The Endangered Northern River Shark Glyphis garricki (family Carcharhinidae) is found only in northern Australia and southern Papua New Guinea. It is a euryhaline species inhabiting tidal rivers and estuaries with all life stages (neonates to adults) recorded in rivers, while adults have also been recorded in marine waters. During surveys under the National Environmental Research Program (NERP) and the National Environmental Science Program (NESP), samples were collected in the Northern Territory in the rivers of Van Diemen Gulf (VDG) and the Daly River, and in the Kimberley region of Western Australia, from Cambridge Gulf and King Sound. Previously, the species was only known from a very limited number of records (~40) and these surveys have shown its range to be more widespread than initially documented. It is also likely that the species will be recorded in other rivers and estuaries with suitable habitat as more surveys are undertaken. For the purpose of this report, the VDG encompasses the Alligator Rivers region west to the Adelaide River. Based on recent genetic population structure research, the VDG population appears demographically separate from other known populations in the western part of its range, which in turn are separate to each other.

The aim of this study was to apply Close-Kin Mark-Recapture (CKMR) to G. garricki to estimate population size within the Northern Territory. Samples were collected from six rivers and creeks flowing into VDG (from east to west): East Alligator River, South Alligator River, West Alligator River (collectively, these three rivers make up the ‘Alligator Rivers’), Wildman River, Sampan Creek, and the Adelaide River. While sampling has been conducted across the Australian range of the species (i.e. Daly River, Cambridge Gulf, King Sound), limited sample size outside the VDG prohibited the application of a CKMR model and therefore the focus of this study is the VDG.