Tonnes of huge carp are regularly returned to Lake Burley Griffin, a move that is in conflict with multi-million-dollar strategies supported by the ACT government to clean up the Murray-Darling Basin and Canberra's lakes.
Tournament rules for coarse fishing (for freshwater fish other than trout and salmon) are at odds with a key strategy of the Upper Murrumbidgee Demonstration Reach clean-up program from Bredbo in NSW to Casuarina Sands in the ACT.
Supporting the Demonstration Reach, which aims to improve the river's health for native fish, are ''carp out'' events, where thousands of anglers take as many carp as they can from where they breed and congregate, especially in Lake Burley Griffin. Native fish, including Murray cod, that eat carp are released in the same water.
Yet on Friday, Saturday and Sunday, the Sydney Coarse Angling Club pulled more than a thousand carp from Lake Burley Griffin and returned every one of them to the water.
The club's spokesman, Howard Hill, said reducing numbers would do more harm than good, because the fewer carp left in the lake would breed more to overtake the number caught and killed.
He said 37 anglers competed over the weekend, while bigger national and Australia-versus-New Zealand events were planned for February. More than 3500 kilograms of carp were caught and released at the last national event in Canberra.
Coarse anglers also got exemptions that allowed them to return noxious fish, including redfin perch, to the rivers and lakes of NSW and Victoria.
The conflict in the ACT had been apparent since 2010, when the Demonstration Reach strategy was released, with policy authors noting that coarse tournaments resulted in the capture and release of several tonnes of carp.
''Such exemptions clearly conflict with the intent of this carp reduction plan,'' they said. ''In NSW there are similar issues with it not being illegal to return carp to the waterway after capture.''
Canberra ecologist and long-time fishing commentator Bryan Pratt estimated that about eight million eggs were taken from a single carp captured in the ACT recently.
He said multiplying the tens of millions of carp in Lake Burley Griffin with the eggs they carried explained why rod-and-line fishing, netting, trapping, even lowering the lake's level during spawning, would never eradicate the pest.
Dr Pratt said other than gene and virus methods, it was unlikely Lake Burley Griffin would ever be free of carp.
''Possible yes, probable no,'' he said.
''No established pest has ever been eradicated from mainland Australia, despite intensive effort, many millions of dollars, and being supported by powerful legislation that requires pests to be controlled.''
Such was the resilience and mobility of carp and the spread of their eggs by waterbirds, a blitz on the pest in spawning hot spots could not be in isolation. Controls were needed across a wide area to reduce numbers in rivers, lakes, urban ponds and farm dams.
The strategy, and an ACT taskforce , recommends targeting spawning or congregating carp in hot spots, such as the mouth of Sullivans Creek, Jerrabomberra Creek and its associated wetlands.
Greens MLA Shane Rattenbury said a deal on waterways contained in his agreement with ACT Labor would include carp reduction. The Greens promised $165,000 for the carp program before the election, but details had not been settled with the government.
He said although it was unknown whether carp could be fully eradicated, substantially reducing their numbers could make a big difference.
Research indicated carp were first detected in the ACT in 1976 when several were caught in Lake Burley Griffin. They were suspected of being introduced when other fish were put into the lake.
CSIRO senior research scientist Dr Ron Thresher believed he had a solution in 5000 potential carriers of a daughterless carp gene.
Not only would breeding carp who could have only sons help tip the balance against the freshwater menace, the technology could potentially have the same impact on cane toads.
Dr Thresher's research team found genes in a carp's relative, the zebra fish, which were female only and could trigger another gene construct that eliminated the females, turned females into males or made the females sterile.
''We could have fish ready for field trials within 12 to 18 months if the funding was available,'' Dr Thresher said. ''With carp, it is a longer generation; we are probably looking at four or five years.''
About $8 million had been invested in the technology that rearranged genes and returned them to the fish. But research had stalled due to lack of continuing funding.
''Having invested millions of dollars to get the work to this stage, it seems short-sighted not to complete the trials,'' Dr Thresher said.