The survival of the Maugean skate, a prehistoric ray species native to Tasmania's Macquarie Harbour, hangs in the balance. Despite efforts to estimate their population, researchers face challenges due to fluctuating data. A drastic decline was estimated in 2023, leading to concerns about the species' severe extinction risk by 2050. However, subsequent estimates showed an incredible rebound, raising questions about the reliability of the data.
Professor Jayson Semmens, an expert from the Institute for Marine and Antarctic Studies, believes a reliable method will soon be established. He emphasizes the scrutiny scientists face and the need for accurate data to make informed decisions. The uncertainty surrounding population estimates has influenced the Commonwealth's decision to maintain the skate's status as endangered rather than critically endangered, despite draft recommendations to the contrary.
What remains consistent across various reports is the impact of salmon farming on the Maugean skate's habitat. The final report from the Commonwealth Threatened Species Scientific Committee unequivocally states that industrial aquaculture is the primary driver of habitat stress for the skate. The Maugean skate thrives in waters with suitable dissolved oxygen levels, typically found at depths of 5 to 15 meters. While it can handle seasonal fluctuations, a prolonged reduction in dissolved oxygen, which occurred from 2009 to 2014, poses a significant threat.
The issue lies with organic carbon. River inflows bring organic carbon, but most settles on the surface. In contrast, organic carbon from salmon farms is more biologically available, released in mid-waters with already low dissolved oxygen, and retained deeper in the water. This has created a critical problem for the skate, pushing it to its environmental tolerance limits.
The Commonwealth's advice warns of catastrophic consequences, stating that reduced water quality due to salmonid aquaculture operations would be devastating for the skate, especially when combined with other threats like dam-impacted river inflows, historic mining influences, and warmer waters.
The risk matrix included in the advice highlights the severity of the issue. In the 'catastrophic consequences' column and the 'almost certain likelihood' row, it specifically mentions "Reduced water quality from salmonid aquaculture operations in Macquarie Harbour."
While mitigation measures have been implemented, their effectiveness is still being determined. The Tasmanian Environment Protection Authority (EPA) has reduced the allowed fish biomass, but in response, fish farms increased feed levels, creating a nitrogen problem. Nitrogen output limits and oxygen mitigation plans were introduced in 2022 and 2023, respectively. Data suggests some improvement in dissolved oxygen conditions at certain sites, but the extent of this improvement is yet to be fully understood.
The Commonwealth is also funding a reoxygenation program, which involves pumping dissolved oxygen bubbles into the harbour via a diesel-fuelled barge. While this technology shows potential, it is still in the trial phase, and its long-term benefits remain uncertain.
The independent science committee believes that an immediate difference can be made by reducing the amount of salmon farmed in the next 12 months. This is the first action listed in their short-term recommendations, emphasizing the need to significantly reduce fish biomass and feeding rates to improve dissolved oxygen concentrations.
Environment Minister Murray Watt has stated that he will follow the science in his decision-making. He recognizes the expertise of independent scientific experts and has committed to implementing the committee's updated conservation advice.
Labor member Anne Urquhart, known for her strong support for salmon farm jobs, believes the final decision rests with Minister Watt. She emphasizes the importance of environmental standards and the need to be guided by science. The fate of the Maugean skate and the future of salmon farming in Macquarie Harbour remain intertwined, with the potential for significant impacts on both the environment and the local economy.
