Historic Dolphin Stranding: What We Learned

BREWSTER – There are plausible theories, but no definitive explanations, for the largest recorded mass stranding of bottlenose dolphins in the northeast U.S. last week, when around 30 of the animals stranded themselves on the flats in Cape Cod Bay between Brewster and Wellfleet

The International Fund for Animal Welfare (IFAW) responded to the strandings around 5:30 a.m. on Monday, July 13, finding as many as 32 stranded bottlenose dolphins between Ellis Landing in Brewster and First Light Beach in Eastham. All but five or six were freed when the tide came in, but five or six died before responders arrived. On Tuesday morning, about 20 dolphins stranded again. Working from the beaches and from a boat, IFAW workers — many of them volunteers — herded them to deeper water, affixing two with real-time satellite tracking tags. By Wednesday, those tracking tags brought good news.

 “Thanks to the Caisley identification tags placed during the rescue response, our team confirmed that the rescued dolphins had successfully integrated into a larger group — a promising indication that at least some of the stranded animals were once again traveling and socializing with wild dolphins in deeper water,” an IFAW update announced.

 Volunteers documented and measured the deceased dolphins, and two necropsies were performed on Wednesday. But the medical examinations yielded no clues about the cause of the stranding, according to Dr. Sarah Sharp, animal rescue veterinarian at IFAW.

 “Findings included no obvious pre-existing disease, near to full-term pregnancies in both, and evidence of trauma from the stranding event (bruising, superficial cuts, froth in the lungs). Preliminary cause of death is attributed to stranding trauma, likely exacerbated by the late-stage pregnancies,” she said. 

Strandings of Atlantic white-sided dolphins are not uncommon on the Lower Cape’s bay shoreline, the crook in the Cape’s bended arm. One theory is that the shallow tidal flats, made up of soft, silty sand, can interfere with the animals’ echo-location and keep them from finding deeper water. But last week’s stranding was unique because the species, bottlenose dolphins, are generally found well offshore. Climate change could be playing a role.

 “As the northwest Atlantic Ocean warms, species that were more commonly found farther south in the past are likely to become increasingly common in our region, and as a result, bottlenose dolphin strandings on Cape Cod have been increasing over the past seven years,” Sharp said. But why this group came so close to shore is a mystery. There were anecdotal reports of bait fish close to the shoreline which might have lured the dolphins into shallow water, “but neither of the necropsied dolphins had stomach contents that would support that theory,” Sharp said.

 “Whatever brought them in, the large tidal fluctuations this past week of over 13 feet in Wellfleet made it a very dangerous situation for them, in an area that is challenging for them to navigate to begin with,” she said.

The IFAW marine mammal rescue team has had a busy month. Between July 6 and July 14, responders managed separate mass strandings involving two other dolphin species: seven common dolphins and 15 Atlantic white-sided dolphins. They also responded to individual strandings of a harbor porpoise and a minke whale. Experts suspect no single cause was involved. But mass strandings test the capacity of stranding teams, Sharp said.

 “Large groups of very large dolphins stranding certainly put our team to the test, especially when they strand simultaneously in multiple locations,” she said. While IFAW has a vehicle that can transport stranded dolphins, using it to move a large number of 800-pound animals to release in deeper waters isn’t possible. 

 “So we have to change tack and provide supportive care and refloat [the dolphins] in place,” she said. “As a result, we have adjusted some of our response supply organization to better support refloat-in-place operations.”