High level of uncertainty in data; little understanding why skates seem easier to catch
New data shows strong correlation between increased salmon farming intensity and drop in estimated Maugean skate numbers
Foreign-owned salmon companies could cut salmon numbers to save Maugean skate and its long-term future
A slight improvement in the precarious Maugean skate population estimates will not save the skate from extinction, especially if there's another extreme weather event in its only home, Tasmania's Macquarie Harbour, in the next 10 years, the Australian Marine Conservation Society (AMCS) said after a new report from the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS) was released today.
In 2019 an upturning event wiped out half the Maugean skate population in Macquarie Harbour. Oxygen-depleted water, caused primarily by intensive salmon farming, rose up from the depths of the harbour, choking skates in their habitat. While salmon farming continues unabated despite scientific recommendations to reduce it, there are fears another storm event will wipe out another half of the current population and lead to extinction.
The IMAS report describes trends in estimated Maugean skate numbers between 2014 and 2024, based on the 'catchability' of skates using a consistent approach to surveying over the years, with a revised population estimate of 4,102. Although the report indicates that the relative estimate of skate numbers in 2024 compared with previous years is around 2014 levels, the authors state that the "Maugean skate population is still at low levels and potentially subject to major environmental events leading to mortality, as inferred in 2019."
AMCS shark expert Dr Leonardo Guida said: "The new report shows a strong correlation between increased salmon farming intensity and a drop in estimated Maugean skate numbers. That's not surprising since we know salmon farming has dramatically depleted the oxygen levels in the Maugean skate's only home, Tasmania's Macquarie Harbour. The fate of the skate literally rises and falls alongside salmon production.
"Salmon stocks in the harbour have dropped from their peak in 2015 at about 20,000 tonnes - the equivalent weight of about 10,000 four-wheel drives - yet we are only seeing a slight rise in Maugean skate estimates a decade later.
"The skates caught in 2024 were mostly old adults that will not be around much longer given they live to around 10-12 years, and there hasn't been enough time to show that the uptick is solely from juveniles surviving to breeding age. The scientists acknowledge huge uncertainty in the data from animals caught in 2023 and 2024 that shows something has happened that's making the skates easier to catch. Without knowing exactly what happened, we're unfortunately no longer comparing apples with apples from previous years.
"What is abundantly clear is that reducing salmon in the harbour is good for the skate. Tragically, salmon stocks have not dropped enough nor quick enough, and the foreign food giants who own the salmon farms have not shown any willingness to cut salmon numbers since the plight of the Maugean skate came to light. These multinational food conglomerates need to ensure they are operating sustainably to secure the long-term future of their workers.
"Maugean skate numbers have not improved enough for the population to survive another extreme weather event in Macquarie Harbour, whose narrow, shallow entry to the sea makes these events likely. An upturning of deep, oxygen-depleted waters caused by storms in 2019 contributed to wiping out half the Maugean skate population, and oxygen levels where the skate lives, particularly in the World Heritage Area, have not improved much despite a giant soda machine trying to inject oxygen bubbles into the water.
"Millions of dollars of taxpayers' money are being used to trial oxygen pumping into the harbour's water, but it's a long way off proving it can restore oxygen levels. The skate has been nothing short of lucky with the past three years of natural injections of fresh oxygen-rich seawater into the harbour.
"Pumping oxygen into the water alone is not enough to repair the damage already done by at least decade's worth of industrialised salmon farming and more than 100,000 tonnes of salmon defecating in the harbour.
"With the endangered Maugean skate struggling to survive in its only home, Tasmania is on the brink of having its highest profile extinction since it drove the Tassie tiger off the planet. Tasmanian produce trades on the state's reputation for its relatively pristine and wild natural areas, and another extinction would heavily damage that reputation."
