The Mekong giant catfish is a threatened species in the Mekong, and conservationists have focused on it as a flagship species to promote conservation on the river. Although research projects are currently ongoing, relatively little is known about this species. Historically, the fish's natural range reached from the lower Mekong in Vietnam (above the tidally influenced brackish water of the river's delta) all the way to the northern reaches of the river in the Yunnan Province of China, spanning almost the entire 4,800 km (3,000 mi) length of the river. Due to threats, this species no longer inhabits the majority of its original habitat. It is now believed to only exist in small, isolated populations in the middle Mekong region. Fish congregate during the beginning of the rainy season and migrate upstream to spawn. They live primarily in the main channel of the river, where the water depth is over 10 m (33 ft), while researchers, fishermen and officials have found this species in the Tonle Sap River and Lake in Cambodia, a UNESCO Biosphere Reserve. In the past, fishers have reported the fish in a number of the Mekong's tributaries. Today,[when?] however, essentially no sightings are reported outside of the main Mekong river channel and the Tonle Sap region.