Endangered sawfish killed in river low in oxygen

December 17, 2015

Some of the dead freshwater sawfish and bull sharks (Pic: Dr David Morgan)

Some of the dead freshwater sawfish and bull sharks (Pic: Dr David Morgan)

A fish kill event in the Kimberley has seen at least 12 critically endangered freshwater sawfish die in water that was depleted in oxygen.

Scientists from Murdoch University’s Freshwater Fish Group & Fish Health Unit were alerted to the deaths in a 15km stretch of the Fitzroy River in and upstream of Geikie Gorge National Park by Darngku Heritage Tours. Nine bull sharks, one freshwater whipray, cherabin (freshwater prawn), freshwater bivalve molluscs and catfish also perished.

Dr David Morgan, Director of Murdoch University’s Centre for Fish & Fisheries Research, who attended the fish kill with fellow researchers, said water samples taken in the area of the deaths revealed that the water below one metre in depth was depleted of dissolved oxygen.

“It is a real shame to see such large threatened species die, but hopefully this is a one off event. It’s not something I’ve seen before in the Fitzroy,” he said.

“We believe the fish kill is linked to rainfall in the area that flowed into this section of the river.

“The water there was black, tannin stained and likely to have carried large amounts of biologically active matter where rapid bacterial decomposition occurred.

“This resulted in low dissolved oxygen and was compounded by extremely high temperatures in the area. When the fish kills were first reported to us in late November, the daily air temperature was 46oC. The river we sampled was also extremely low, the lowest in the 15 years we have been conducting fish research in the area.”

Dr Morgan expected fish kill events to become more common under drying trends particularly when water temperatures were incredibly high in late spring and summer.

Dr Morgan shot the following footage when he and fellow researchers were first alerted to the deaths.

YouTube Preview Image

The Fitzroy River is a global refuge for freshwater sawfish and Murdoch University researchers have been studying various aspects of their ecology for over a decade. Team Sawfish, which includes the researchers and many local groups, has tagged hundreds of sawfish in the river.

Two of the dead sawfish and one of the bull sharks had previously been tagged by Dr Morgan and his team.

One of the sawfish was a 2.6 metre male, and is the biggest male ever recorded in the freshwaters of the river. One fish, which was tagged in 2011 (the biggest year of sawfish pups ever known) had grown from a one metre pup to 1.9 metres at the time of the fish kill.

The Department of Water’s Phytoplankton Ecology Unit analysed water samples that were collected form the gorge and found no known toxic microalgae. The fish were too degraded pathologically for analysis.

 

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Comments (One response)

Jimi Morgan January 9, 2016

This is a classic case of fish kills due to 'inversion' of the water column due to thunderstorm activity. This happens alot in aquaculture ponds and lakes David Morgan. Cheers

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