Critical sawfish nursery habitats identified in Fitzroy River

Murdoch University researchers have identified the key habitats of endangered sawfish species in Western Australia’s Fitzroy River.

The eight-year study into the movement of sawfish in the isolated freshwater reaches of the north western waterway has identified that deep pools and shallow environments, like glides, are important habitats for the Freshwater Sawfish (Pristis pristis), but that restriction of flow or altering of river pathways could jeopardize 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 Murdoch researchers have been working to identify the habitats and conditions that need to be considered in conservation and management decisions for the region.

Their work has been supported by the Western Australian Marine Science Institution, Chevron Australia, Western Australian Government’s State Natural Resource Management Program and the people of the Kimberley, including the Nyikina-Mangala Rangers.

Lead researchers Mr 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, located between 120 km and 150 km upstream from the river mouth.

“We monitored the movements of 32 juvenile 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,” Mr Whitty said.

“This study demonstrated for the first time that juvenile Freshwater Sawfish are least active when they occupy deeper runs and pools near large woody debris, by day. 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.

“It appears unlikely 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 current 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.

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.

Professor Morgan said the accelerometers will help Ms Lear understand what the sawfish are doing in their natural environment.

“The accelerometers will help us to 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,” he said.

A journal article on the research was published in Endangered Species Research.

Source: Murdoch University

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