Most Australians have never visited the Murray River, and even fewer know that it ends in a vast and shallow freshwater lake. The freshwater lake is separated from the Southern Ocean by 7.6 kilometres of barrage that were built in the 1930s across the five channels that converge on the Murray River's sea mouth.
The barrages dammed the estuary and the artificial lake now sits almost 1 metre above sea level and covers an area of about 650 km² (250 mi²). This artificial lake is so vast that you can't see from one side to the other, and it evaporates the equivalent of about two Sydney Harbour's full of freshwater each year. It is full of carp, a pest fish introduced from Europe.
Before the barrages were built mulloway, Agyrosomus japonicas, were a mainstay of the local fishery. Milang, a little port on the shores of Lake Alexandrina was home to a hundred mulloway fishermen who routinely sent off several hundred tons of fish to the Adelaide and Melbourne fish markets. Back then the central basin of the wave-dominated barrier estuary was sometimes full of freshwater and sometimes full of salty water, the nature of the mix depended on the tides, the winds and Murray River flows.
According to South Australian historian J.C. Tolley it was the ocean that most affected the size and position of the Murray River's sea mouth:
The position of the channel at the mouth is governed principally by the ocean... During the great 1956 flood, the highest ever recorded on the lower Murray, the river outlet, although wider and deeper than normal, was situated in the easterly section of the overall movement pattern and was in a similar position as the situation of the mouth during the dry year of 1914.
However in April 1938, during a violet storm the mouth doubled its width in a few days and a great deal of sand at the western extremity was washed away. Within two months the channel had narrowed and when surveys were carried out 12 months later the position of the outlet was in almost the same situation as before the storm. During this period there was no great fluctuation in the volume of fresh water coming down the river.
Murray River flow was usually good in spring, but by mid summer it had often slowed and if conditions were calm a sandbar would quickly form and sometimes block the Murray's sea mouth. Then usually by March a south westerly wind had picked up. The old fishermen say, that at that time of year the mulloway would hangout in the underwater canyons beyond the Murray's mouth, as though reluctant to come in. Then on the big tides, always with the full moon, large schools would race through the Murray's sea mouth.
The year the barrages were sealed, the mulloway came in and then were trapped, on each ebbing tide, churning in the channels below the barrages. There is an old photograph of the Goolwa wharf groaning under 160 ton of dead mulloway.
Now the dominant fish species in Lake Alexandrina is the pest, European carp, Cyprinus carpio. This is a freshwater species, it wouldn't exist in the lake except for the barrages that dammed the estuary and stopped the tide.
Henry Jones a commercial carp fisherman from Lake Alexandrina told ABC Rural not so long ago that,
In 1981 they [carp] hit our area and they exploded in the next two years. The commercial fishermen called a summit and they invited all our prospective customers who they thought would buy carp…
They picked out cray fishermen, because we thought they could pay a bit more for them, and the human consumption market… we've got so many orders for carp we can't fill our orders.
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