UNSW scientists say the floods in March are essential for rivers and nativefish species in NSW, but they fear that European Carp numbers will repopulateto even greater levels and reverse eradication efforts.

A fish ecologist from UNSW Science’s Centre for Marine Science and Innovation,Derrick Cruz, co-authored a study on fish after flood events.

He says carp thrive in floods and will now have access to a huge area ofinundated floodplain, allowing them to expand across habitats.

“European Carp are able to travel huge distances, and floods allow them tomigrate across water bodies that may not have been accessible before thefloods, because many rivers are now connected,” the fish ecologist in theSchool of Biological, Earth and Environmental Sciences says.

Carp is an introduced pest which already accounts for 90 per cent of theweight of all fish, or biomass, in some areas of the Murray-Darling Basin.

It causes significant environmental and economic impacts because it competeswith native fish.

Prior to the floods, carp eradication efforts in certain riverways includedcarp musters, electrofishing and trapping, exclusion of carp from breedingareas, and stranding spawning carp in floodplain habitats by lowering waterlevels.

Mr Cruz says carp can have extremely large numbers of offspring.

“If only one percent of carp eggs survive to adulthood, we’re still facing aninflux of potentially millions of carp across the Murray-Darling over thecoming years,” he says.

The floods may allow carp to “outcompete native fishes through habitatdisturbance and competition for limited resources”.

Carp can also survive blackwater events whereas native freshwater fish oftenperish in the events, and this may happen in coming months, Mr Cruz says.

Blackwater events occur when a body of water which hasn’t been inundated withwater in a long time is flooded, causing a build-up of biomatter that degradesand promotes high microbial activity, which can starve the water of oxygen.

“Carp have a special adaptation to air breathe. They can actually rise out ofthe water and take gulps of air, like mammals,” Mr Cruz says.

“They’re able to use that air to survive in very oxygen deprived environmentsthrough this adaptation, but our native freshwater species of fish don’t havethat ability and often don’t survive.”

European carp are native to both Eastern Europe and Western Asia and werereleased in Australia in the mid-1800s and early 1900s.

The spread of carp throughout the Murray-Darling Basin was assisted bywidespread flooding in the mid-1970s.

Mr Cruz says events like these recent large floods can reverse eradicationefforts.

“We need to identify innovative ways of restricting carp, while allowingflooding,” he says.

Director of the Centre for Ecosystem Science at UNSW, Professor RichardKingsford, says the way in which the rivers and ‘delivery channels’ forirrigated agriculture and towns have been operated has favoured carp and thepest’s ability to breed during floods.

However, Prof. Kingsford says the focus should be on building up nativespecies and conditions.

“Floods are essential for the rivers,” Prof. Kingsford says.

”We need to reinstate the flooding on the floodplain and try and build up thenumbers of native fish species.”

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