SCIENCE -- February 16, 2012 at 3:59 PM EDT
Marine Experts Flummoxed by Mass Dolphin Strandings
Scientists and volunteers respond to stranded dolphins on the shores of Cape Cod. Video by the International Fund for Animal Welfare.
Since January 12, 179 dolphins have been found stranded on the sandy shores, mud flats, and shallow waters of Cape Cod, unable to swim back to sea.
Strandings have been recorded for centuries on Cape Cod; an average of 228 marine animals -- that includes dolphins, whales, porpoises and seals -- are beached on Cape Cod during an average year. But the magnitude -- more than half the annual average in one month alone -- has baffled marine experts, along with the strange fact that it's been limited exclusively to one species: the common dolphin.
"All 178* are common dolphins," said Katie Moore, manager of the marine mammal rescue effort for the International Fund for Animal Welfare, the group leading the effort. "Which is what's scary. If I look over the last 13 years, the average number of common dolphins we see in a year is 38. This is an enormous event."
So what's causing it? The possibilities are long-winded, but the short answer is simple: they don't know.
Certain factors are common to every mass stranding on the Cape. Dolphins are social animals, and their tendency to travel in large groups -- a trait that helps them forage and defend against predators in the wild -- has an unfortunate side effect: stranding in large groups. And the common dolphin species is known for living in especially large groups.
The craggy hook shape of the Cape has a channeling effect that can direct the animals inland, and its long, gently sloping shores that translate to barely any water at low tide keep them there.
The Cape is like an arm," said Trevor Spradlin, marine mammal biologist with NOAA Fisheries Service. "On the bay side, it's very shallow with very extreme tides, and on the ocean side, it's very deep."
There are no indications that tidal patterns have changed and no signs of disease in the 10 dolphins that have undergone detailed necropsies.
"Once the animals are able to get into deep water, they're fine," Spradlin said. "It could be as simple as these animals are just lost. It could be that they're not used to navigating in shallow waters, and they're just victims of the tides."
Warmer water temperatures and behavior of the animals' prey have also been cited as possible factors.
Of the 179 stranded, 108 were found dead, according to the latest data from the International Fund for Animal Welfare. Among the 71 remaining, 53 were successfully released and 18 died, including three that were released and then restranded and four that were euthanized, due to severe injury. That's about a 75 percent release rate, extremely high compared to rates in the past, Moore said.
Stranded dolphins usually die from the sudden onset of gravity, said Gretchen Lovewell, manager of the Stranding Investigations Program at Mote Marine Laboratory. They're designed to be buoyant, and pressure from gravity on land can crush their bodies, causing organ failure. "Their organs get congested with fluids and blood - and then it's a downward spiral."
The team of vets, biologists and volunteers on the Cape have developed a tight and fast response system. When a stranded animal is reported, volunteers rush to the scene, where they keep the gulls from scavenging the beached dolphins. If needed, they shift the animal so it's resting on its stomach, where it's easier to breathe. Team members peform triage, and if the dolphin is still alive, they move it to a medical trailer, where they check for wounds and lesions, monitor heart rate and respiration and draw blood to determine if the bloodwork falls within a normal range.
"We're basically trying to see if these animals look good, if they're behaving normally - as much as they can behave normally when caught out of water," Moore said.
Animals in good shape are transported over land to the open water on the outer Cape, away from the sloping beaches and estuaries. All animals are tagged on their dorsal fin, and select animals get satellite tags for radio tracking. (Satellite tags are saved for a select few due to the cost, which can range from $2,000 to $4,000 a pop.)
Every stranding scene is different, said Tracy Plaut, who has volunteered with marine mammal strandings for 24 years. She says she's found dolphins stuck in the mud, dolphins lying on their backs on dry sand with bellies exposed, and dolphins cut up by oyster shells.
Volunteers and staff members, who have been working around the clock for weeks now, describe cold, wet conditions, dry skin and aching hands. But the situation is especially stressful for the animals.
"I think our handling them, even though we're gentle and methodical and purposeful in what we do, is very stressful for them and very frightening," Moore said. "It may be an anthropomorphic way for me to look at it, but they're probably very frightened."
*The number of total dolphins was upgraded to 179 later in the day.