(CNN) -- At least 20 dolphins have died after washing up near several Cape Cod towns, an International Fund for Animal Welfare spokeswoman said Monday.
Between 40 and 50 common and Atlantic white-sided dolphins have been found stranded close to the shore since Thursday, and the number will likely rise, said IFAW spokeswoman Kerry Branon. Some animals were released Monday near Provincetown, bringing the total number of animals saved to 19.
"It may not sound like a high success rate, but when you consider that 27 were alive when they washed up, I think we're doing pretty well." The remaining eight that were stranded alive died.
Stranded living animals are given full health assessments, including ultrasounds and hearing tests; dead animals are given CT scans and necropsies, Branon said. Five released dolphins have been equipped with satellite tags on their dorsal fins to track their location and to see if they're surviving.
Despite January-April being "high season" for dolphin stranding near Cape Cod, IFAW experts aren't exactly sure why so many dolphins are appearing now.
One theory is that the animals get caught in currents when they come close to land to feed. Another theory is that, as social animals, if one dolphin is sick or injured, the whole group will stay with that animal. Finally, the topography of Cape Cod may create areas where the dolphins can get stuck.
Katie Moore, a Cape Cod dolphin rescue veteran of 15 years, says that this is only the second time she has seen this many dolphins washing ashore.
"Sometimes they come up one at a time, other times we see them 10 at a time," said Moore, manager of IFAW's rescue team.
The six-member rescue team is on-call 24/7 and will continue searching this week.