Two of those bottlenose dolphins have died in recent months.
Schoelkopf said the remaining dolphins now have company. A harbor seal is in the southern end of the Shrewsbury River near Monmouth Beach. It is acting normally and appears to be in good health, he said.
Schoelkopf said the seal shows there is still food in the river.
"The photo we have of it is with a menhaden in its teeth," he said.
Whether there is an adequate food supply for the dolphins in the river is a key element that federal authorities are weighing in trying to decide whether to intervene by either scaring the animals out to sea, or netting them and taking them there.
Both options have so far been rejected as too risky.
Federal officials say they hope that when water temperatures fall, the bait fish the dolphins survive on will leave the river, and the dolphins will, too.
Researchers recently placed underwater microphones in the river to record sounds that might be scaring the dolphins from heading back out to sea.
The recordings are being analyzed as scientist continue to monitor the estimated 12 remaining bottlenose dolphins that have been feeding and frolicking in the Shrewsbury and Navesink rivers since the beginning of summer.
A key question is whether construction work on the Route 36 bridge - a major thoroughfare leading to and from the northern Monmouth County oceanfront - is scaring the dolphins or discouraging them from heading back out to sea.
The dolphins passed underneath the bridge on their way into the Shrewsbury River in early June and must go back out the same way in order to reach Sandy Hook Bay and the ocean.
Animal advocates have wanted for months to coax or shoo the animals back out to sea, citing several previous instances in which dolphins took a wrong turn, ended up in the river and died when weather got too cold.
They 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.
