We’ve been out on the West Alligator River for a second survey of sawfish and river sharks. The first survey took place 12 months ago and discovered a significant population of northern river sharks in this remote Kakadu river. We’ve now shown that the Alligator Rivers (the East, West and South Alligator) support the most significant known population of this endangered species.
On the recent visit to the West Alligator River, we caught mostly northern river sharks – which were measured, and tagged with a small PIT tag before being released. The tag provides a unique number identifier when scanned by a handheld scanner (this is very similar to microchips used in domestic pets). One northern river shark tagged in October 2013 was recaptured, indicating long-term site fidelity to the river between dry seasons. A single juvenile bull shark was also caught, tagged and released – but we failed to find any of the critically endangered speartooth sharks or the undescribed whipray species found in October 2013.
The NERP threatened euryhaline elasmobranch (sawfish and river sharks) project is investigating the population size and movements of the threatened river sharks, along with sawfishes, to learn more about the structure of the populations, and what rivers they move in and out of.
We also performed maintenance on acoustic receivers at the mouths of the Wildman River and West Alligator River – to ensure they will remain in place this wet season. These are part of a network of receivers at each Kakadu river mouth to examine between-river movement of the sharks. The main acoustic receiver array is situated along the entire length of the tidal portion of the South Alligator River.
One of the things we want to discover is whether the sharks from one system move to adjacent rivers or if each river system contains its own population with no or little exchange between rivers. If each river represents a unique population then management needs to occur at the river-scale, in order to protect each population. No sharks tagged in the South Alligator River have yet been detected on the Wildman River receivers, but two speartooth sharks, two northern river sharks and one bull shark tagged in the South Alligator River have been detected on the West Alligator River receivers. The project runs until May 2015 – and we’ll bring you more updates on what we learn and how it helps us do our work.
Sawfish and river sharks are rare species worldwide, protected under national environmental law in Australia, with Kakadu National Park home to some of the best surviving populations.
Dr Peter Kyne, Charles Darwin University
This research is conducted under the Australian Government’s National Environmental Research Program (NERP) Marine Biodiversity Hub and Northern Australia Hub, through a partnership between CSIRO, Northern Territory Fisheries and Charles Darwin University with support from Kakadu National Park and traditional owners.
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