The dolphin is one of about 15 that have been spotted in the river since June and is the third known to die there. It was found dead on Thursday.
National Oceanic and Atmospheric Administration spokeswoman Teri Frady says that the animal was an older female with an empty stomach. She says the same bottlenose dolphin was observed to have been in good condition in November.
Frady says it could take some time to determine whether the dolphin had an illness.
Some environmental groups want the government to approve a plan to move the remaining dolphins from two rivers near Asbury Park, fearing ice on the rivers could kill them.