(CNN) -- The number of unexplained dolphin deaths on Cape Cod, Massachusetts, rose Friday, with rescuers tallying the toll at 103.
About 160 common dolphins have been found since the animals began stranding themselves in early January, said Michael Booth, a spokesman for the International Fund for Animal Welfare, the organization leading the rescue the effort.
The beachings have puzzled investigators, as rescue team members struggle to treat, tag and transport the living dolphins to the outer Cape Cod coast to be released.
While dolphin strandings are not uncommon on the cape, Katie Moore, a marine manager for the group, said earlier this week the event is extraordinary in its "protracted" nature as well as the number of dolphins involved.
"There is a large variability year to year," she said, but this event represents "more than half my annual average in a month," said Moore.
Once beached, a dolphin is vulnerable to predators and susceptible to organ damage and sunburn. If a dolphin is still alive, the responders get it onto its stomach, if it is not already, for easier breathing. They keep away seagulls that would otherwise peck at it and warm it with blankets or cool it with water as necessary, Moore said.
Necropsies had been performed on at least nine of the dolphins, and blood and microbial swab samples have been taken from some that were found alive, Moore said.
So far, no pattern of disease or trauma has been found that would point to a cause.
Although the winter and early spring are the normal time of year for dolphin strandings to occur, the weather this season has been unusually warm, leading to speculation about climate change and subsequently low "distribution of prey" as possible causes.
Wellfleet harbormaster Michael Flanagan had earlier explained that usually, in the winter, "the harbor ices over and inhibits the animals from coming close to the shore. But now that the water is warmer, we're seeing lots more dolphins washing up than ever before."
Moore cautioned that, while climate or other external factors such as acoustic disorientation can't be ruled out, "we don't have a single answer."