By Jeff Whitty and Dr David Morgan

Murdoch University’s Freshwater Fish Group (Centre for Fish & Fisheries Research) in conjunction with the Nyikina-Mangala Rangers, are unravelling the mysteries of one of the most threatened fishes in the world, the freshwater sawfish (Pristis pristis).

A WAMSI project funded by Chevron Australia, ‘Team Sawfish’ is helping to protect one of the world’s largest fishes that is found in freshwater. The freshwater sawfish has declined globally, and in Australia is listed as Vulnerable on the Environment Protection and Biodiversity Conservation (EPBC) Act.

Facing multiple threats including fishing pressure, often by means of bycatch and habitat modification, the numbers and ranges of all sawfishes have greatly declined. In Western Australia, the freshwater sawfish inhabits rivers as juveniles and as such it is likely to be impacted by habitat modifications such as instream barriers (e.g. dams), which may obstruct their migrations into freshwater nurseries. Murdoch University researchers are investigating what impacts these barriers may have on the freshwater sawfish.

In August 2015, Team Sawfish, consisting of Murdoch University researchers and the local Nyikina-Mangala Rangers, set out to continue their work studying the impacts of such barriers in the Fitzroy River, Western Australia.

Team Sawfish measuring a freshwater sawfish

Starting at 360rkm (i.e. 360 kilometres upstream of the river mouth), Team Sawfish systematically sampled pools for sawfish as they moved downstream, including those pools located in close proximity to  the various barriers on the river and ending within the estuarine pools near the river mouth.

The aim of this trip was to catch and tag freshwater sawfish with acoustic transmitters in order to monitor and thus better understand how anthropogenic barriers may affect the movements and/or behaviour of these fishes during the wet and dry seasons, noting movement over the barriers is only possible during peak flows during the wet season (December-April).

Nyikina-Mangala Rangers preparing to deploy an acoustic receiver to monitor the movements of tagged sawfish

During their sampling efforts, Team Sawfish found pools along the river to have become filled in and shallow, a likely result of the lack of flushing of introduced sediments during the past few small wet seasons.

The small 2014-2015 wet season also seemed to have led to the capture/presence of very few sawfish and no young of the year (those pupped within the 2014-2015 wet season).

This finding was congruent with findings from previous years, which suggested that the relative abundance of sawfish within the freshwater pools of the river is positively correlated with the size of the previous wet season.  

The Freshwater Sawfish that were captured were limited to size classes that would have been pupped in 2011-2012. Observing sawfish from the 2011 year class to still be present within the river provided further evidence that some juvenile sawfish do inhabit the river for more than four years, as previous data suggested.

The monitoring of sawfish continues and the team is continuing to tag and record freshwater sawfish in spring of this year.

If you catch a tagged sawfish, or would like to know more about these mysterious creatures, please contact fish@murdoch.edu.au.

 

Sawfish Project