More than 30,000 humpback whales now splash across the western Indian Ocean, according to newly released preliminary data. That’s up from fewer than 600 in the late 1970s, after nearly two centuries of whaling decimated populations in the region.

A team led by marine biologist Chris Wilkinson, technical manager for the Mammal Research Institute at the University of Pretoria, sampled the number of humpback whales in one population by counting them as they swam past Cape Vidal, on South Africa’s east coast. The whales migrate each year from feeding grounds in Antarctic waters to breeding areas near Mozambique. From their 2018 survey and data from previous years, the researchers extrapolated to estimate a population of more than 30,000 humpbacks in the entire western Indian Ocean.