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Why we need to fix cold water pollution for our native fish

I spent much of the late 1990s studying Environmental Science at the University of NSW. It was a time when the idea of “cold water pollution” in Australian streams was just really starting to be explored by ecologists.

I had grown up in Wellington, in the Central West of NSW – where summertime meant water skiing on nearby Burrendong Dam and swimming in the freezing cold Macquarie River just downstream of the dam. It was a logical choice that my honours thesis (though never published) would be one of the first studies on the cold water pollution impacts downstream of Burrendong.

Cold water pollution is the lowering of water temperatures that commonly occurs downstream of large dams. This is because the outlets for most dams are at the bottom of the dam wall, resulting in the release of the cold water from the depths of the dam. It’s estimated in NSW that water temperatures can be between 10 and 17 degrees colder as a result of cold water pollution.

The breeding cycle of many native fish species is finely tuned to the spring warming of water, which triggers spawning.  Native fish generally have well defined, and often narrow temperature windows for spawning, and for the successful recruitment of juvenile fish. The cold water released from the bottom of dams means that often the temperature window for spawning is never – or rarely achieved.  Unsurprisingly, many invasive fish species – like carp - are able to breed very successfully in the colder temperatures. 

Even if native fish are able to breed, cold water pollution can significantly reduce their chance of survival. A study by NSW Fisheries at Burrendong Dam showed that silver perch had a 100% survival rate in in the warm water releases (18 to 24oC) compared to a 25% survival rate in the cold releases (12 to 14oC).

In 2014, the NSW Government embarked on an innovative – and as it turned out – award winning engineering project to fit a flexible “curtain” around the intake tower of Burrendong.  This enabled the warmer water on the surface of the dam to be released.  At a cost of just $3.4 million, early results from projects assessing the impact of the curtain are showing a 3 degree warming.

Raising water temperatures offers a high probability of inducing significant beneficial response within aquatic ecosystems: higher productivity at all trophic levels; a greater number of native fish breeding events; more successful breeding events; and greater diversity of aquatic invertebrates, fish and other cold-blooded animals such as turtles and frogs
NSW Cold Water Pollution Interagency Group (2012): Cold Water Pollution Strategy in NSW - report on the implementation of stage one, NSW Department of Primary Industries, a division of NSW Department of Trade and Investment, Regional Infrastructure and Services

The Murray-Darling Basin Authority is soon to finalise its recommendation on the Northern Basin Review. 

In the NSW portion of the Northern Basin, Pindari, Copeton & Keepit have all been identified by the NSW Government as dams with significant potential to cause cold water pollution.  Glenlyon Dam, in the upper reaches of the Dumaresq River on the NSW Queensland Border is another.

If, as the NSW Government strategy suggests, that fixing cold water pollution from dams is a no-brainer for improving environmental outcomes, then complementary measures such as this must be part of the MDBA’s recommendation.  We’ve known for decades that fixing cold water pollution is important for particularly for improving outcomes for native fish.  Surely the time has come to do something #morethanflow?

Jack Knowles is the NFF's Natural Resource Manager.

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