Course:FNH200/Projects/2026/Flat Oyster (Endangered Food

Introduction (What are Flat Oysters? Where did Flat Oysters Originate? Where can you get Flat Oysters from?)

Ostrea edulis

The primary focus of our project is on the European flat oyster (Ostrea edulis). This oyster is an edible bivalve mollusc recognized by its distinctive flat, oval-to-roundshell. It is native to the coastal waters of Europe and can be found from Norway to Morocco in the North-Eastern Atlantic, as well as throughout the Mediterranean and Black Seas. The species can grow to over 20 cm and is harvested and farmed for human consumption. However, Ostrea edulis has experienced significant population declines due to factors such as overharvesting, disease, habitat loss, and environmental changes, making its conservation and safe consumption important topics to consider.

In the past, Ostrea edulis was overfished excessively, resulting in the natural beds of the oysters becoming depleted. The cultivation efforts for the species started in 1849, while Coste, a French scientist, continued in his efforts in 1855, developing the oyster farming methods. In the early 1920s, a death outbreak among the oysters led yet to another decline. After that, the numbers of oysters became larger, with the production of the oysters being at about 15,000 to 20,000 tonnes in 1968 to 1970. Moreover, these flat oysters can be obtained from oyster farms, fisheries, aquaculture operations, seafood markets, fishmongers, and some restaurants. For this project, we will further discuss the harvesting, processing, preservation, regulations, and food safety surrounding Ostrea edulis.

Yearly Production of Ostrea edulis from 1950-2003

Endangerment of Flat Oysters

Ostrea edulis once created extensive beds along the Europe's coastline, which helped increase biodiversity along the coasts, as well as food security for the European population to consume. During the late 1700s and 1800s, millions of oysters were being harvested each year to feed the growing urban population of the Industrial Revolution and were seen as "poor man's food". In the 19th century, about 700 millions oysters were consumed annually just in London, and the beds near the Scottish water yielded up to 30 million oysters annually, which made native oyster fisheries extremely profitable. This shows how important Ostrea edulis was for consumption and the local economies. However in the 20th century, there was more demand for Ostrea edulis and combined with coastal degradation, water pollution, and disease outbreaks, it led to the population of the species declining and eventually their beds becoming rare and absent. Factors such as over-exploitation, illegal harvesting, diseases, and introduced pests and parasites also led to the decline in stocks, which meant that if the population density falls below a certain level, the species are unlikely to be able to reproduce successfully and not be extinct (Allee effect). Hence, they are now considered as a 'Threatened and Declining species' by the OSPAR convention for the Protection of the Marine Environment of the North-East Atlantic.

Flat Oyster Larva

Because of the endangerment surrounding Ostrea edulis, there are now adopted national legislation and policies by the OSPAR member nations to protect and conserve the species, with aims of achieving biodiversity goals, restoring ecosystem functions and strengthening the existing population to be resilient against human threats and climate change. As well, there has been growing interesting in restoring the population of Ostrea edulis for the past couple years due to their ecological importance in contributing to a healthy marine ecosystem. To conserve the species, the OSPAR member nations are planning to follow two recommendations: protecting, maintaining and expanding the remaining Ostrea edulis population, and restoring the species to the areas they once occupied. The population also depends on larval (baby-like phase) development as the quantity produced depends on larval density. In Scotland, there have been a large-scale, long-term project by the Dornoch Environmental Enhancement Projected to restore Ostrea edulis to the Dornoch Firth on the east coast, as well as smaller community led projects working to restore the population on the west coast. Overall, there is a collective effort Europe to protect, conserve, and restore the Ostrea edulis population to what it once was.

Harvesting

Commercial production and harvesting of Ostrea edulis requires basic techniques that involve the collection of spat (young oysters) and transferring them to a growout system. The final growout production of the oysters then occurs in two techniques: on-bottom cultivation and off-bottom cultivation.

On-bottom cultivation:

On-bottom cultivation is where one-year-old oyster spats that approximately 1 cm in diameter, are re-laid by boat on-bottom on the subtidal ground. Before the spats are seeded, predators are cleared out from these growout areas using tangles and toothed steel frames and mats may be used in order for the them to not sink. In order to maximize growth and reduce the impacts of bonamiasis, spats are seeded at low densities (50-100 kg ha^-1) in the water, and are harvested when they are about 2 years old and weigh about 60-80 g.

Submerged collectors in the harbour of Brest to allow the flat oyster larvae to settle and begin their development

Off-bottom cultivation:

Off-bottom cultivation is where oysters are maintained in deeper water through the use of buoyed lanterns, plastic trays, or attaching medium size oysters, about 5cm in diameter, to ropes by cement. Workers will occasionally clean the oysters by removing algae and mussel seed. Another off-bottom technique to growing Ostrea edulis is growing them in mesh bags or pouches raised off the bottom on steel trestles, where they are thinned out as they grow.

For bottom-grown oysters, farmers will use toothed or toothless dredges about 3.5-4m wide and 2m deep with 3-5cm teeth blades that are operated by hydraulic or pneumatic winch to harvest them. Whereas for oysters that are grown being suspended, they are harvested by hand.

Post-Harvest

Oysters are sold as live products and as processed meat. Live oysters need to be protected from heat, drying, and physical damage during transportation while being kept cool. Oysters should have some way of tracing back their harvesting location and date.

Depuration:

Defined in Canada as "the process of using a controlled, aquatic environment in a depuration establishment to reduce the level of microbiological contamination in live shellfish." Depuration is used for flat oysters intended for fresh markets in some production areas before being transported to market. Depuration does not eliminate every possible hazard and some contaminants cannot be reliably removed through depuration so the quality of the harvesting area must be controlled as a further safety measure.

Oysters are put into clean water tanks that are self-draining to continuously to filter the clean water which lets microorganisms and/or contaminants in their digestive systems to be expelled. Depuration processes vary depending on the operation and effectiveness depends on factors like water quality, temperature, size, equipment, and the time between harvesting and depuration. The water used is treated for safe use, typically with UV light systems or less commonly chlorination/dechlorination and ozonation/deozonation.

Freezing:

Freezing the flat oyster extends shelf life, kills some parasites, eases transportation, and changes meat texture. Oysters should only be frozen while still alive but are no longer alive after thawing so it is used differently from live oysters marketed for fresh consumption. The meat becomes softer and are only acceptable for cooking purposes. Freezing slows microbial and enzymatic activity which allows oyster to be stored for a longer period of time than a refrigerated fresh oyster. Freezing does not eliminate all bacteria or viruses. The liquor/natural juice in oysters help against dehydration in cold temperatures.

Frozen whole oysters are packed in polyethylene bags and good condition can be maintained for 6 months if kept stable at -30°C. The meats of frozen whole oysters begin discolour to yellow/green and develop rancid flavours if held at -15°C after 3 months. Frozen oyster meats are packed with a vacuum in a film with flexibility and resistance to vapour and oxygen. Frozen meats will also keep for 6 months in suitable packaging at -30°C or at least below -12°C.

In Shell/Whole Oyster Freezing: Can be frozen in single layers in an air blast freezer. Freezing may cause shells to open/lossen muscles which makes shucking easier.

Half Shell Freezing: Frozen with half of their shell also in single layers in an air blast freezer with plastic film to cover the half open oysters.

Meat Freezing: Cheaper to freeze just the shucked meats as they can be frozen individually or in blocks but are definitely not meant for raw consumption. Shucked meats are also more thoroughly washed to get rid of any sand, debris, or discard of any ragged meats. They are then drained of excess water and also frozen in an air blast freezer before being packaged. However, they can also be first packaged with a vacuum into a rigid container before freezing.

High Pressure Processing:

HPP uses high hydrostatic pressure to eliminate pathogens commonly found in seafoods, slow spoilage microorganisms' growth, and extend shelf life without exposing the food to high temperatures. Instead, it uses pressurized cold water to only make minor changes in sensory and nutritional properties and aids in preserving these properties for extended durations in the fridge. It may also make shucking the oyster easier because it relaxes its' muscles. It is important to note that HPP is not a complete defence against every microorganism or food safety hazard.

Storage/Packaging:

Live flat oysters are kept in things like mesh bags, waxed cartons, ventilated boxes, and insulated containers with ice packs for transport. Packaging aims to keep the oysters cold and protected from physical damage. Oyster packaging also must have some way to trace back where the oyster was harvested from, with harvest records and identification that allows people to see where and when it was harvested if there was a contamination in the area. Live/raw shellfish should be kept refrigerated after purchasing below at least 4°C and should not be completely enclosed in a container. Fresh live shellfish like oysters should last a few days in the fridge.

Canadian Regulations of Flat Oysters

The regulation of oyster harvesting and food safety in Canada is primarily administered through the Canadian Shellfish Sanitation Program (CSSP), which is jointly managed by the Canadian Food Inspection Agency (CFIA), Fisheries and Oceans Canada (DFO), and Environment and Climate Change Canada (ECCC). These agencies and programs regulate the conservation, food safety, and disease control of oysters. Shellfish growing waters are routinely monitored and classified based on their sanitary quality to minimize potential risks associated with consuming contaminated oysters. Oysters intended for consumption must be harvested from areas which are appropriately classified and open for harvesting. These controls are particularly important due to the risk posed with the consumption of shellfish from water contaminated by pollutants. Pollutants such as those associated with fecal pollution can accumulate in oysters resulting in potential health risks if consumed. Checks are routinely performed which can result in previously approved harvesting areas to be temporarily closed if contamination poses a food safety risk. Following the harvest of oysters, additional controls are in place to ensure that they are properly handled, they are kept at appropriate temperatures, and they can be traced back to their harvesting location if a food safety issue arises.

Food Safety Surrounding Flat Oysters

Flat oysters (Ostrea edulis) can pose food-safety risks because they are filter feeders. They filter surrounding water for food and can accumulate pathogens and toxins present in the water. These hazards can cause illness, particularly when oysters are consumed raw or undercooked .

One important bacterial hazard is Vibrio. Vibrio parahaemolyticus can cause diarrhea, abdominal cramps, nausea, and vomiting, while Vibrio vulnificus can cause severe bloodstream infections and may be fatal, particularly in people with certain health conditions . Norovirus is another concern because sewage contamination can introduce the virus into shellfish-growing waters. Raw oysters have been linked to outbreaks of norovirus and gastrointestinal illness in Canada .

Oysters can also accumulate marine biotoxins produced by harmful algae. These toxins can cause serious illnesses, including paralytic, amnesic, and diarrhetic shellfish poisoning . Therefore, monitoring water quality, sewage contamination, and marine biotoxins is important for oyster safety

Exam Question/Fact

How do environmental conditions impact the food safety surrounding oysters?

A) Oysters are filter feeders that filter surrounding water for food, allowing them to accumulate pathogens, pollutants, and marine biotoxins present in the water. (correct answer)

B ) Oysters are filter feeders, but their filtering process naturally removes most pathogens and toxins from the water, making them generally resistant to environmental contamination.

C) Environmental conditions have little impact on oyster safety because oysters obtain most of their nutrients from the sediment rather than the surrounding water.

D) Oysters can only become contaminated when they are improperly stored or handled after harvesting; pathogens and toxins present in the surrounding water do not accumulate in oysters.

CORRECT ANSWER FURTHER EXPLAINED: Oysters are filter feeders so they use surrounding water for food and can accumulate pathogens and toxins present in the water. They can also accumulate marine biotoxins produced by harmful algae which can cause serious illnesses, including paralytic, amnesic, and diarrhetic shellfish poisoning. Monitoring water quality, sewage contamination, and marine biotoxins is important for oyster safety.

An explanation why your question should be on the final exam

• Shows how food safety isn’t only about processing but also where it comes from
• Encourages us to evaluate how we treat our environment because ultimately the state of the environment determines the state of our fuel
• understand that food safety involves the entire food system, from the environment where food is produced to how it is harvested, processed, stored, and consumed
• Filter feeder is an interesting and unique concept
  • even if oyster looks fresh and is handled properly after harvesting, the environmental conditions where it was grown can still affect its safety
• Eating raw oysters is not simply whether the oyster was cleaned or refrigerated
  • quality of the water, contamination of the harvesting area, and environmental conditions can all influence the potential food safety risk