TRENTON, N.J. -- Wildlife officials say the wayward beluga whale known as Helis may be making its way out of the Delaware River.

It was moving south Sunday and was last seen around the Tacony-Palmyra Bridge, or about 15 miles from where it has been in recent days. And officials are optimistic that the whale is finally ready to head back to the ocean.

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A federal fisheries official said observations and photographs suggest the whale is healthy. Officials had no plans to try to intercept the whale to examine it.

"Visually, he looks to be healthy," said Jamison Smith of National Oceanic and Atmospheric Administration's National Marine Fisheries Service.

Officials said they were reassured by the whale's attempts to avoid the water patrol following it. "It's a good sign, because it's normal, adult beluga behavior," Smith said.

Authorities continued to ask boaters to stay at least 150 feet from the whale. "(Boats) could alter its behavior or cause it to become disoriented or stressed or stay in the area," Smith said.

Researchers in Canada believe the whale is a beluga they've dubbed Helis (pronounced ay'-LEE), derived from the French word helice, or propeller. Experts believe a scar on Helis came from a boat propeller.

There's no shortage of theories on why a whale indigenous to North Atlantic waters ended up in a river 1,200 miles south.

Some researchers think the beluga may have followed schools of food fish into the river, then stayed after stumbling upon an abundance of herring and shad. Others believe belugas sometimes head to fresh water to shed dead skin, Smith said.

Old age may have drawn Helis to the Delaware, said Robert Michaud of the nonprofit Group for Research and Education of Marine Mammals in Quebec.

The whale is near the end of its 30- to 35-year life span and may have been driven from its pod by younger males.

Trying to relocate the whale would be difficult, said Teri Frady, a spokeswoman for NOAA Fisheries Service.

"Let me tell you, we're not going to go to the handbook of how to get a beluga out of the river, because it's never been done probably in recent history," Frady said.