Critical sawfish nursery habitats identified in Fitzroy River

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An eight-year study into the movement of critically endangered sawfish in the isolated freshwater reaches of northwestern Australia’s Fitzroy River has identified the habitats that are important to their survival.

A group of Murdoch University researchers has found that deep pools and shallow environments, like glides, are important habitats for the Freshwater (Largetooth) Sawfish (Pristis pristis) and that restriction of flow or altering of river pathways could jeopardise these environments.

Little is known about the movements of these sawfish which rely on the intermittently flowing rivers and estuaries of the Fitzroy River as a globally significant  nursery. Locally they have been recorded up to 400 kilometres from the coast.

With increasing pressure from fishing and the impact to their migration by instream barriers the researchers, supported by the Western Australian Marine Science Institution, Chevron Australia, Western Australian Government’s State Natural Resource Management Program and people of the Kimberley, including the Nyikina-Mangala Rangers, have been working to identify the habitats and conditions that need to be considered in conservation and management decisions for the region.

Lead researchers Jeff Whitty and Associate Professor David Morgan explained the study, published in Endangered Species Research, was conducted over the 2008 to 2015 dry seasons (May or June to November), when the Fitzroy River is transformed into a series of isolated reaches in which the sawfish are trapped.

Two freshwater reaches of the Fitzroy River were monitored that were located between 120 kilometres and 150 kilometres upstream from the river mouth.

The researchers monitored the movements of 32 juveniles sawfish (952 to 2510 mm in length) using acoustic telemetry, with sound-emitting transmitters tracked by a series of loggers over an eight-year period.

 

 

“This study demonstrated for the first time that juvenile Freshwater Sawfish are least active by day, when they occupy deeper runs and pools near large woody debris,” Mr Whitty said. “They are most active during night-time and twilight hours in shallow water such as glides, pool edges, and shallow runs, when their prey are also more abundant in shallow waters.

“More observations need to be done, however, on why the juvenile sawfish move to the deeper pools in the day,” Mr Whitty said. “It appears unlikely that they are moving to deeper water only to conserve energy in cooler water because, at least during the early dry season, there’s little difference between surface and bottom water temperatures.”

Some of these questions are being addressed by PhD researcher, Karissa Lear, a Forrest Foundation Research Scholar, who is using accelerometers (the same technology that is in FitBits, smart phones, and other smart devices) to study sawfish behaviour.

 

PhD student Karissa Lear with a sawfish pup temporarily held at Kimberley Training Institute in Broome to examine metabolic rate. (David Morgan, Murdoch University)

 

Ms Lear is also examining the metabolic rate of sawfish while they are wearing their accelerometers, which tells how much energy each specific behaviour costs.

“These studies will allow us to put accelerometers on wild sawfish in the Fitzroy, look at what they are doing in their natural environment, estimate how much energy they are using based on their behaviour, and importantly, see how their behavioural patterns, feeding effort, and energy use change throughout the dry season as temperatures increase,” she said.

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