The Lower Lakes were once part of the much larger River Murray estuary that included the Coorong. The barrages prevent seawater from mixing with the freshwater of Lake Alexandrina, which has resulted in the destruction of 90% of the pre-barrage estuarine environment. In particular, the installation of the barrages to retain freshwater in Lake Alexandrina has had a devastating effect on the mulloway fishery of the Coorong. (Olsen 1991).
The Murray River Barrages Environmental Flows Report (1.75MB), produced by the Murray-Darling Basin Commission in 2000, identifies the following culprits that are a direct result of the barrages:
the current estuary, comprising the Coorong and Murray Mouth areas, is 10% of its former size
brackish and saline habitats have been lost due to freshwater incursion
abrupt changes in the water environment due to the drastic reduction in tidal range
On 22 September 2009, DEH announced a $500,000 fish down, with the goal of killing large numbers of carp before they die in Lake Albert.
Carp, in particular, have become a serious cause of biodiversity loss. Carp were introduced into the Murray-Darling system in 1964 and reproduce at such a high rate (up to 5 million offspring per year) they readily out compete native species. The 2000 report recommended carp controls, which have unfortunately been largely ignored till now.
In 2008/2009 the Lower Lakes and Coorong Fishery harvested 792 tonnes of carp at a value of $863,000. Carp is used as lobster bait and fertilizer.
Fish kill near Ewe Island, early September 2009