The agency says it counted only five bottlenose dolphins Monday and Tuesday in the Shrewsbury River, and none in the Navesink. There were 12 dolphins spotted at the beginning of December; the agency says eight appeared to have lost weight.
The whereabouts of the other dolphins are not known.
Three have died so far, including the most recent one found on Christmas Day.
"NOAA expects additional mortalities or live strandings among the remaining animals may occur as the winter progresses," the agency said in a written update issued Wednesday afternoon. "We also continue to believe that these animals are not candidates for intervention and relocation."
Teri Frady, a spokeswoman for the agency, said in an e-mail Wednesday that "We do not improve their prospects by attempting to force them from the area. Distance, numbers, acclimation to the area, high likelihood of mortalities in a relocation attempt argue against it."
"What we have been doing is protecting their ability to use the very instincts and behaviors that also ensure the long-term health and survival of individuals in the larger population," she wrote.
"They may fail, but that, too, is a common and important pattern in nature."
Frady said it is possible that as many as eight dolphins have made it out to sea on their own, but cautioned that can't be proven.
"They have not been resighted, alive or dead, and are not matched to any recovered carcass," she wrote. "Anything else is speculative."
The dolphins are at the center of a tug-of-war between federal wildlife officials - who plan to leave them alone unless they appear to be in imminent danger - and animal rescuers who want them either removed or coaxed out of the river and back out to sea.
The animals first showed up in the two rivers in early June, feeding on the abundant mossbunker that fill the rivers during the summer.
Animal advocates have wanted the dolphins removed since before the July 4th holiday, citing heavy boat traffic in the river. They also worried whether ongoing repair work on the Route 36 bridge between Highlands and Sea Bright - under which the dolphins passed on their way in - was scaring them from swimming back out to sea.
"It would have been nice to see this earlier - an admission that something not good is happening with these animals, that they're not happy and healthy - while we still had time to do something about it," said Bob Schoelkopf, co-director of the Marine Mammal Stranding Center in Brigantine.
Scientists are split on whether the dolphins can survive the winter. A panel assembled by NOAA in mid-December said there is evidence of dolphins surviving winter in cold water climates.
Local rescuers, though, worry that waiting too long could invite a replay of a scenario that resulted in the deaths of four dolphins that lingered in the river in 1993. Ice eventually closed in on them and they drowned.
Whether there is enough food for the dolphins to survive the winter has been a key question for scientists and rescue advocates.
The animal found dead on December 25 was an adult female carrying a third trimester fetus. There were lesions on the adult's lungs, and it had nothing in its stomach.
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