“This is a hard conversation”: Endangered Maugean skate headed for extinction in a decade unless Macquarie Harbour fish farms rested

Marine scientist calls for urgent action after skate falters following fish farm boom
 April 18, 2023
Published:  April 18, 2023
A juvenile Maugean skate. Image: Jane Rucker, Institute for Marine and Antarctic Studies.

A leading Tasmanian marine scientist has called for Macquarie Harbour fish farms to be left fallow for several years to prevent the extinction of the endangered Maugean skate.

“There’s lots of evidence to say that the population is in very serious decline,” Neville Barrett, an associate professor at the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS), said last November. “I personally predict it’s likely to be extinct within a decade on the current trajectory.”

Speaking at an ocean summit hosted by the Australia Institute, Barrett said Tasmania faced a choice between allowing the skate to go extinct or potentially losing “several hundred million dollars revenue” if the aquaculture industry could not move their fish elsewhere if leases were rested for several years.

“It’s a hard decision. This is a hard conversation. But you know, it’s been really hard to say this stuff because it is difficult for the general public to access and understand all of this,” he said.

Associate Professor Neville Barrett at the University of Tasmania's Institute for Marine & Antarctic Studies. Photo: Bob Burton.

The skate, dubbed by Barrett as the “thylacine of the sea”, was discovered in Bathurst Harbour in 1988 and subsequently in Macquarie Harbour. Recent IMAS research suggests the Bathurst Harbour population is either extinct or comprises just a few individuals.

It added urgency to the push to ensure the Macquarie Harbour population survives. In October 2022, the federal environment minister, Tanya Plibersek, unveiled a 10-year action plan to prevent 110 priority species, including the skate, from becoming extinct.

The government says it aims to have measures in place by 2027 that put the priority species on track for “an improved trajectory” by 2032.

But the skate might not have that much time.

In 2016 about 3200 skate were estimated to live in Macquarie Harbour, but recent reviews emphasise a “high degree of uncertainty” with that figure. Underlying the uncertainty were the sudden deaths of 11 of 25 tagged skate in 2019 in conditions not considered unusual.

“We have seen mortality events, and they are sudden and impossible to predict … They have the potential to remove … a large proportion of animals,” Associate Professor Jayson Semmens, the head of the skate research program at IMAS, said.

A further factor causing alarm is the apparent decline in the number of young skate in the population. Research suggests this could be due to eggs on the harbour floor being exposed to unfavourable oxygen levels. Maugean skate do not mature until they are about four years old and live to about ten. It means a decline in the population could occur relatively quickly.

The skate occupies a narrow environmental niche, mostly living in brackish water sandwiched between a surface layer of fresh water from the Gordon and King River catchments and saline bottom water pushed in through the narrow ocean entrance by tides and storms.

While it can briefly tolerate warmer surface or saline bottom waters, the species prefers zones between 7.5 and 12.5 metres deep with dissolved oxygen levels of between 60 and 80 per cent.

Adult Maugean skate. Photo: Institute for Marine and Antarctic Studies.

Those conditions are becoming far less frequent. If the skate seeks out oxygen-rich shallower water, it is at greater risk of being snared in gillnets set in some areas by recreational fishers to catch flounder and escaped salmon. Fluctuations in hydro generation affect oxygen levels in the harbour and climate change adds another layer of complexity to restoring the harbour’s health.

But in Barrett’s eye, fish farming remains the biggest threat to the survival of the skate.

Boom and bust

When fish farms were established in Macquarie Harbour in the 1980s, they were small-scale. By 2005, soon after the skate had been recognised as endangered by federal and state governments, there were about 2000 tonnes of farmed fish in the harbour. By 2011, production in the harbour by the state’s three fish farming companies – Petuna, Huon Aquaculture and Tassal – had more than quadrupled.

Despite the decline in oxygen levels, the companies lobbied the state government to approve new leases and increase stocking rates at existing fish farms to reset the biomass cap at 29,500 tonnes.

At the time, the government knew relatively little about the skate. A joint environmental impact statement the companies submitted in late 2011 to support the proposed expansion devoted a little more than one page to the potential risks to the species. It said: “Modelling undertaken for dissolved oxygen and nutrients predicts that the proposal would result in no significant effect in areas inhabited by the species, i.e. shallows less than 10 metres.”

Environment Tasmania, the state’s peak environment group, warned that the skate was “likely to suffer” from the expansion and “the huge increase in nutrients and related decrease in dissolved oxygen levels could lead to marine life not being able to survive in the area”.

Their concerns were brushed aside. The then minister for primary industries and water, Labor’s Bryan Green, approved the expansion in May 2012, pointing to the potential for 160 long-term jobs. “The key to this project is that the expansion is sustainable for the long-term,” he said.

 “We need to remove the polluting fish farm pens and allow the oxygen levels to recover. The science says so.”

Philip Cocker, Environment Tasmania.

A few months later, federal environment minister Tony Burke decided the proposal did not require further assessment under the Environment Protection and Biodiversity Conservation Act and approved the expansion, subject to a short-term cap and additional monitoring requirements to protect the World Heritage Area and the skate. To support the expansion, the then federal minister for regional development, Anthony Albanese, provided a $7 million grant in mid-2013 towards the cost of required infrastructure.

Within a few years, almost 20,000 tonnes of salmon and ocean trout were in the 10 fish farm leases in the harbour.

Barrett told the summit: “As we predicted from the outset, oxygen declined straightaway. At times there has been little oxygen below 10 metres in the harbour, almost zero levels and sometimes little below five metres at its worst.”

Scientists have also flagged that low oxygen levels could affect the population of crabs and shrimp that the skate primarily feeds on.

As the magnitude of the environmental problems in Macquarie Harbour grew, the united front of the fish farm companies splintered. In 2017 Huon Aquaculture unsuccessfully sued the federal government for approving the expansion.

But with damage to the World Heritage Area and the declining health of the harbour in public view, the government was under pressure to respond. Over the last decade, the biomass cap on the fish farms has gradually been lowered and is now almost back to where it was before state and federal governments approved the expansion. Bottom-dwelling fauna have slowly recovered, but oxygen levels have not.

Too little, too late?

As the future of the skate has become more precarious, the state government has committed to developing a conservation action plan by the end of this year that will include research priorities and mitigation actions.

Restrictions on gillnetting were introduced in 2015 and tightened late last year to reduce accidental by-catch of skate. But restoring oxygen levels remains the biggest challenge.

In September last year, the Environment Protection Authority (EPA) changed the regulatory standards for fish farms in Macquarie Harbour from a 9500 tonne biomass cap to an emissions limit of 500 tonnes a year of dissolved nitrogen. It estimated this could reduce nitrogen pollution from fish farms by 10 per cent.

Maugean skate

Map of skate tracking receivers, harbour depths and fish farm leases. Image: Institute for Marine and Antarctic Studies.

Scientists have welcomed the switch, but the benefit is uncertain. “The reality is that we don’t know what the impact will be and over what time period it will occur,” said David Moreno, a research fellow at IMAS.

A recent IMAS report to the EPA “strongly recommended” setting an oxygen limit for mid and bottom waters in fish farm environmental licences. It warned the current limit on dissolved oxygen at 2 metres was “insufficient to protect the flora and fauna of Macquarie Harbour, most of which is below 2 metres”.

A tagged skate relays data to an array of sensors in Macquarie Harbour. Image: Institute for Marine and Antarctic Studies.

The issue is under consideration, but any change is unlikely to happen soon. The Department of Natural Resources and Environment (DNRE) recently concluded a round of public consultation on a draft environmental standard for fish farming across the state. The EPA says environmental licences for the fish farms in Macquarie Harbour will be reviewed and updated in 2024 after DNRE has finalised the standard.

In mid-2022, DNRE created a working group of government agencies and Salmon Tasmania, the peak industry lobby group, to advise on measures to improve water quality in the harbour. Discussions are also underway with Hydro Tasmania to minimise the impact of freshwater flows from power generation on water quality.

Asked about the possibility of allowing the leases in the harbour to lie fallow, a spokesperson for Petuna said the company believed there was a “need for collective, holistic discussion and action among key stakeholders from all the industries, including aquaculture, that operate in and around Macquarie Harbour”. Tassal and Huon Aquaculture did not respond to questions.

A decade after having warned of the risks of expanding fish farming in the harbour, Environment Tasmania said it feared there would be further delays in cutting pollution from the fish farms.

“We don’t need government time-wasting committees, nor industry ideas and spin to save this million-year-old species,” Philip Cocker, the group’s spokesperson, said.

“We need to remove the polluting fish farm pens and allow the oxygen levels to recover. The science says so.”

Bob Burton is a Hobart-based author, researcher, editor and freelance journalist. He is the Editor of CoalWire, a weekly bulletin on global coal industry developments for the US-based non-profit group Global Energy Monitor. His freelance journalism has been published in a wide range of news outlets from the British Medical Journal to the US-based PR Watch.

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