Last year, farmed oysters in Japan’s Hiroshima prefecture began dying off in large quantities. The Ministry of Agriculture, Forestry and Fisheries (MAFF) reported catastrophic mortality rates of up to 90 per cent in key regions, threatening both current harvests and the long-term viability of the industry.
After a visit to Hiroshima Prefecture, a key oyster producing region, Minister Norikazu Suzuki said producers were facing a “dire situation”.
“I witnessed with my own eyes the mass mortality of newly harvested oysters. I had heard about it, of course, but seeing it with my own eyes and holding open oysters in my hands – they had been in the ocean for two years and were just about ready to be harvested – really left me speechless, especially from the perspective of the oyster producers,” he said in November.
Experts have suggested causes such as rising sea temperatures, increased salinity, and ocean acidification, which create lethal conditions for vulnerable larvae.
Speaking to AgTechNavigator, Professor Hongjie Wang said human activities in coastal areas have influenced the environment and oysters have been particular sensitive to the changes.
“For example, oysters are sensitive to changes in water temperature, oxygen, and water acidity, through changes in metabolism and disease susceptibility, and uncertainty of food quantity and quality through affecting their growth rates,” said the director of the Ocean Carbon Laboratory at University of Rhode Island’s (URI) Graduate School of Oceanography.
She added that environmental stressors rarely occurred in isolation. Low oxygen conditions frequently coincide with low pH, while disease outbreaks often intensify under environmental stress, particularly during periods of higher-than-average temperatures.
Today, the oyster industry’s primary challenge is maintaining oyster health amid increasingly variable and stressful environmental conditions and oyster juveniles are the most susceptible.
“The frequency, duration, and timing of these extreme events are concerning, especially for larvae and post-settlement juveniles, which are the most vulnerable life stages. The weaker shells of early life stage oysters make them highly sensitive to low pH, meaning that despite the greater resilience of adult oysters, younger individuals may not survive,” said Wang.
“From this perspective, nursery and hatchery sites are effectively the first line of exposure to future changes in environmental conditions, especially ocean acidification. This has already been observed on the West Coast of the United States, where naturally low-pH upwelled waters caused larval mortality.”
Ensuring the future oyster farming
Against this backdrop, Wang said sustaining oyster production in the long term would require more targeted research into how oysters respond to multiple, interacting environmental stressors.
“Most real-world stress events involve a combination of these factors rather than a single driver, yet many studies still focus on one variable at a time.”
Jacqueline Rosa, who is pursuing her master’s degree in oceanography from URI’s Graduate School of Oceanography, recently spent 18 months conducting field work on how water quality and farming practices impact these mollusks.
Wang, who is her academic advisor said the research “fills a critical data gap” by setting baseline water quality conditions for oyster-farming sites
“Our hypothesis is that oyster mortality is linked to specific, abnormal environmental conditions, such as low dissolved oxygen and/or pH. By pairing continuous water quality observations with oyster performance data, this project provides the foundation needed to evaluate whether observed mortality events are environmentally driven,” said Wang.
She acknowledged a lack of familiarity with the specific situation in Japan; however, she emphasised that ocean acidification was a global phenomenon affecting coastal systems worldwide.
“Even though our study does not target that region, it may still shed light on similar issues in other parts of the world.”
Measures for long-term resilience
As oyster farmers balance environmental risk, biological vulnerability, and operational costs, a practical response is to reduce exposure during the most sensitive life stages.
“Given that early life stages are the most sensitive to environmental stress, farmers may need to start with larger seed or nursery-reared juveniles to reduce mortality by bypassing the most sensitive stages and increase resilience to short-term environmental variability. This comes at a higher upfront cost, but over time, this may increase survival probability and yield stability.”
Wang also identified stock selection as a critical sustainability consideration. Many aquaculture operations depend on seed produced in nurseries located in different environmental regions, which may not be well adapted to local conditions.
“Longer-term resilience may come from selecting stocks with higher tolerance to harsh environmental conditions. This does not mean a single “best” oyster, but rather matching stock to place-specific environmental conditions, rather than relying on a single stock. Excessive reliance on a narrow genetic line may reduce resilience and undermine long-term adaptability.”
Furthermore, gear selection and placement also play an increasingly important role.
“Gear depth influences exposure to variations in temperature, oxygen, salinity, and pH. Collecting baseline water quality data specific to on-farm conditions allows growers to make informed decisions about how and where to deploy gear, and to strategically leverage gear placement to reduce exposure to known environmental risks,” said Wang.
The second phase of Rosa’s study involves involved studying the equipment and methods used for farming, with the goal of informing best management practices, reducing operational costs, and enhancing the long-term resilience of the local oyster industry.
“Aquaculture gear is rapidly evolving, making it critical for farmers to select equipment that is most effective for their operations,” she said.
“Traditional grow-out methods are highly susceptible to biofouling, which can reduce growth rates, restrict water flow, and increase mortality. Surface and bottom gear are also labour and time intensive to maintain.”