The Maugean skate, or Port Davey skate (Zearaja maugeana), is a ray found only in Tasmania. It is heading for extinction and may soon become the world’s first marine fish extinction in modern times because of human activities.
Threatened Status in Australia
- The Maugean skate (Zearaja maugeana) is listed as endangered under both Tasmania’s Threatened Species Protection Act 1995 and the Environment Protection and Biodiversity Conservation Act 1999.
Location:
- Maugean skate: Macquarie Harbour, Tasmania
The Maugean skate has inhabited Australian waters since T-Rex roamed the Earth. The skate lives only in Macquarie Harbour, on the remote south-west coast of Tasmania. This harbour covers an area of 300 square kilometres, making the Maugean skate one of the most geographically restricted shark or rays in the world.
Ongoing impacts of salmon farming, climate change, manipulated river flows for producing hydroelectricity, and other historical impacts could directly cause the world’s first modern-day marine fish extinction within 10 years. Emergency conservation action is needed.
Maugean skate, Dipturus maugeana. Image: Institute Marine and Antarctic Studies, Tasmania.
What are the numbers?
Current numbers of the skate are thought to be around 1,500 individuals following unfortunate mass-mortality events in 2019 that wiped out half (47%) the population. Seven years ago the skate’s numbers were estimated at 3,200.
What’s causing this? Why is the skate choking to death?
Although Macquarie Harbour is naturally low in dissolved oxygen, its water quality has crashed primarily from the oxygen depleting effects of intensive salmon farming and manipulated river flows to maintain hydroelectricity production. The skates and any eggs they lay are deprived of the oxygen they need to survive.
Salmon Farms: Intensive aquaculture in the harbour results in uneaten fish-feed and fish faeces sinking to the bottom, driving microbial activity that consumes oxygen and further depletes the naturally low-oxygen levels in the harbour. Such is the extent of this, that ‘dead zones’ have been created throughout the harbour’s deeper waters.
Manipulated river flows: Damming of major rivers to optimise power generation means that natural high and low flow events are rarer. This prevents the harbour from ‘recharging’ naturally with oxygen rich seawater.
Stylised diagram of how altered river flows create a ‘freshwater cap’, making it difficult for denser seawater to enter and recharge oxygen-depleted waters of Macquarie Harbour. Note how fish waste exacerbates oxygen depletion at depth.
Severely low oxygen levels (0-20%) are thought to be a major factor in the 2019 mass mortalities, when a ‘mixing event’ driven by coastal storms pushed seawater into the narrow entrance of the harbour. This displaced less-dense and oxygen-depleted bottom waters up through the harbour’s water column. The harbour was effectively turned ‘upside-down’ and the shallows where the skate prefers to live and lay eggs became dangerously low in oxygen.
Giving the harbour a rest from salmon aquaculture and regulating river flows to support increased oxygen levels deep in the harbour, is critical to provide resilience against deadly ‘mixing events’ that will likely increase with warming climate.