Garig Gunak Barlu National Park Green Sawfish (Pristis zijsron) aggregation surveys

Christy-Louise Davies, Thomas Tothill, Jessica J. Meeuwig, Peter M. Kyne

Research output: Book/ReportCommissioned report - ERA-eligiblepeer-review

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Abstract

Sawfishes are one of the most threatened families of vertebrates with Australia now widely recognised as a global ‘lifeboat’ for the group. Despite this, significant knowledge gaps remain and the local distribution and areas of critical importance for sawfish are poorly defined. Drone footage captured in late 2018 showed an aggregation of sawfish in the shallow waters of Garig Gunak Barlu National Park in the Northern Territory. This project aimed to characterise this sawfish aggregation and determine if it represented a nursery area by deploying baited remote underwater video (BRUV) stations and unmanned aerial vehicles (drones).

A number of survey sites were identified within the National Park and two survey trips were undertaken. Prevailing weather conditions and limited water clarity restricted usable video footage to that captured by drones at one site (Lidarnardi) during Survey Trip 2. The deployment of BRUVs proved to be unsuccessful during this project.

Drone surveys recorded Green Sawfish on seven out of eight survey days (88%) and on eight of 26 transect flights (31%). Recorded sawfish numbers ranged 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.

Despite the challenging field conditions and failure of BRUVs to obtain usable video footage, the drone surveys confirmed the occurrence of juvenile Green Sawfish in the shallow intertidal waters of Garig Gunak Barlu National Park. Given the number of small individuals recorded, this area likely represents a nursery area for the species although this could not be validated with widely-used criteria for defining elasmobranch nursery areas. Despite this, park waters clearly represent critical habitat for the 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.
Original languageEnglish
Place of PublicationDarwin
PublisherCharles Darwin University
Commissioning bodyCommonwealth Department of Agriculture, Water and the Environment
Number of pages33
Publication statusPublished - 15 Feb 2022

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