent and charismatic creatures as a series of lifeless and bloodied corpses spread out along the waterline. The circumstances that caused the stranding are still unknown; although the negative impact of the whales’ deaths on New Zealand’s fragile orca population is certain. Unfortunately, while a pod of orcas beaching like this one is something of an anomaly, February’s photos served as a grisly reminder of other cetacean strandings, a phenomenon that occurs several times a year all over the world, while also reminding us how little we actually know about why they occur in the first place. The term “mass stranding” refers to the simultaneous beaching of more than two unrelated individuals, sometimes resulting in the deaths of hundreds of animals at a time. In rare cases, stranded whales are rescued and returned to the ocean, but too often their own weight, once on land, causes fatal internal injuries from which they cannot recover. Although any cetacean has the potential to become stranded, the phenomenon seems to primarily affect pelagic or deep-water cetaceans, such as pilot whales, sperm whales and false killer whales.

Several theories regarding the causes behind stranding have been suggested over the years, though very few have been explored sufficiently enough to become more than conjecture. Some argue that these events are simply a natural, if tragic, occurrence; Aristotle regarded the reasons behind mass whale strandings more than 2,000 years ago. He wrote about whales running aground “rather frequently when the fancy takes them and without any apparent reason,” suggesting that strandings have happened without the interference of man for millennia. The gregarious nature of cetaceans, which live and travel in pods connected by strong familial and social bonds, may be responsible for some stranding cases. Some scientists believe that if sickness or inexperience causes a single whale or dolphin to become stranded, the rest of the pod often falls victim to the same fate. Whether this happens as a result of failed attempts to help their distressed family member, or simply because they refuse to abandon the stranded whale and subsequently become trapped in shallow water by receding tides, is uncertain.

Other theories blame deep-water species’ inexperience with shallow coastal waters, particularly when bad weather results in extreme tidal fluctuations or strong onshore winds. Certainly, there have been several cases of whales beaching themselves in the aftermath of severe storms: In 1999, 54 Atlantic white-sided dolphins became stranded on the shores of Cape Cod after a period of exceptionally stormy weather. Cape Cod sees more than its fair share of strandings each year, which leads to another theory: Areas with confusing or unusual coastal topography, such as jutting promontories of land, gently sloping beaches and complex estuarine systems, are stranding hotspots. This theory makes sense, as similar conditions to those found in Cape Cod also exist in other areas with disproportionate stranding records, including parts of South Africa, Australia and New Zealand. In November 2011, 20 sperm whales stranded on Tasmania’s Ocean Beach, the latest chapter in a recurring tragedy that has seen numerous cetacean pods meet their fate in the same location. The beach is the point at which ferocious winds and appalling sea conditions in the Kerguelen Triangle finally meet land, perhaps explaining the frequency with which mass strandings occur upon its shores.

Several other theories cite natural causes to explain the strandings, including underwater seismic activity, magnetic field anomalies and cetacean disease. However, while natural causes are presumably responsible for the beachings of the past, the increasing frequency of these events points to the role of human activity. Mass strandings of beaked whales have been documented in scientific literature since 1874, with 136 incidents being reported between 1874 and 2004. Of those 136, a staggering 126 occurred after 1950, coinciding with the introduction of mid-frequency sonar in military and mapping exercises. Figures like these strongly suggest that the widespread and indiscriminate use of sonar since the 1950s is directly responsible for the increase in cetacean mass strandings, a theory that many scientists support, and which has been proved in specific cases.

Sonar creates a picture of the underwater landscape, useful in marine mapping projects and for the detection of submarines from a military perspective. Some of the systems project over 235 decibels of sonar sound waves, which can travel for many hundreds of miles through the water. The range of sonar used in naval testing off the coast of California was so extensive that the resulting sound waves were detected on the opposite coast of the northern Pacific. It is thought that it still far exceeds the level known to affect large whales as far as 300 miles from the sonar’s source. The exact effect of sonic waves on cetacean physiology is not known for certain, but scientists think that mid-frequency sonar can cause whales to dive beyond their limits in an attempt to escape the noise pollution it causes. The resulting injuries resemble those suffered by divers with decompression illness; cetaceans involved in strandings associated with the use of sonar display symptoms including bleeding in the brain, ears and internal organs.

During active sonar and frequency transducer testing by NATO in 1996, 12 Cuvier’s beaked whales became stranded along the Greek coastline. A scientific paper researching the incident was published two years later, in which the author determined that the likelihood that the stranding occurred as a direct result of the sonar testing was better than 99.3 percent. Naval activity was also deemed to be the most likely cause of an incident in the U.K. in 2008 that resulted in the deaths of 26 short-beaked common dolphins in Falmouth Bay. A government investigation into the beachings of four separate species of whale in the Bahamas after the U.S. Navy used mid-frequency sonar there in 2000 also found military activity to be responsible for the strandings. Commercial use of sonic waves is also to blame; last year an independent scientific review of an incident that resulted in the deaths of 100 melon-headed whales in Madagascar concluded that the sonar mapping activities of an oil company survey vessel triggered the tragedy.

Research published last year from the University of St. Andrews in Scotland confirmed that both beaked whales, and unexpectedly, certain species of baleen whale, suffer disrupted feeding patterns as a result of sonar interference. In fact, Cuvier’s beaked whales exposed to 200 decibels of sonar from distances of between 2 and 6 miles showed an aversion to the sonar so extreme that they fled the area, diving deeper and for far longer than they normally would. These changes in behavior could result in unsafe diving patterns and harmful interruptions of feeding habits, making that the fallout of sonar use much broader than the strandings themselves. It is possible that the strandings are simply the visible manifestation of a problem that has much wider ramifications throughout the marine ecosystem, resulting in more unseen cetacean deaths at sea.

There has been no evidence so far to suggest that the orcas stranded last month in New Zealand were the victims of either military or commercial sonar. The extent to which sonar is used in global naval activities is huge; if sonar were solely responsible for mass cetacean strandings, one would expect the number of incidents to be much higher. Perhaps sonar amplifies natural risk factors where they already exist, culminating in a fatally dangerous cocktail. That it has directly caused the death of large numbers of cetaceans in some instances is undeniable, but natural circumstances also play a role. Until there’s more conclusive research, we cannot know for sure why so many whales and dolphins die on the world’s beaches each year. In the meantime, organizations like the National Resources Defense Council (NRDC) are going to court to defend cetaceans against military sonar navies around the world.