FAQs about strandings

What is a stranding?
NOAA’s National Marine Fisheries Services (NMFS) defines a stranded animal as any dead marine mammal on shore, any live dolphin or whale cast ashore or unable to return to its natural habitat, or any live seal that cannot leave shore due to injury or poor health.

A single stranding occurs when one dolphin, whale, porpoise, or seal comes ashore by itself dead or alive and in need of intervention.  A mass stranding is an event where two or more dolphins or whales (other than a mother/calf pair) strand at the same time in close proximity to one another.  (Seals and baleen whales do not mass strand.)  Mass strandings sometimes involve over 100 individual animals. On Cape Cod, we have frequent single and mass strandings, averaging 226 stranded animals per year.

What types of marine mammals are stranding on Cape Cod?
Four species of seals and fifteen species of cetaceans (dolphins, whales and porpoises) have stranded on Cape Cod’s shores since the stranding network began consistently documenting these events in 1998.

Why are there so many mass strandings on Cape Cod?
Cape Cod is one of only a few places in the world where mass strandings (multiple whales, dolphins or porpoises beaching simultaneously) occur on a regular basis.  There are many theories regarding the causes of these strandings throughout the world.  Often, the causes of these large events remain a mystery.  While we may not know definitively what caused a given mass stranding event on Cape Cod, there are several factors that contribute to their occurrence:

  1. Social bonds - Some cetacean species have very strong social bonds.  These cohesive social groups make a lot of sense in the wild because there is safety in numbers.  Animals evade predators and hunt more effectively in a group.  However, these same social bonds can lead to trouble when one individual ventures into tidally influenced coastal waters.  Whether it is a dominant animal, a sick individual, or a very young animal that heads toward shore, the whole group will often follow leading to a mass stranding when the tide falls.
  2. Location, location, location – There are a handful of places on the planet that experience mass strandings on a regular basis.  Cape Cod usually ranks among the top three sites each year for the frequency of events.  Many of these sites share similar traits with Cape Cod, including a hook-shaped land mass jutting out into the water, gently sloping beaches and convoluted estuarine systems.  All of these factors can contribute to a mass stranding.
  3. Tides – The species that commonly mass strand are pelagic, or offshore animals.  In the Cape Cod region, this includes Atlantic white-sided dolphins, common dolphins, pilot whales, and Risso’s dolphins.  Since these animals spend most of their lives in the deep waters of the open ocean, they are not accustomed to tidal fluctuations.  As a result, when they venture into nearshore coastal waters, they can be caught high and dry when the tide recedes.
  4. Weather – Scientists also theorize that extreme weather may play role in causing some mass strandings and we believe this is true for some of the events that have occurred on Cape Cod.  High winds, coupled with extreme tides may push animals closer to shore, leaving them stranded when the tide falls.

FAQs about Seals
FAQs about Dolphins, Porpoises and Whales