SANYA -- At least 15 whale species were spotted during a recent scientific expedition carried out by Chinese researchers in the northern waters of the South China Sea.

A team of researchers from the Marine Mammal and Marine Bioacoustics Laboratory of the Institute of Deep-sea Science and Engineering under the Chinese Academy of Sciences conducted the scientific expedition. After a 20-day voyage spanning over 3,500 km, they returned to Sanya in South China's Hainan province on July 15.

The researchers used a combination of visual observation and passive acoustic monitoring, supplemented by environmental DNA collection, during their research. They observed at least 15 whale species including 10 deep-diving whale species and spotted five dolphin species living far out at sea.

This latest scientific expedition provides further evidence of the rich whale biodiversity within the surveyed waters. As a flagship species of marine ecosystems, whales are indicative of the local environmental conditions and of great research value for better protection of the ecological environment in the South China Sea, the researchers said.

Starting in 2019, the laboratory has carried out six scientific expeditions in the South China Sea for five consecutive years. Their research results are expected to provide a scientific basis for further protection of the endangered and rare marine animals in the waters.