Pilot trials injecting oxygen into Tasmania’s Macquarie Harbour show promising results that could provide a lifeline for the endangered Maugean Skate (Zearaja maugeana).
A pilot project underway in Macquarie Harbour, on Tasmania’s west coast, is providing evidence that mechanical oxygenation methods can improve the level of dissolved oxygen in the water. This could prove crucial in improving conditions for the endangered Maugean Skate, whose only known remaining population makes its home in Macquarie Harbour.
Low oxygen levels
Ongoing monitoring in Macquarie Harbour during the past decade has shown a decline in the levels of dissolved oxygen in the water, particularly at depth.
Based at the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS), marine ecologist Associate Professor Jeff Ross has worked on several environmental and monitoring research projects in the harbour over the past decade. He says cumulative factors that have contributed to low levels of dissolved oxygen.
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Water temperature has increased by an average of 1.5 to 2° Celsius, which has reduced the amount of oxygen the water can hold
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Atlantic Salmon aquaculture has accelerated the use of available oxygen via the break down fish and feed wastes
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Variations in natural and regulated river inflows into the harbour also contribute to oxygen levels. Natural recharge of oxygen into the harbour from the ocean often coincides with periods of little freshwater input
Jeff says the fall in dissolved oxygen has been identified as a key threat to the skate.
Urgent action last year established a National Recovery Team for the Maugean Skate and workshops in late 2023 identified several options for intervention. From these, pumping oxygen into the bottom waters was selected as the most viable option to trial.
Emerging from this, FRDC and Salmon Tasmania have funded a $7.2 million Macquarie Harbour Oxygenation Project (2023-087), to trial mechanical oxygenation techniques, with Jeff as the project leader.
Feasible, safe and scalable
Three trials have been conducted in 2024, in February, April and August. The first trial ran for a week, pumping up to 750 kilograms of oxygen a day into the water.
The second trial ran for six weeks, pumping up to 3000 kilograms of oxygen a day. A third trial in August and September is expected to run for six weeks and pump up to 5000 kilograms of oxygen a day – the likely limit of the available equipment.
Jeff says the oxygen has stayed at depth, as they had hoped, rather than forming large bubbles and rising to the surface. The mapping of the oxygen plumes generated by the pumping continues to be refined as part of the project. However, monitoring shows increases in dissolved oxygen by five to 10 per cent, with plumes travelling horizontally through the water for up to 500 metres out from where the oxygenated water is discharged.
“The results so far have exceeded our expectations,” says Jeff. “We set out to prove that it was feasible to do this and that it was environmentally safe. We’ve been able to demonstrate that, and the loads we are adding are quite significant.”
Targeted improvements
Jeff says the project is not trying to oxygenate the whole harbour (Macquarie Harbour is six times the size of Sydney Harbour). An initial aim is to offset the oxygen used by the aquaculture sector. But the technique could also be used to elevate dissolved oxygen levels in important skate habitat.
“It’s a highly collaborative project, drawing in expertise from government, researchers and industry partners. The ex-Navy barge we use, the ‘Wombat’ has been provided by one of the aquaculture companies involved.
“It’s also been incredibly fast-moving in terms of innovation and adaptation to challenges, and that has allowed us to scale up far more quickly than we thought possible. For example, we’ve improved the energy efficiency of the pumping operation five-fold since we began, and we expect to improve that further.
“At the same time, the role of the project team is to make sure the project is driven by science. We’re taking a very careful staged approach and conducting detailed monitoring for any adverse environmental impacts.”
The FRDC project will run for two years, but Jeff says it was important to have some positive outcomes in the first six months, to provide both a short-term boost for the Maugean Skate. It also provides longer-term confidence in the oxygenation technique as a tool, or lever, that managers can use to improve oxygen levels and manage the environment in Macquarie Harbour more broadly
“This isn’t the only option being considered. It’s part of a package of actions needed to protect the Maugean Skate,” says Jeff.
Related FRDC Project
2023-087: Macquarie Harbour Oxygenation Trial
