The bodies of the four dolphins have been recovered in the past three weeks - three in the rivers and one in the ocean off Manasquan. One was in the Navesink, just outside the Middletown home of rocker Jon Bon Jovi.
No cause of death has been established for any of them, said the NOAA, a federal agency whose mission, in part, is to conserve and manage coastal and marine resources.
"We can't show it conclusively at this point, but it's likely these two are from the Shrewsbury-Navesink group of last year," NOAA spokeswoman Teri Frady said.
The dolphin recovered off Manasquan probably wasn't part of the river group based on a comparison of its dorsal fin with photos of the river dolphins' dorsal fins, she added. A bottlenose dolphin's dorsal fin is a unique identifier, almost like a human's fingerprints.
So far, at least six of the 16 dolphins from last year have died.
And it's possible a new group could be back in the rivers within a month. Last year, they showed up in early June, and this is the time of year that coastal migratory dolphins arrive in New Jersey after wintering in the Carolinas.
NOAA scientists and some marine biologists along the East Coast said they think bottlenose dolphins may be trying to expand the limits of their range northward, which would explain why they periodically show up in the Shrewsbury River, a narrow tributary that runs off Sandy Hook Bay and connects with the Navesink in Monmouth County.
Largely because of that belief, they declined to intervene last year despite calls by animal rescue groups to coax or scare the dolphins out of the river before it froze over in winter.
The fates of the remaining dolphins from last year are unknown.
Workers at a riverfront restaurant where the river meets the bay said they saw three to five of the dolphins swim out of the river into the bay on the day before the river froze in January.
The strongest proponent of an intervention to remove the dolphins from the river last year was Bob Schoelkopf, co-director of the Marine Mammal Stranding Center, a nonprofit organization based in Brigantine that tries to rescue and rehabilitate stranded animals. He cited two previous instances when groups of dolphins wandered into the rivers, were left to their own devices and died.
He said the Shrewsbury runs from north to south and feels like the ocean to the dolphins, which don't know it's a dead end.
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