Australia spent decades pulling timber out of its longest river to move irrigation water faster, before it began putting wood back, with 4,450 snags from highway clearing placed along a 110-kilometre stretch of the Murray. That is according to Dr Jarod Lyon of Victoria’s Arthur Rylah Institute for Environmental Research, whose research, published in Ecological Applications, found three times as many Murray cod in the restored stretch after seven years of tracking.
“Knowing that our restoration attempts actually work to increase the number of native fish gives us hope,” Lyon and co-author Professor Corey Bradshaw of Flinders University said. Golden perch numbers also doubled in the restored stretch, whilst Murray cod numbers fell by 40 per cent in a similar stretch below Mulwala Weir that received no new timber.
More than 25,000 large snags were pulled from the Murray River below Lake Hume between 1950 and 1980, and the researchers described the riverbank and in-stream habitat left behind as degraded. The replacement timber came from trees cleared for the Hume Highway extension between Albury and Tarcutta in southern New South Wales, according to Lyon, with most pieces weighing more than a tonne.
Snags are “the riverine equivalent of marine reefs”, he wrote, and Murray cod use them as cover to ambush prey and as sites to spawn. Of the fish fitted with radio tags, 36 more moved into the restored stretch than left it, and most came upstream from Lake Mulwala and the Ovens River, where nothing blocked their way.
The researchers concluded that returning wood can lift the number of large native fish in degraded rivers, “assuming there is connection to a source population”. They argued that habitat work complementing environmental water should have broad appeal, particularly where human demand limits how much water reaches a river.
It comes as 385 snags have been returned to the Darling-Baaka River between Bourke and Louth in far western New South Wales, a river once stripped of millions of trees and logs for paddle steamers. The office of the Commonwealth Environmental Water Holder, Dr Simon Banks, put $900,000 towards the work after finding that Murray cod in that reach “needed more than water to recover”.
“Sourcing the right type of wood can be a barrier to re-snagging efforts,” South Australia’s Department for Environment and Water states, because a snag will float away unless its tree was cut down recently. Juvenile Murray cod and golden perch were already being recorded within their first snags at Bookpurnong, Banrock Station, and Overland Corner in the Riverland, according to the department’s Glenn Shimmin.
Red gum pins that lock the root balls down “had to be sourced from interstate and trucked to South Australia” for those first placements, Shimmin said, until two Riverland wineries offered trees from their own woodlots. Both wineries had planted native trees, such as river red gum, in those woodlots, which were watered with wastewater from their operations. “We recognise the need for timber to go back into the river to provide important native fish habitat, and repurposing these trees fits the bill,” said Tim Field, who managed Accolade Wines’ Banrock Station wetland at the time.
