Two species of fish have recovered from the Millennium drought in record numbers

This year marks a decade since the end of the Millennium drought, when floodwaters reached the mouth of the Murray River in 2010. For 1,200 days earlier, Australia’s most iconic river had ceased flowing to the sea, causing populations of fish and other aquatic animals plummet.

In particular, native migratory fish, including congolli (Pseudaphritis urvilli) and lamprey (Geotria australis), have been severely affected by barriers to migration, such as dams and weirs, and by lack of river flow.

However, our research has shown that some clever engineers and increasing volumes of water for the environment are helping the Congolli and the bagged lamprey bounce back in record numbers.

With native fish in the Murray-Darling Basin only a fraction of what they were before European colonization, rebuilding populations will be a long process. But learning from successes like this along the way will help in the journey to a healthier river.

What happened to fishing during the Millennium drought?

From 2001 to 2009, Southeast Australia experienced the worst drought ever recorded in history.

Low rainfall and water abstraction for irrigation and unprecedented human consumption have reduced water flows in the Lower Murray by approximately 70%. The water level in the lower lakes at the end of the river system has dropped to more than a meter below sea level.

To prevent the ocean’s saltwater from mixing with critical freshwater deposits, tidal weirs (dam-like structures) were closed and the Murray River was disconnected from the sea.

This has been a major problem for a number of migratory species, including sac lamprey and Congolli, which need to migrate between fresh and salt water to complete their life cycle.

During the Millennium drought, no lamprey was observed in Lower Lakes and Coorong, while the number of young Congolli declined. After more than three years of dam closure, local populations were threatened with extinction.

But in late 2010, both species were saved from severe flooding as the Murray returned to the sea and abundances have continued to improve steadily over the past decade.

Several management initiatives have also been instrumental in supporting the recovery, even during the most recent drought. In particular, the installation of fish ladders and better water management. Fish ladders are water-filled channels with a series of steps that allow fish to swim around or over dams and weirs.

Supporting fish migration

The native fish populations in the Murray-Darling Basin are estimated to be around 10% of those pre-European colonies. Barriers to fish movement and altered river flow are the two main causes of the decline.

The Murray Barrages were built in the 1930s, regardless of the passage of fish, and in 2003 the first fish ladder for 70 years was built.

In 2020, there are now 11 fish ladders scattered across Murray Barrages and our research has shown that they effectively support viable migrations.

Other fish ladders were built on the upstream dikes, together opening up more than 2,000 kilometers of the Murray River for fish migration.

However, water must be available for fish scales to work, and this is where ambient water plays a role.

In 2009-10, approximately 120 gigaliters of environmental water were delivered across the basin. In 2017-18, this volume was over 1,200 gigaliters and included substantial volumes across the Murray Barrages.

This increase allowed the Murray River to continuously flow out to sea, restoring its natural characteristics, albeit at a greatly reduced volume.

Furthermore, water for the environment has supported the continued operation of the barrage fish ladders since 2010, a big win for lamprey and Congolli.

The rebound

Since the drought lows of the Millennium, this year we have so far caught a record 101 individual pouched lamprey moving through fish barrage ladders and upstream. This is up from last year’s catch of 61 fish.

The Congolese populations are also booming. From 2007 to 2010, we sampled a combined total of just over 1,000 Congolese. Compare this to the summer of 2014-15, when we sampled over 200,000 of them passing through the fishways.

Congolli is today one of the most abundant fish in the Coorong and upstream of the barriers of the Lower Lakes.

What the rest of the pelvis can learn from this

Fish ladders and ambient water have been successful in supporting fish migration to Murray Barrages, but across the Murray-Darling Basin, thousands of reefs remain and more have been considered, particularly in the Northern Basin.

These reefs can hinder the movement of fish that migrate entirely in freshwater, such as the golden perch (Macquaria ambigua) and the threatened silver perch (Bidyanus bidyanus). This includes reproductive migrations of adults and downstream dispersal of juveniles.

Mitigating the impacts of existing and new structures on fish movement is critical to restoring native fish populations in the Murray-Darling Basin.

To help restore migratory fish throughout the basin, there needs to be a greater understanding of the movement requirements of all stages of fish life, building effective fish ladders, and river currents need to be sufficient to facilitate movement at valley, including eggs and larval fish. Removing barriers may also be a viable option.

In any case, after 15 years of experience in the lower Murray River, we have learned that the protection of migratory fish is best achieved when researchers, the community, water managers and river operators work together closely. Such partnerships are the basis for establishing a healthier river.

This article was republished by The Conversation under a Creative Commons license. Read the original article.