A pod of rare freshwater dolphins has been spotted at a biosphere reserve in southern Vietnam, conservation officials said on Monday.

About 20 of the Irrawaddy species were sighted near the Ba Lua Islands by researchers, according to a report released by the Vietnam Institute of Tropical Biology.

“The number of dolphins sighted was larger than schools in the Malampaya Channel in the Philippines and the Mekong River, where populations of between seven and 10 have been sighted,” researcher Vu Long said.

It was the third recorded sighting in Vietnam since 1973. The distribution area of the Orcaella brevirostrisis species is discontinuous in tropical and subtropical regions between the Indian Ocean and Pacific Ocean. It can live in fresh, brackish or saltwater.

Freshwater populations are known in the Ayeyarwady River of Myanmar, the Mahakam River in Indonesia and the Mekong River that runs through Laos, Thailand, Cambodia and Vietnam.

The species is listed as vulnerable by the International Union for Conservation of Nature due to dramatic declines in population.