Giant blue-grey sei whales that vanished from Argentina's Patagonian coast a century ago due to hunting are starting to flourish once again, demonstrating how species can recover when measures to protect them are put in place.

In the 1920s and 1930s regular whaling ships along the shores of Argentina, and beyond, saw populations dwindle. In the last 50 years, global bans on commercial whaling have helped populations of sei and others revive.

"They disappeared because they were hunted, they did not become extinct but were so reduced that no one saw them," said Mariano Coscarella, biologist and researcher in marine ecosystems at the Argentine state science body CONICET.

Coscarella added that it had taken decades for numbers to recover enough that the whales had again been sighted, which only started to happen again in recent years.

"In this case it took over 80 years," Coscarella said. "They breed every 2 or 3 years and so it took almost 100 years for them to have appreciable numbers for people to realize they were there."

The team last month worked to fit some sei whales with satellite trackers to map their migration patterns, with funding from the National Geographic's Pristine Seas project. They recorded footage of them from boats, drones and below water.

"We can consider this a success of conservation on a global scale," Coscarella said, adding that the global moratorium on whaling was critical to numbers improving. He warned that countries pulling out of this agreement would jeopardize the recovery.

"After hunting that reduced the (sei whales) population to a minimum, almost 100 years later, this population started to bounce back and now they come to the same places they used to before they had been hunted."