Every year thousands of whales and dolphins – some dead, some ill or injured, some perfectly healthy – are found on beaches worldwide.
This phenomenon – called stranding or beaching – isn’t new. It’s been happening for thousands of years, at least.
Even Aristotle wrote about it back in the 4th century BCE. “It is not known why they sometimes run aground on the seashore: for it is asserted that this happens rather frequently when the fancy takes them, and without any apparent reason,” he wrote in Historia Animalium.
A stranding occurs when one or more cetacean species (whale, dolphin or porpoise) becomes marooned on land, usually on a beach.
Most scientists agree on the cause of single strandings – the individual has become ill, and either died at sea (its carcass then floating and washing ashore) or it has stranded because illness has led to weakness, distress or disorientation. Mother/calf pairs are also classified as single strandings.
But when it comes to multiple strandings, more commonly known as mass strandings, scientists don’t have a definite answer.
Image credits: National Oceanic & Atmospheric Adminstration (NOAA), Public domain, via Wikimedia Commons; Auckland Museum, CC BY 4.0, via Wikimedia Commons
Why pilot whales?
While we don’t know why cetaceans strand en masse, we do know which species are most susceptible.
Mysticetes (baleen/toothless) species rarely mass-strand. It is the odontocete (toothed) whales and dolphins that beach themselves in large numbers. In Australia and New Zealand, pilot whales strand in greater numbers than any other species.
But why?
Theories
Scientists have been studying the cause of strandings for centuries. In that time many theories have emerged – some popular and some extremely contentious among experts.
When it comes to pilot whales, specifically, there are several prominent modern theories.
The answer is most likely a combination of these:
Natural causes:
Strong social bonds
Pilot whales, like most toothed-whale species, form pods with extremely strong social cohesion. This means if one member of the group heads towards shore, the rest could follow, their herding instinct kicking in. Similarly, if one becomes stranded, the group will hear that individual’s distress calls and could strand themselves alongside them in solidarity.
Navigational errors
Toothed whale species use echolocation to navigate their underwater world, sending out pulses of high-frequency sound and using the sound’s reflections to map their surroundings.
But sometimes these sonar signals fail, and could do so for a number of reasons.
Bad weather, rough seas and/or dirty water could reduce sonar effectiveness. Echolocation also works best in deep water. This means the whales can run into problems when they encounter a sloping sea floor as masses of sand can absorb the sonar signals too fast. This could make it hard for the whales to realise the sloping sand is there until they are very close. By this time the whales may have already strayed too close to shore, or be trapped.
Confusing tidal patterns
Coastal regions with shallow topography and unexpected tidal ranges could also confuse the pilot whales. In these types of locations, sometimes called ‘whale traps’, the animals may not only have lost their navigational skills due to the shallow water, but can be caught in a race against the confusing outgoing tides.
Chasing prey
Pilot whales could easily make mistakes while hunting, most likely not realising they are entering shallower waters in pursuit of prey, until it is too late.
Food shortages could also be to blame, forcing the whales to travel into less familiar waters to chase animals they wouldn’t usually prey on.
Fleeing predators
Just as pilot whales are themselves hunters, they are also hunted by larger and more aggressive species, such as orcas.
When being chased by these predators, pilot whales might strand themselves in a panic to avoid being caught.
Spooked
It’s not just predators that pilot whales could be fleeing. They could also swim in the wrong direction or even strand themselves after being frightened by a loud sound or strange movement.
Human-made causes:
Although whale strandings have been occurring long before human activities could be to blame, they may now be happening more often, due to human-made disturbances in our oceans.
Noise pollution
This could be the cause of some modern-day strandings.
Loud vessel and other human-made noises might not only spook the whales, but they might also interfere with the effectiveness of their echolocation systems.
Other, more extreme human-made noises, such as those emitted from military sonar activities and seismic surveys, might also cause acoustic trauma in pilot whales, causing long-term hearing problems and, again, affecting their echolocation systems.
Injury
Other human activities, including fishing, boating, underwater construction and polluting, are responsible for many injuries or illlnesses that can cause a pilot whale to strand.
Overfishing also deprives pilot whales of their main sources of food, leading them to hunt different animals closer to shore.
Silver linings
While scientists haven’t yet reached a shared conclusion as to what causes pilot whales to strand, one thing they can all agree on is that every stranding brings them a step closer to discovering the reason.
As distressing as each stranding is, scientists are able to collect invaluable data from each one. This comes from both physical samples taken from the deceased animals, and observations made about the behaviour exhibited by the whales before, during and after each stranding event.
With scientific technologies constantly advancing, and everyday citizens now able to capture footage of mass strandings from every angle, one day soon the mystery could very well be solved, and with it, hopefully, a solution found to prevent them.
