SYDNEY, Nov. 7 (Xinhua) -- Human activity has severely jeopardized the number and recovery of North Atlantic right whales, which have been in decline for several years and are now a critical conservation concern, according to latest Australian-linked research.

The baleen whale species, once a top target for whalers with their high yields of oil, has declined for years and at its peak numbered less than 500, The Royal Society organization said in a statement about the Curtin University-linked study on Wednesday.

When the number of whales was rising, it did so at about 2 percent a year but their recovery "was being jeopardized by deaths of adult female whales" with the deaths "primarily caused by people", it said.

"Without these deaths, the species' numbers could have been almost double what they are now."

The study included analyzing whale calf numbers and population projection modeling, annual survival estimates and necropsy records.

North Atlantic right whales "were nearly exterminated by historical whaling. Their abundance slowly increased ... to a maximum of fewer than 500 whales, and since then they have been in decline", the researchers reported in the Royal Society Open Science scientific journal.