A pilot project to stimulate dissolved oxygen levels in Tasmania's Macquarie Harbour has successfully passed another milestone – and it could improve this last known habitat of the unique and endangered Maugean skate.
Marine ecologists at the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS) have been monitoring the environmental conditions in Macquarie Harbour for over a decade.
Their research has shown that the harbour is naturally a low dissolved oxygen environment in the mid-bottom waters, and that these oxygen levels declined from 2009 to 2013 – with salmonid aquaculture, natural and controlled river flows and climate change influencing the harbour's dissolved oxygen level.
Oxygen levels have fluctuated since this time, and most recently have increased due to summer conditions in three of the past four years being conducive to natural recharge of oxygen from the ocean.
The decline in oxygen levels raised concerns about the potential impacts on the viability of the endangered Maugean skate. Among a suite of projects underway to address these concerns, IMAS was tasked with leading the scientific evaluation for a project trialling technology aimed at increasing oxygen levels in the waters of the harbour.
The Macquarie Harbour Oxygenation Project (MHOP) is designed to evaluate the feasibility, scalability and environmental efficacy of mechanical oxygenation as a tool to help increase the levels of dissolved oxygen. It is a joint initiative of the Australian Government's Fisheries Research and Development Corporation (FRDC) and Salmon Tasmania to support conservation efforts for the Maugean skate.
"We know similar approaches have been successful overseas at increasing levels of dissolved oxygen in water, but Macquarie Harbour is quite unique, so a staged approach is critical," IMAS Associate Professor Jeff Ross said.
"We need to carefully assess this approach, to make sure there is not likely to be adverse ecological effects and the oxygen is delivered and retained where it is required."
Monitoring has identified that the preliminary trials have been successful after demonstrating that the oxygen is delivered and retained at depth, with no adverse ecological effects.
"In February, we began cautiously with a one-week trial that delivered 500–700 kg of oxygen a day. Based on positive results, we then transitioned to a six-week trial from April through to June," Associate Professor Ross said.
"For this trial, we progressively increased oxygen loads from 1,000–3,000 kg a day over six weeks, with a detailed examination of nutrient and heavy metal concentrations before, during and after the trial – and found that the microbial community and sediment health showed no adverse environmental effects."
The third trial began in the first week of August, with the project team looking to increase the oxygen load delivered daily to 5,000 kg.
"The loads and scale of delivery and effect are now quite significant, especially in the context of potentially improving bottom-water dissolved oxygen levels in target areas that are important Maugean skate habitat, and for offsetting the estimated demands of salmon aquaculture," Associate Professor Ross said.
An important focus of the project is to improve energy efficiency in running the oxygenation plant, particularly if the system is ultimately scaled-up and ongoing.
"As the oxygenation load has increased for each phase, we have seen a five-fold decrease in the energy requirements for every kilogram of oxygen delivered – and achieving more energy efficiency gains is the focus of our engineering team, as we continue to increase and assess the scalability of the system."
Fast Facts
- In July 2023, the National Recovery Team for the Maugean Skate was formed to develop a conservation plan and coordinate its implementation. The team includes representatives from governments, industry, research organisations and the community, who have a strong interest in Maugean skate conservation.
- The Macquarie Harbour Oxygen Project (MHOP) was developed in response to one of the urgent conservation actions to reduce or offset the impacts on dissolved oxygen from salmon farming. The project centres around three packages of work:
- Engineering and operating the oxygenation plant and how the oxygen is delivered into the water column
- Measuring the ecosystem response to oxygenation across nutrient concentrations, metal availability, microbial activity, and sedimentary and mobile fauna
- Computer modelling to predict the diffusion of the oxygen plume and spread, both locally and more broadly.
