A qualitative risk assessment of climate-driven aquatic health risks in Australian oyster aquaculture

The ocean is not a static backdrop for human industry; it is a living, breathing system that changes with the seasons and, increasingly, with the shifting climate. For the people who farm oysters along the Australian coast, this environment is both their workplace and their livelihood. These shellfish are filter feeders, meaning they constantly pump water through their bodies to extract food, a process that makes them highly sensitive to what is happening in the water around them. When the water gets too warm, too fresh from heavy rain, or filled with harmful microscopic algae, the oysters can become sick or die. This vulnerability is the central concern of a new study that asks a critical question: as the climate continues to change, how will the risks of disease and toxic algae blooms affect the future of Australia's oyster industry?

Researchers from several Australian universities and government agencies set out to answer this by looking at the two main types of oysters farmed in the country: the native Sydney rock oyster and the introduced Pacific oyster. They focused on three major production regions: Coffin Bay in South Australia, Narooma in New South Wales, and Norfolk Bay in Tasmania. To understand the future, they did not just rely on computer models; they combined a deep dive into scientific literature with the direct knowledge of experts who have spent decades studying these waters. They gathered insights from scientists, government health officials, and industry leaders to build a picture of what might happen by the year 2040. Their goal was to identify which specific threats were most likely to worsen and to understand how confident we can be in those predictions.

The team narrowed their focus to four specific threats that could harm the oysters. Two of these are biological agents that cause disease: a bacterium called Vibrio harveyi and a parasite known as Marteilia sydneyi, which causes a condition called QX disease. The other two are types of microscopic algae that can form harmful blooms, known as Karenia and Prorocentrum minimum. These algae can produce toxins that kill marine life or make oysters unsafe for humans to eat, forcing farms to close. By interviewing experts and reviewing past data, the researchers assessed how likely each of these threats was to cause significant problems under current conditions and how that likelihood might change as the climate warms.

The results paint a clear picture for one of the bacterial threats. The risk of Vibrio harveyi causing disease is expected to rise across all three regions by 2040. The experts were very confident in this prediction. As ocean temperatures increase and marine heatwaves become more frequent, the water becomes a more hospitable environment for these bacteria, which thrive in warmth. The Pacific oysters, which are already farmed in warmer waters, are particularly vulnerable because they filter more water than their native counterparts, potentially taking in more bacteria. The researchers found that the risk for this specific disease is likely to move from a low level today to a moderate level in the near future, a shift that could have serious economic consequences for farmers.

In contrast, the outlook for QX disease, which affects only the Sydney rock oyster, is more stable but still concerning. In the Narooma region, where this disease is a known issue further north, the risk is expected to remain at a moderate level, unchanged from today. The experts noted that this parasite spreads very slowly and its movement depends on a host animal that has not yet been identified. Because the parasite does not jump quickly between locations, the risk of it suddenly appearing in new areas is not expected to spike dramatically, even as the climate changes. However, the uncertainty surrounding its intermediate host means that farmers must remain vigilant.

The situation with harmful algae blooms is more complex and harder to predict. For the region of Coffin Bay in South Australia, the risk of a bloom that could disrupt oyster farming is currently moderate and is expected to stay at that level. This assessment was bolstered by a massive bloom that occurred in South Australia in 2025, which gave experts recent, concrete data to work with. In the other regions, the risk is currently low but may rise slightly. The experts expressed a lower level of confidence in these predictions because the conditions that trigger these blooms are incredibly complicated. It is not just about temperature; it involves a mix of rainfall, ocean currents, and nutrient levels that are difficult to forecast. The researchers emphasized that while we know blooms are becoming more frequent globally, predicting exactly when and where a specific type of algae will explode in numbers remains a significant challenge.

Perhaps the most reassuring finding concerns the algae known as Prorocentrum minimum. Despite being a known organism in these waters, the experts agreed that it is unlikely to cause significant harm to oyster health or production in the future. While it can form blooms, there is little evidence that it poses a major threat to the industry in Australia, and its risk is expected to remain negligible.

The study also highlighted that temperature is the single most important factor experts consider when judging these risks, but it is not the only one. Rainfall, salinity, and the overall health of the ecosystem play crucial roles. For instance, heavy rains can wash nutrients into the ocean, feeding algae, or wash fresh water into estuaries, stressing the oysters and making them more susceptible to disease. The researchers found that while we have a good understanding of how temperature affects bacteria, our knowledge of how climate change will alter the behavior of algae and the spread of parasites is still incomplete.

To help the industry prepare, the paper suggests several practical steps. Farmers could benefit from better monitoring systems that track water quality and the presence of harmful agents in real time. Breeding programs that select for oysters that are naturally more resistant to heat and disease are already showing promise and could be expanded. Changing how and where oysters are farmed, such as moving them to deeper water to avoid extreme surface temperatures or heavy rain, could also reduce risk. The study concludes that while the future holds new challenges, a combination of better science, smarter farming practices, and close cooperation between researchers and farmers can help the industry adapt. The path forward relies on filling the gaps in our knowledge, particularly regarding the unpredictable nature of algae blooms, to ensure that Australia's oyster industry remains resilient in a changing climate.