Of five dolphins stranded on a Cape Cod beach at low tide last week, four survived, and were returned to the ocean. The five dolphins were juvenile common dolphins, clearly distressed, vocalizing and thrashing in the sand. They were found in the early morning by walkers on the Provincetown Harbor Beach, according to the International Fund for Animal Welfare (IFAW), which was called to the scene by the Cape Cod police department. One dolphin died before being rescued; the manager of IFAWs marine mammal rescue unit explained that dolphins experiencing the effects of gravity for the first time may go into shock from the trauma, leading to a fatal shutdown of the body.
The surviving four dolphins were transported by IFAW by trailer to Herring Cove beach, where they were examined and blood-tested to ensure that they were healthy enough to survive before being released. The beach where they were released was chosen for its favorable underwater topography – it gets 20 to 30 feet deep within a few feet of the shore. The dolphin that died will be autopsied and the resulting information shared with other marine mammal researchers. The four dolphins who survived the stranding on the Cape Cod beach were released together to enhance their chances of survival; due to their social nature, they tend to do things in groups. During the rescue, volunteers take advantage of this fact, calming the marine mammals by positioning them so that they can see one another.
Dolphins become separated from larger groups, and may become caught in shallow waters between sandbars, or swim up creeks and become trapped by a tidal fluctuation. IFAW responds to 200 to 500 marine mammal strandings a year, although a stranding of five dolphins at a time is unusual. On average, 226 animals strand themselves on the Cape Cod shoreline; this year the number of animal strandings is somewhat lower. So far this year, the region’s marine mammal response team has responded to 44 animal strandings, only two of which involved dolphins.
Scientists offer numerous theories on why the marine mammals get stranded. It is possible that because of their social nature, the stranding of a single animal due to sickness or injury leads other animals who refuse to abandon the pod member, to strand themselves as well. Mass strandings of deep-water species of whales are more likely than mass strandings of other whale species (such as orcas) that live in more shallow waters. Mass strandings of dolphins are less common than whale strandings.
Strandings may occur due to the use of low- and mid-frequency sonar on US Navy ships. The intense underwater sonic blasts in use by the military have been linked to serious injury and death among marine mammals. In year 2000, four species of whales were found stranded on beaches in the Bahamas after the use of mid-frequency sonar in the area by the U.S. Navy: a government investigation found the Navy’s use of sonar at fault in the strandings.
Whales in strandings caused by sonar are often found with physical trauma, such as hemorrhaging in brain, ears and internal tissues. Whale strandings associated with use of sonar may show symptoms of decompression sickness, known as “the bends” – evidence that the animals have resurfaced too rapidly after a deep dive to avoid the noise. Scientists offer other possible causes of strandings such as weather conditions, magnetic field abberations, disease, seaquakes, or unfamiliar underwater topography. It is possible that more than one cause may exist.
Over the years, several rescues of stranded dolphins, seals, and whales have taken place on Cape Cod – something of a “hotspot” for these events – enabling the development of strategies for such situations. The dolphins who survived last week’s stranding on the Cape Cod beach are the beneficiaries of a store of knowledge accumulated about how best to help the animals.
By Laura Prendergast
Nature World News