Why Do Whales Strand?Whale stranding is one unresolved mystery on this planet–or is it?
Whales have stranded since ancient times, although we do not know how many of those were live strandings or how often they stranded. What historical strandings mean is that certain species are prone to strandings that can be triggered by both man-made and natural causes or a complex combination of both. For us, historical strandings do not mean that anthropogenic noise producers are off the hook, but instead mean that certain species are extremely vulnerable.
Live Strandings: Current Hypotheses
Since pelagic species are prone to panicking, whalers have taken advantage of this behavior for some time (Brownell, 2008), most recently to herd dolphins (i.e., in Taiji, Japan). Sperm whales were observed to flee in straight lines, stay their course for hours, and even hit objects that obstruct their path (Mead, 1993). Furthermore, we hypothesize that numerous—if not most—live strandings are caused by panic, for often species that strand are prone to panic.
Whales and dolphins may become disoriented for various reasons. Kogias and common dolphins have magnetite crystals in their brains that might be used in navigation, though this idea has barely been studied. We also do not know whether cetaceans use geomagnetic data in navigation. We do know, however, that even pelagic species are occasionally seen inshore and in coastal waters but without being stranded, so they can clearly navigate in these environments in at least some circumstances (Mead, 1993).
Problems with Echolocation
The echolocation used by dolphins and toothed whales might become distorted in shallow waters. However, baleen whales that live strand do not use echolocation, so such distortion does not fully explain strandings. Experiments have shown that even pelagic cetaceans can adjust their echolocation to adapt to their surroundings. Furthermore, pelagic animals occasionally appear inshore without becoming stranded.
In some cases, live stranded cetaceans are very sick; some might even have large open wounds or neural damage. However, data show that these animals are a minority when it comes to live strandings. Often necropsy is inconclusive, and one study reported that “(92%) of mass stranded animals necropsied presented with no significant pathological findings” (Bogomolni et al., 2010) Even though some animals show symptoms of chronic disease, this does not automatically indicate the cause of stranding, for many wild cetaceans have the same diseases but do not strand. At the same time, problems found in stranded cetaceans could be due to the stranding event itself, such as for example cardiomyopathy due to stress of sranding or evidence of drowning as a result of rolling in surf.
Though social cohesion is often discussed as a cause of strandings, it does not explain live strandings of solitary species, such as baleen whales or kogias. Plus, even highly social species such as sperm whales, pilot whales, and common dolphins often strand alone; in 2013, we recorded six single live sperm whale strandings and 11 single live pilot whales strandings. Why did their pods not follow them? A recent genetic study (Oremus et al., 2013) has shown that in mass strandings of pilot whales, the animals are often not related, which casts doubt on the role of family ties in live strandings. In fact, this finding is more consistent with the panic hypothesis.
Weather conditions are often blamed for live strandings. Indeed, there have been isolated instances of a single stranded dolphin that could not fight the strong surf during the storm, or a juvenile whale that could not navigate sandbars at low tide. However, live strandings occur in all kinds of weather, of which the most remarkable example is well-documented mass stranding in Brazil that happened during pristine weather conditions. Overall, it appears that weather conditions can explain only very few isolated cases of live strandings.
Complex topography can be a factor of live strandings, since many mass strandings occur in hotspots such as Cape Cod, MA, US, or Farewell Spit, NZ. However, pelagic cetaceans visit hotspots occasionally without stranding. It is important to understand that trap-like topography can impede rescue by making whales and dolphins feel trapped, which exacerbates their panic.
Anthropogenic factors have been documented to contribute to both live mass and single strandings. Anthropogenic factors that affect and cause strandings include an array of noise-producing activities, the most well-known of which is sonar. However, many other activities can affect live strandings, including rockets, torpedoes, missiles, explosives, low-flying aircraft, seismic surveys for oil and gas, hydrographic surveys and mapping, and other noise-producing activities. These activities threaten cetaceans, create panic, create acoustic walls that they cannot overcome, and in another sense, act as ancient whaling techniques in which whalers banged on metal pipes to herd whales and dolphins onshore.
Our Opinion on the Causes of Live Strandings
We believe that a small portion of live strandings involves very sick animals, while some others involve confused, weak, and lost calves and juveniles. However, most live strandings and nearly all live mass strandings are panicked responses to threatening stimuli or to the confusion of being lost (e.g., a juvenile humpback stuck on a sandbar).
Though our opinion somewhat differs from mainstream hypotheses, we believe enough data support the panic hypothesis of live strandings. Panic explains both why some species strand and why others do not, as well as why whales and dolphins restrand. It also explains historical strandings before anthropogenic factors were introduced and how these modern anthropogenic factors affect cetaceans. The panic hypothesis is also consistent with inconclusive necropsies, or necropsies which found nothing abnormal. At the same time, it is also the scariest hypothesis because it means that a perfectly normal pod can be dead in no time at all and because it demonstrates how truly vulnerable cetaceans are to anthropogenic influences. The panic hypothesis also makes the currently widespread denials of rescue, medical attention, and euthanasia more sinister.
We also believe that, in some cases, stranding could result from decompression sickness that affects pelagic and deep-diving species. It is unclear whether cetaceans can recover from such sickness, though recovery most likely depends on the extent and severity of embolism.
Note: We believe that earthquakes have nothing to do with live strandings. Terrestrial animals can detect quakes in advance, and there is no reason to believe that cetaceans cannot do the same. There are no data showing that earthquakes cause strandings, though data do show a fin whale fleeing a quake without stranding by swimming in the opposite direction of the epicenter for 200 km.