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Blackwater kills fish

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Duncan McLay with rescued crayfish
DPI Narrandera Fisheries’ Duncan McLay with some of the rescued crayfish. Photo: Narrandera Argus

Hundreds of fish died including Murray Cod, Golden Perch, Bony Herring and Australian Smelt as well as Murray Crayfish in a disastrous black water event in Bundidgerry Creek near Narrandera late last week.

The incident created a storm on Facebook, with anglers, residents and shire councillors alike calling for action and explanations.

A local fisherman observed that in most years in the Narrandera area there was not a 100mm-plus of rain in two days in January and a very wet December leading up to it, with similar amounts right across the Riverina over the same period.

“ The water coming in from Cowabbie Creek and the flood plains that feed this creek equate to thousands of hectares and once an area that size is inundated with such a high rainfall in such a short period of time the grasses, bark, cow and sheep manure general crap all enter the waterways with such pace and force it’s a difficult task to deal with.

“It creates havoc in all areas it enters with the depletion of oxygen generally very quickly (black water) below is one part of the creek that is still running in but at a much slower pace.”

NSW DPI Fisheries confirmed this week that it had received a report of the fish kill event on January 10, including observations of Murray Crayfish walking out of the water.

Murray Crayfish are listed as a vulnerable species in NSW. Native fish species affected included Murray Cod, Golden Perch, Bony Herring, Australian Smelt along with Murray Crayfish. Several introduced species including Carp and Redfin Perch were also affected.

DPI Fisheries staff concentrated their rescue efforts on Murray Crayfish and were able to rescue and relocate almost 100 Murray Crayfish between January 12-14 under a collection permit to DPI’s Narrandera Fisheries Centre, where they will be held until conditions improve and they can be returned to Bundidgerry Creek.

The 100 rescued and relocated Crayfish ranged in size from large mature adults to smaller juveniles. “Murray Crayfish can survive short periods of low oxygen levels by walking out of the water, although this makes them particularly susceptible to predators. Long-term exposure to critically low oxygen levels can result in death, with hypoxic blackwater previously contributing to population declines in some regions,” the DPI Fisheries spokesperson said.

Blackened crayfish
Photo: Narrandera Argus

The suspected cause was a large rainfall event the preceding days that inundated the floodplain and washed organic material into the waterway.

As a result of the large volumes of organic material entering the waterway, a rapid reduction in Dissolved Oxygen (DO) levels occurred, commonly referred to as a hypoxic blackwater event.

DPI Fisheries worked with environmental water managers and river operators to rapidly respond with environmental water releases sent down Bundidgerry Creek on Monday January 10 in efforts to dilute the hypoxic blackwater and reduce the impact on native fish.

Regulators at both the Seven mile (Oak Creek) and Five mile (Bundidgerry Escape) were opened to allow the hypoxic water to exit Bundidgerry Creek and to prevent downstream impacts on properties.

Oxygen levels in Bundidgerry Creek improved downstream of Cowabbie Creek as a result of this release, although sections of the Five mile and Rocky Waterhole saw no initial improvement.

Water quality continues to be monitored in the affected reach, as well as several other local creeks and DPI Fisheries continues to work with environmental water managers to support native fish outcomes in the area.

Community members are encouraged to report any fish kills or observations through the Fishers Watch hotline on 1800 043 536.

For more information on fish kills and the key causes, visit www.dpi.nsw.gov.au/fishing/habitat/threats/fish-kills

Narrandera Argus 20 January 2022

This article appeared in the Narrandera Argus, 20 January 2022.

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