Not just fish poo – why Macquarie Harbour has an oxygen problem
IN late 2016, ABC TV’s Four Corners program raised concerns that overstocking of some salmon farm leases in Tasmania’s Macquarie Harbour was damaging the ‘clean green’ image of Tasmania’s fisheries.
In the ensuing public debate, the sustainability of the state’s salmon farming industry, worth more than $700 million a year, was called into question.
This year, the debate escalated, when 1.35 million farmed salmon and trout in Macquarie Harbour died over the 2017-18 summer. That event coincided with higher than usual temperatures and low dissolved oxygen (DO) levels in the harbour and the presence of a fatal fish disease – POMV (pilchard orthomyxovirus).
Critics of the harbour’s salmon-farming industry say the low oxygen levels observed in recent times have largely been due to waste from the introduced Atlantic salmon stocks accumulating below the pens.
But scientists are now beginning to understand the role that the harbour’s environment and history – influenced by topography, tides, salinity, temperature, wind, freshwater inflow from rivers, and human activity in the catchment over the past century or so – play in the harbour’s oxygen cycle.
Back to the science
Macquarie Harbour is a large indented lagoon, roughly 33 km long by 9 km wide, and with a 276 sq km surface area. Estimated to be six times the size of Sydney Harbour, the harbour is protected from the big seas off Tasmania’s remote west coast by a narrow, shallow entrance.
One-third of the harbour falls within the internationally recognised Tasmanian Wilderness World Heritage Area, through which the King and Gordon-Franklin rivers flow.
The harbour is also home to the endangered Maugean skate, which lives deep below the surface, where dissolved oxygen levels are especially critical, as life forms here lack access to atmospheric oxygen.
In 2016, researchers from the University of Tasmania (UTAS) and CSIRO began a rigorous study of the oxygen cycle in Macquarie Harbour’s waters and its impact on living organisms, including the Maugean skate and farmed salmon and trout. The research is supported by the federal government through its Fisheries Research Development Corporation (FRDC), and by the state government through its Environment Protection Authority (EPA) and Department of Primary Industries, Parks, Water and Environment (DPIPWE).
The CSIRO team focused on using 3D hydrodynamic modelling, along with regular monitoring of the water column, to understand the drivers of estuary circulation and oxygen. Where does the oxygen at depth come from, for example? How is oxygen circulated vertically and horizontally? How does the estuary respond to freshwater inflows from rivers and salt-water intrusions from the ocean?
Modelling and measurement
CSIRO’s Dr Karen Wild-Allen, a biological oceanographer, and Dr John Andrewartha, a hydrodynamic modeller, initially used the model to formulate a hypothesis of water circulation in the harbour. Over time, they have been able to calibrate and fine-tune the model with actual daily measurements of salinity, temperature and oxygen to improve the model’s accuracy, and thus its predictive capacity.
CSIRO has collected data via two techniques. The first is an automated profiling mooring in the centre of the harbour that samples temperature, salinity, oxygen, and other water-quality properties every few hours. This data is transmitted via the net to CSIRO computers in Hobart, where Wild-Allen‘s team can access and analyse it via an on-screen dashboard.
Data has also been collected via an autonomous underwater vehicle, known as Starbug X, which carries sensors on board and can rove throughout the water column, sampling prospective low-oxygen ‘hotspots’.
“Previously, people had measured low oxygen at different spots, and our model had suggested these isolated patches were connected,” says Wild-Allen. “But we couldn’t confirm this until we actually ran an oxygen sensor through them.”
One harbour, three environments
The results of CSIRO’s work show that Macquarie Harbour has a long flushing time, driven by saltwater influx through the mouth and freshwater inflows from rivers.
“Macquarie Harbour has three stratified layers,” explains Wild-Allen. “The surface water (0–10 m deep) is dominated by the river inflows. This water is fresh and buoyant with high levels of oxygen, and is rapidly discharged to the sea through the harbour entrance.
“The sub-surface layers (below 10 m) are much older and stay in the harbour for much longer. Because they’re older and isolated from the atmosphere, they’re depleted of oxygen due to respiration and biological processes in benthic organisms.
“In other words, the harbour’s deeper water is naturally low in oxygen.”
Oxygen in the mid-water layer (10–20 m below the surface) can be even lower in concentration than in the bottom layer, below 20 m.
This is largely due to seawater influx through the narrow harbour entrance, that displaces and exchanges a portion of the deep water. “Every now and then, an intrusion of salty and dense seawater cascades over the sill at the entrance and under the fresher water above,” adds Wild-Allen.
“This heavy seawater sinks to the bottom of the harbour, providing an injection of fresh, oxygenated water into the deepest layers.
“So, the very bottom of the harbour often has a higher oxygen concentration than the mid-water layer.”
The low-oxygen mid-water layer is especially critical to salmon farming operations, because it overlaps the bottom of the fish pens – indeed, any upward displacement of this layer by the influx of new seawater along the harbour bottom is a major concern for the industry.
As Wild-Allen emphasises, even without taking waste from salmon farming into account, Macquarie Harbour is naturally low in oxygen – not just due to its hydrodynamics, but also due to discharge of sediment and organic material by local industries in the nineteenth and twentieth centuries.
“It was not a pristine environment even before the salmon farming,” says Wild-Allen.
“Fish farming is just one of a series of human impacts on the harbour over many years. Historically, there was a lot of mining and forestry in the catchment, and these could also have reduced oxygen levels in the harbour.”
Exploring out to sea
The next phase of the FRDC work will involve the development of evidence-based decision support systems for use by the EPA and DPIPWE.
“We’ll be adding the biological drivers to this story,” says Wild-Allen. “That will allow us to put together a full biogeochemical model of the harbour that also includes plankton and detrital respiration.”
Wild-Allen says CSIRO has received funding for the next three years to explore the feasibility of offshore salmon farming off the southern Tasmanian coast.
“That science will be used to underpin best practice in the relocation of salmon farms from inshore to offshore waters in future,” she says.