Fish, undammed

You wouldn’t be a freshwater fish for quids in this world.

As human culture and ingenuity has progressed, the construction of instream barriers such as dams and weirs has grown as a significant threat to freshwater fish species worldwide. 

There are more than 2 million dams and other stream barriers in the US alone.

Now, a team of researchers from the University of New South Wales (UNSW) has found a way to get fish around – or rather over – barriers, reconnecting them with breeding grounds and preferred habitats.

They’ve developed a “fishway” that utilises the acceleration of flows in a tube system to pump the fish – protected by a cushion of water – vertically through a tube up and over an obstruction, thus delivering them safely into the water on the other side. 

“A challenge that we are just commencing to address is getting the fish downstream in the river,” explains Bill Peirson from the Water Research Laboratory at UNSW.

“This is not important for small structures – they just swim with the flow. However, for high structures they can be killed by impacts in the overflow and we need to find gentler ways for them to descend large dams.”

The team reported their research in the Journal of Ecohydraulics in 2019, before trialling their prototype at the end of 2020 in two large tanks on a slope at the university campus. Two tubes connected the tanks, pumping water to and fro.

A small Australian bass (Percalates novemaculeata) was coaxed into the tube chamber in the bottom tank, then propelled at high speed through a Perspex tube into the second tank, which sat further up the slope, eight metres higher in elevation.

The method of transporting fish through tubes isn’t new. The prototype adds to previous engineering feats in the US, which see fish being carried over dams by a waterless chute.

The team found that with aerated water in the tube, the fish have a layer of bubbles that provides them with oxygen and cushions them against any pressure variations as they travel through.

Importantly, no fish were injured during the trials and all piscine study subjects remained healthy after a monitoring period of 14 days.

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Snapshot: freshwater fishes in decline

• Freshwater fishes are among the most threatened vertebrate taxa in the world
• The IUCN reports that global freshwater fish populations declined by an average of 83% between 1970 and 2014
• Water extraction for human use and associated infrastructure such as dams and weirs are a primary cause

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In Australia alone, development in the Murray Darling Basin has led to the construction of more than 10,000 barriers, restricting fish species’ access to breeding grounds and preferred habitats. 

“Freshwater fish populations have declined by more than 80% over the last four decades across the globe,” explains Richard Kingsford from the Centre for Ecosystem Science at UNSW.

“This is partly due to the hundreds of thousands of our dams, weirs and barriers stopping their movements. If we could reconnect our rivers and give fish the ability to navigate our rivers safely, we would see more breeding and healthier native fish populations.”

However, Peirson says there’s no reason we can’t have both dams and healthy river systems.

“Dams are important for maintaining our water supplies during droughts, but they greatly disrupt our river systems,” he says.

“There is now a real prospect that we can allow fish to migrate as they should and significantly restore our river ecosystems.”

Following a successful demonstration of the prototype, the research team is now in talks with the City of Paramatta Council in Sydney, with the aim of installing a tube fishway at the Marsden St Weir, on the Parramatta River.