The Marine Mammal Stranding Center said two of the dolphins died right away.
The other six were assisted by a veterinarian, the center said, and their conditions were "rapidly deteriorating."
"The decision was made to humanely euthanize the dolphins to prevent further suffering, as returning them to the ocean would have only prolonged their inevitable death," the MMSC said.
All eight dolphins have been taken to the NJ state lab for necropsies.
"We share in the public's sorrow for these beautiful animals, and hope that the necropsies will help us understand the reason for their stranding," the MMSC said.
The dolphins were found along the beach sometime before 11 a.m.
Chopper 6 was overhead during the effort to save the dolphins. People brought buckets of water from the ocean to douse the animals.