SAVANNAH, Georgia (CNN) -- Only two mother 'right' whales and their calves have been spotted this winter off the coasts of Georgia and north Florida, further evidence that the most endangered of the big whales is on the brink of disaster, scientists say.

Normally, several mother-calf pairs and single whales have been seen by January. More than two dozen visited the area last year.

"It's very scary," said Chris Slay of the New England Aquarium, a researcher who tracks whales for the National Marine Fisheries Service.

The slow, shallow-diving whales indigenous to the North Atlantic Ocean became known as the "right" whale to kill during whaling times because they were easy to harpoon and floated a long time after death.

The whales migrate as far south as Florida in the winter before returning to Canada in the summer.

Researchers say the El Nino-La Nina weather cycle of last year could be partly to blame, disturbing the deep ocean currents and making it more difficult for the whales to find food and each other.

Experts say no right whales have been killed by ships during the past two migrations.

Worldwide, their numbers have dwindled to between 350 and 400. Researchers say the 1999-2000 mating season could be critical for the species' future.

The Associated Press contributed to this report.