Representatives of the Marine Mammal Stranding Center are trying to recover the animal.
The center's co-director says it is a common dolphin, similar to one that was found dead yesterday, several miles north in the river. He says the dolphin looks bruised and in bad shape.
The center says the dolphin is not part of a pod of 15 bottlenose dolphins that have been swimming in the Shrewsbury and Navesink rivers since June.
Two of those dolphins have died in recent months.
Federal authorities are deciding whether to try scaring the animals out to sea or netting them and taking them there.