The U.S. Coast Guard's Atlantic City station and scientists from the National Oceanic and Atmospheric Administration surveyed the Shrewsbury and Navesink rivers in Monmouth County for any sign of the bottlenose dolphins. They didn't find any.
Teri Frady, a NOAA spokeswoman, says the lack of dolphin sightings could mean the last five have escaped from the river.
"Clearly, the fact that there were no live animals in the rivers and that there were no carcasses spotted is a good sign," she said.
The owner of a restaurant on the banks of the Shrewsbury says he and some of his employees saw the dolphins swim out into Sandy Hook Bay last month.
Three of the original 16 dolphins spotted in the river last June have died. The fate of the others is unknown.
The dolphins had been at the center of a tug-of-war between federal wildlife officials - who said they would leave the dolphins alone unless they appeared to be in imminent danger - and animal rescuers, who wanted them either removed or coaxed out of the river and back out to sea.
Local rescuers worried 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.
The restaurant owner who said he saw the dolphins swim out to the bay said they did so right before the Shrewsbury froze over.
NOAA scientists say they believe the dolphins originally swam into the rivers to pursue fish they feed on.
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