Garig Gunak Barlu National Park Green Sawfish (Pristis zijsron) aggregation surveys
Thom Tothill | February 2022
New publication
A collaboration between Charles Darwin University and the Marine Futures Lab has successfully utilised drones to characterise Critically Endangered green sawfish (Pristis zijsron) aggregations in Garig Gunak Barlu National Park in the Northern Territory.
Drone footage captured by Park Ranger Alan Withers in 2018 showed an aggregation of sawfish in shallow intertidal waters off the Cobourg Peninsula. In 2019, Thomas Tothill led two expeditions with Christy Davies from Charles Darwin University to the region, deploying stereo-BRUVS and conducting drone transects to characterise the aggregation and determine if it represented a nursery.
Drone transects proved to be an effective way to characterise green sawfish aggregations. Drone surveys recorded sawfish numbers of 1–8 individuals per transect and sawfish density ranged 3.8–30.5 sawfish per hectare. The size of individual sawfish was 57–167 cm total length (TL) with most in the size range 60–100 cm TL. Green sawfish can grow larger than 700 cm and reach maturity at approximately 430 cm, indicating that all sawfish identified were juveniles.
The density of sawfish is the highest reported anywhere in the world. Given the number of juvenile green sawfish recorded, this area likely represents a nursery area for the Critically Endangered species.
The green sawfish aggregation is within a protected area affording it some refuge from major threatening processes. However, similar inshore intertidal habitat is not well represented in northern Australian protected areas.
Citation: Davies C, Tothill T, Meeuwig JJ, & Kyne PM (2022). Garig Gunak Barlu National Park Green Sawfish (Pristis zijsron) aggregation surveys. Report to the National Environmental Science Program, Marine Biodiversity Hub. Charles Darwin University, Darwin.
01
Fieldwork team
Drones transects and Baited Remote Underwater Video Systems (BRUVS) were utilized to collect data at Garig Gunak Barlu National Park.
02
Green Sawfish
Drone image of Green Sawfish (Pristis zijsron) aggregating in the shallow water off Lidarnardi (Record Point).
03
Drone still
Drone image flown at a height of 10 m recorded a Green Sawfish (Pristis zijsron)
04
Survey
Garig Gunak Barlu is a protected area located in the Northern Territory of Australia on the Cobourg Peninsula about 216 kilometres north-east of the territory capital of Darwin.
Funding and Acknowledgements
This work was undertaken for the Marine Biodiversity Hub, a collaborative partnership supported through funding from the Australian Government’s National Environmental Science Program (NESP). NESP Marine Biodiversity Hub partners include the University of Tasmania; CSIRO, Geoscience Australia, Australian Institute of Marine Science, Museums Victoria, Charles Darwin University, the University of Western Australia, Integrated Marine Observing System, NSW Office of Environment and Heritage, NSW Department of Primary Industries.
This research was prompted by drone footage captured by Garig Gunak Barlu Senior District Ranger Alan Withers; we thank Alan for sharing his knowledge of the area, accumulated over more than 2 decades working in the region. We also thank Traditional Owners and other residents for their knowledge in the form of sawfish sightings. Fieldwork would not have been possible without the onground support of rangers Alan Withers and Robbie Risk, who assisted us in every way imaginable. We extend our sincere thanks to Naima Rodriguez Lopez, Greg Williams, and Fredrick Baird for assistance in the field. Finally, we thank the Traditional Owners and the Cobourg Peninsula Sanctuary and Marine Park Board for granting us permission to visit and work on their country.