The scientists, who are using equipment attached to the North Atlantic right whale to track its movements, said they will launch another rescue mission if the animal moves back toward the coast.
The 34-foot yearling was first found entangled off the Florida coast last week and was dubbed Kingfisher in honor of a Coast Guard cutter that aided the rescue effort.
Scientists managed to attach the tracking buoy with a satellite transmitter to Kingfisher but could not free it and decided to return this week.
While whales often don't need to be disentangled from debris, scientists worry the lines around Kingfisher could kill it by tightening as it grows.
"There are only 300 right whales left in the world and every single right whale is incredibly valuable to the Right Whale Recovery," Laura Engleby, a marine mammal biologist with the National Oceanic and Atmospheric Administration's Fisheries Service, said Wednesday on CBS News' The Early Show.
Kingfisher has been traveling in and out of the Gulf Stream, the powerful oceanic current that moves north along the North American coastline.
The crew of the University of North Carolina-Wilmington's 70-foot Research Vessel Cape Fear, and scientists from the National Oceanic and Atmospheric Administration and its National Marine Fisheries Service had planned to set out in search of the whale early Friday.
They planned to use a smaller boat to get close enough to sedate it and then disentangle it, which could take several days.
But Barb Zoodsma, the NOAA biologist in charge of the project, said Kingfisher had moved so far from shore that rescuers would not reach it until late afternoon, as darkness was setting in.
The whale was about 18 miles off Cape Lookout at 8 a.m. Friday and traveling at 3 knots, faster than scientists had anticipated.
The Gulf Stream is about three miles offshore from Florida to North Carolina, then shifts farther out to sea, where a rescue is less practical, officials said.
"Then everything becomes a little less clear," Zoodsma said. "All of the unknowns start looming a little bit larger."
About 60 percent of right whales become entangled in fishing lines, according to the nonprofit Center for Coastal Studies, which does applied research and marine mammal rescues.
Last year, the center confirmed 35 reports of whale entanglements and freed 12, program coordinator Joanne Jarzobski said.
In 1991, researchers spent more than $250,000 trying to rescue a right whale tangled in rope but failed to save it. A female right whale became ensnared in fishing line off Canada in 2002, and was spotted off Cape Cod last March, still entangled.