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Slots in dyke walls, won’t fix mulloway fishery

By Jennifer Marohasy - posted Tuesday, 3 June 2014


Hardly a week goes by without another announcement from government that additional millions of dollars will be spent ostensibly improving the environment of the Lower Murray. Such announcements are popular with the general public in South Australia. They mostly believe the propaganda that the Lower Lakes really do represent a wetland of international importance and that government expenditure can somehow buy further improvement. In reality their dammed estuary, known as Lake Alexandrina, surrounded by rabbit-infested farmland and overpriced housing estates is brimming with invasive European carp (Cyprinus carpio) and the voracious predatory English perch, otherwise known as redfin (Perca fluviatilis). A few more fishways, as proposed in the latest $2.9 million funding announcement from the South Australian Minister for the River Murray, Ian Hunter, won't change this dynamic. Key estuarine species, once important to the local fishery need more than distressed tepid water, they need a tide.

The Minister claims in the media release that the fishways will provide: "Greater access to breeding areas and different feeding grounds while facilitating the spread of rare native species in the Coorong and Lower Lakes area… Ultimately, it will help to ensure sustainability for more than 30 species of native fish and restore Murray-Darling Basin fish populations."

But there is no evidence to sustain such a pronouncement, so quickly parroted by those also keen to believe the reports and papers authored by SARDI (South Australian Research and Development Institute) employees that claim the Lakes and Coorong Fishery could support a healthy mulloway (Argyrosomus japonicas) fishery if only environmental flows were restored. Indeed, these publications are exceedingly deceptive because they combine the dammed freshwater Lake Alexandrina fishery that is dominated by carp, with the salty Coorong fishery dominated by mulloway and then refer to estuarine species from the Lakes and Coorong Fishery (LCF). In reality, there are few estuarine species in the Lakes and no mulloway. That's right. No mulloway have been recorded from the lakes in any of the many surveys over recent decades.

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Furthermore while the popular claim, repeated in the peer-reviewed papers, often with Greg J. Ferguson as senior author, claim populations of mulloway have been depleted by inadequate environmental flows, data in one of the same papers (Aquatic Living Resource, volume 21, pages 145- 52, figure 4) clearly shows that the collapse of the mulloway fishery in the 1940s occurred with completion of the barrages, which function as sea dykes. Prior to the barrages, good catches occurred even in drought years.


Click on the image for a large better view.

Indeed, the year with the highest recorded mulloway catch, exceeding 600 tonnes, was 1938. This was a drought year during a dry decade in the Murray Darling with low flow to South Australia. The following year 1939, despite the persistent drought, 595 tonnes of mulloway were harvested.

Ignoring these early records, Greg Ferguson and colleagues claim annual flow down the Murray River can explain 43 percent of the variability in the mulloway catch in the LCF. Conveniently, at least in terms of the politics, they choose to restrict their analysis to the years 1984 to 2005. But a longer record, that precedes construction of the barrages, shows that after the sealing of the Goolwa barrage in February 1940, the mulloway fishery never recovered to previous levels despite the extraordinary large flows to the Lower Lakes and Murray mouth in 1956 and 1974.

In short, it is absurd to suggest that more environmental flow will fix the mulloway fishery while the barrages remain in place. But this is the advice from Australian mulloway experts, who draw a salary from government.

Before the barrages, when the south westerly wind picked up, the sea would often pour in through the Murray's mouth and work its way across the lake. So Lake Alexandrina was often fresh in spring and summer, but salty by autumn. In his book Poor man river: memoirs from the River Murray estuary Alastair Wood, a fisherman from Encounter Bay, tells how the mulloway would spend winter to the west amongst the mangroves of the Gulf of St Vincent. When the new season's winds wafted up the gulf the mulloway returned to the Murray mouth.

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The schools lead by large fish, forty pounders and heavier, would settle in the offshore underwater canyons. Within sight of the Murray's mouth and smell of the river, Mr Wood describes how on a full moon and a big tide, larger cows would swim through the surf, in through the Murray's mouth, and along the channels to the lake proper where they would gorge on the little bottom-dwelling fish, the congolli (Pseudaphritis urvillii).

Research from the Shoalhaven estuary in NSW, where there are no obstructions to fish movement, shows that mulloway use the entire 48 km long estuary with high flow events driving fish downstream towards the mouth at Shoalhaven Heads (Matthew Taylor et al., DOI: 10.1371/journal.pone.0095680). The fish follow the shift in the location of the saltwater wedge. Of course such a wedge is absent from the lower Murray River, because the estuary was destroyed with the construction of the barrages.

Studies of mulloway in the 12 km long Great Fish Estuary in South Africa show that mulloway move up and down that estuary corresponding with the speed and direction of the falling tide or, the fish remain stationary in deeper and structured habitat within the estuary (Tor Naesje et al., Marine Ecology Progress Series, volume 460, pages 221-232). The barrages prevent the tide entering Lake Alexandrina, once the central basin of the lower Murray estuary.

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This article was first published on Myth and the Murray.



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About the Author

Jennifer Marohasy is a senior fellow with the Institute for Public Affairs.

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