The release of water for the environment into the lower Lachlan River, NSW in early spring stirs the mighty Murray cod out of their homes to search for a mate to breed.
University of Canberra researchers working on the Science Program for the Commonwealth Environmental Water Holder (CEWH) - Flow Monitoring, Evaluation and Research (Flow-MER) - are currently finding high numbers of Murray cod larvae when compared to recent years. They point to releases of environmental water providing stable flows in the lower Lachlan River contributing to more Murray cod breeding in the river.
Environmental water managers are trying to create ideal conditions for the Murray Cod after three years of high and variable flows in the Lachlan River, which can lead to lower numbers of the fish surviving to adulthood.
Murray cod are a territorial fish. They don't often move too far from home, even to chase prey. But come springtime all that changes. In late September or early October, when the water temperature rises, they leave the safety of their home to find a mate, spawn, and lay their eggs on hard surfaces such as a rock or hollow log.
A rise in water levels in spring is believed to initiate spawning behaviour. This higher water level allows the fish to migrate up and down the river in search of a partner. When this pulse is followed by a period of lower and stable flows to keep eggs safely underwater, conditions are ideal for Murray cod spawning.
The CEWH, which provides water for the environment across the Basin, is trying to help the cod's recovery in the Lachlan River by contributing small pulses in the water flow when the time is right, and by extending the duration of natural flows, particularly leading up to cod-breeding season.
Under Flow-MER, aquatic ecologists have recently been on the lower Lachlan monitoring newly hatched Murray cod larvae, and early results are promising.
"On our first sampling trip this season in the lower Lachlan River we've found Murray cod larvae at all three monitoring sites which is great," said Mr Ben Broadhurst, a Senior Aquatic Ecologist at the Centre for Applied Water Science, University of Canberra.
"It's too early to accurately determine the numbers of cod larvae, but they were common captures across in traps and drift nets at each site. Though, I can confidently say that abundances are higher than the last few years."
The researchers measured and estimated age of a small number of the Murray cod larvae and determined that spawning occurred in the first two weeks of October, shortly after the release of water to the environment in the lower Lachlan River.
"These very early results are a positive sign for Murray cod recovery and water management on the lower Lachlan River," Mr Broadhurst said.
Water for the environment continues to be delivered to the region, with a flow pulse currently underway to help Murray cod larvae disperse from their nests and provide a boost to primary food production in the river.
The Murray-Darling Basin's biggest fish, the Murray cod, were once much more abundant in rivers across the Basin than they are today.
Disruption of their ability to move up and down the river and changes to water flows caused by human water use and climate change have been key contributors to the Murray cod's decline. In 2003, the Murray cod was listed as vulnerable and while the downward decline of cod is showing signs of recovery, they remain listed as vulnerable to this day.
The CEWH's use of science to guide the delivery of environmental flows to improve the populations of cod and other native fish is an important step in the right direction.
