Determining the spatial and trophic niche of tropical euryhaline elasmobranchs is critical to understanding their functional role as apex predators within riverine and estuarine ecosystems. Euryhaline elasmobranchs of the South Alligator River, in the World Heritage National Park, Kakadu, Northern Territory, Australia, include Glyphis garricki, Glyphis glyphis and Carcharhinus leucas. The Glyphis species are rare and have been listed as threatened species. This combined with the fact that their taxonomy was only recently resolved has resulted in a paucity of data in their ecology that encompasses dietary preference, spatial and seasonal axes of variation. Using a novel combination of techniques, we will establish both the trophic (stable isotopes and fatty acid analysis) and spatial (acoustic tagging) niches. It is anticipated that this data will indicate the niche occupied by these species to reveal links between these predators and their prey across wet and dry seasons along a series of key habitats throughout the length of the South Alligator River.