Why marine mammals strand

The causes of strandings
Single strandings, strandings of only one animal, can occur for a number of reasons. Dolphins, whales, porpoises, and seals may strand alone when the animal becomes lost or disoriented or is suffering from an illness, infection or injury. Newly weaned animals sometimes have a hard time thriving on their own and may strand as a result of their own inexperience.  Older animals may also die of natural causes.  Unfortunately, our actions as humans can often affect marine mammals.  Some animals strand and/or die as a result of human interaction. The detrimental interactions include entanglement if fishing gear and marine debris, being struck by ships, being shot, and even being harassed by well-meaning beachgoers.  Over the last ten years, almost 7% of the marine mammals stranding in our response region suffered from some form of Human Interaction.

The phenomenon of mass strandings affects only cetaceans (dolphins and whales, infrequently porpoises). Mass strandings occur when two or more of these animals strand (excluding a mom/calf pair) within the same general geographic region and within the same tidal cycle.  There are usually multiple factors that play a role in causing these events.  The one constant thread among all mass strandings is that like humans, the species of whales and dolphins involved are highly social animals that depend upon the safety and resources of the group in order to survive. This group mentality that is so helpful to these animals at sea can unfortunately cause otherwise healthy animals to strand en masse when they are near shore. When one animal enters shallow water or strands, the entire group may follow.

Other factors that may contribute to a mass stranding include predator evasion, complex topography, tidal fluxes, extreme weather, geomagnetic anomalies, and sonar or other acoustic disturbances.  Mass strandings of whales and dolphins have occurred on Cape Cod for hundreds of years, thus it is unlikely that the events in this region are the result of modern human activities. 

Social structure:
The species of cetaceans (dolphins and whales) that typically mass strand are often pelagic (offshore) animals that form tight social groups. Unfortunately, the very social bond that serves them so well at sea may be the cause of their downfall as they come close to shore.  These animals stick together at all times, so if one animal becomes sick, injured or disoriented, the entire group may strand instead of just the one affected dolphin or whale.

Scientists and researchers believe that threats by predators such as sharks and orcas may cause marine mammals to swim closer to shore where they are at increased risk of stranding.

Complex topography:
Animals come near shore at different times of the year and may become disoriented and trapped by complex inlets and the hook-like shape of areas such as Cape Cod, Massachusetts.  Wellfleet Bay is an additional hook of land within Cape Cod Bay, and this added level of topographic complexity is likely the reason that 60% of all mass strandings in this area have taken place in Wellfleet.  Researchers have also found similarities in the substrates of areas around the world with a high propensity for mass strandings: these locations tend to have gently sloping sandy or muddy flats that may inhibit the animals’ ability to navigate.

Extreme tidal fluxes:
Mass strandings often coincide with full and new moon tidal cycles. The extreme high and low tides during full moons allow animals to swim farther inshore than normal, leaving them high and dry when the tide turns.  The tides in Wellfleet Harbor can fluctuate up to 12 vertical feet (high tide to low tide) during a full moon cycle.   

Extreme weather:
High winds and stormy seas can cause a storm surge, allowing animals to go farther inshore than usual, making them more likely to become stuck when the tide recedes.  It is also thought that these conditions may increase the likelihood that animals become disoriented in complex coastal areas. 

Sonar and other acoustic disturbances:
Since many dolphins and whales rely on sound for navigation, underwater acoustic disturbances from oil exploration, military sonar or other sources may cause disorientation that could result in a stranding.