CAPE COD, Mass. (CNN) -- At least 81 dolphins have been found dead or died shortly after being discovered on Cape Cod in a series of largely unexplained strandings that began early last month, according to the National Oceanic and Atmospheric Administration.
More than 100 dolphins, identified as common and Atlantic white-sided, have been found stranded along the rocky Massachusetts shoreline since January, said NOAA spokeswoman Teri Rowles and activists involved in the rescue effort.
The mammals have at times washed up in groups of as many as 10, added Katie Moore, a manager for the International Fund for Animal Welfare. Activists say they are transporting the living dolphins by trailers, after they are tagged, to an outer Cape Cod coast where they are being released.
While dolphin strandings are not uncommon on Cape Cod, the animal welfare group's program director called the recent number of dead dolphins "disturbing."
"Cape Cod sees an average of 120 dolphin strandings per year," said A.J. Cady.
It's not clear what's prompted the mass deaths, but recent changes in water temperature could be a contributing factor that has disoriented the mammals, said Wellfleet harbormaster Michael Flanagan.
Once beached, they are vulnerable to predators and susceptible to organ damage and sunburn.
"Usually in the winter, the harbor ices over and inhibits the animals from coming close to the shore," Flanagan said. "But now that the water is warmer, we're seeing lots more dolphins washing up than ever before."
Still, the strandings and causes of death are not clear, said Rowles, who added that the region has not experienced this kind of phenomenon in over a decade. In a similar incident last year, dozens of dead seals were found along the Maine and northern Massachusetts coastlines.