Blog by Team Sawfish
Sweltering heat, mosquitoes, angry crocodiles and swarms of flies characterise the typical late dry season (September-November) in northern Western Australia. And it’s here that Team Sawfish has found itself for the past 15 years , as we continue our research on sawfish. This past October, Dr Ruth Leeney of Protect Africa’s Sawfishes and Dr Barbara Wueringer of James Cook University decided to brave these conditions and joined our team during our month long expedition on the Fitzroy River.
The main objective of our trip was to investigate how unnatural instream barriers, such as dams, may impact on the movement and health of the largetooth sawfish (Pristis pristis), which inhabits freshwater rivers and lakes as juveniles.Dams and other barriers in rivers and estuaries can block the movements of fish and which may prevent them from accessing food, spawning grounds or other important resources.
The Camballin Barrage, a small weir on the Fitzroy River used for irrigation for cattle and crops.
This work is part an ongoing project that is gathering information from several rivers throughout Western Australia.
The Fitzroy River is an ideal site for this study. It’s one of the largest known nurseries of the largetooth sawfish and it has instream barriers of various sizes, including a small weir (a type of dam often used for irrigation) and several road crossings.
During our October trip we sampled various pools within the river that were located between the river mouth and 380 km (236 miles) upstream from the river mouth. From these pools we captured seven different sawfish. This is fewer fish than we typically see, and was largely due to the lack of captures of young of the year (YOY; fish pupped within the year). The number of YOY that we captured earlier in the year was also relatively low, which suggests this may have been a poor year for YOY.
All captured sawfish were only 2-3+ years old but still had an average length of 205 cm (6.7 feet)! In addition, the majority of these fish were recaptures of those that we had previously tagged with individually numbered Rototags (i.e. cattle ear tags). Two of these sawfish were initially tagged 2 to 3 years ago! Long-term recaptures like these allow us to gather some important information on the distribution, movements and growth of this species.
Although we can collect some of this vital information through recapturing sawfish for the few months of the year that we’re in the field, we also rely on local and visiting fishers to report their encounters. (see instructions below on how to report an encounter with a sawfish).
The sawfish that we did capture were also tagged with small acoustic transmitters. These transmitters emit a unique acoustic signal that is picked up and recorded by receivers, which we have installed throughout the river. Data from these transmitters and receivers will help us to better understand sawfish movements and how dams and river levels affect them.
Although we tag the animals in the dry season, it is the data from the wet season we are most interested in. Only during the wet season are the barriers submerged by rising river levels, and we want to know at what river level sawfish move beyond these barriers.
Release of a tagged largetooth sawfish
As well as capturing the largetooth sawfish, we had hoped and expected to find the smaller dwarf sawfish (Pristis clavata), which we have been monitoring over the last few years. Although the largetooth sawfish is the only sawfish that lives in freshwater for long periods of time, the dwarf sawfish, which lives in marine and estuarine environments, is attracted to the estuarine pools of the Fitzroy River in the late dry season, when these pools become more salty (around 35 ppt; similar to the ocean). This increase in salt in the late dry season is due to a decrease in freshwater flow and the constant transport of ocean water into the pools by the large tides. Typically during this time we primarily find dwarf sawfish in the estuarine pools, as the majority of the largetooth sawfish appear to leave the area. This year however, we were surprised to find a couple largetooth sawfish but unfortunately did not find any dwarf sawfish.
After a month in the field our food supplies, energy levels and time ran out. As we left the sawfish and the river behind, our team agreed that in spite of a few minor setbacks our trip was a success. We were able to tag a few new sawfish and collected some important recapture data. This will help us to better understand and inevitably help to advance conservation efforts for these threatened sawfish.
Although, we had to finally part ways with our esteemed and much appreciated colleagues, we had enjoyed the time spent with them and the memories of fly bites, tasty porridge, not so tasty tree sap and most importantly, the sawfish.
Pictured is the October 2014 Team Sawfish field crew. Missing are Dr David Morgan and James Keleher.
To report a sawfish captured in Western Australia and for more information regarding Team Sawfish please visit http://www.freshwaterfishgroup.com/team-sawfish.php. If you encounter a tagged or untagged sawfish outside of Western Australia please visit the Sawfish Conservation Society at www.sawfishconservationsociety.org, or the International Sawfish Encounter Database at www.flmnh.ufl.edu/fish/sharks/sawfish/sawfishdatabase.html.
This research was funded by Chevron Australia and was made possible by the Western Australian Marine Science Institution.