Oyster reef restoration refers to the reparation and reconstruction of degraded oyster reefs. Environmental changes, modern fishing practices, over harvesting, water pollution, and other factors, have resulted in damage, disease, and ultimately, a large decline in global population and prevalence of oyster habitats. Aside from ecological importance, oyster farming is an important industry in many regions around the world. Both natural and artificial materials have been used in efforts to increase population and regenerate reefs.
Ecosystem The first stage in an oyster's life cycle is the free-swimming larval stage. After about three weeks, the larva attaches to a hard substrate—surface area to attach to—such as prop roots, dock pilings, natural rock, and other oysters becoming an oyster spat—oysters that have just settled to the bottom. A large number of oysters often join together, thus forming an oyster reef (also referred to as oyster bed, oyster mat). Once attached to a surface, oysters will stay there for the remainder of their lives. Oysters are classified to be a part of the Ostreacea family, in the bivalve class and part of the phylum Mollusca. Oysters are shellfish that are born individually but grow up to build reefs. Oysters are born drifting on tides and are free swimming with a vertical mobility, however their locomotion quickly changes when oyster reefs secrete a smell. Baby oysters swim above oyster beds and smell a chemical that is emitted throughout tides and is considered a signal for the oysters to get fixed in one place; sessile. Oyster reefs are formed when individual oysters overlap one another and attached itself to their own kind.
Oyster beds provide a number of ecological, economical and recreational benefits to the environment. Oyster beds provide numerous benefits to the ocean life, like supplying habitats for other marine species. Foraged fish, shrimps, crabs, invertebrates and over 300 species live predominantly within oyster beds. Not only do oyster reefs provide a safe nursery to valuable species but they also improved the oceans water quality substantially through a process called filter feeding. Oyster reefs collect their food source by intaking water through their gills and collecting the nutrients and contaminants from the ocean water, while pumping out the cleaned water from their gills back into the ocean. Oyster reefs have the ability to filter out 50 gallons of water per day. The process of filter feeding helps restore the oceans overall health, while also collecting debris for other species to fuel on. Additionally oyster beds provide cohabitation for mussels, barnacles, sea anemones and more. It has become a huge concern that the population of oyster reefs have been decreasing at a rapid rate. Many species are losing their habitat, animals are undergoing a decrease in food consumption and the ocean is experiencing less filtration.
Background
Global Oyster reefs, vital coastal ecosystems, have suffered a dramatic decline worldwide. Centuries of unsustainable exploitation, including intense over-harvesting and destructive fishing practices, coupled with increasing pollution, and disease have severely degraded these habitats. By 2011, it was estimated that approximately 85% of global oyster reefs had been lost, highlighting the scale of this ecological crisis. This decline is documented across diverse regions, encompassing Australia, the United States, Europe, and Asia, including China and Japan, as well as coastal Africa. Few healthy oyster reefs remain on the coast of South America, at around 50% of their initial abundance. Overall, oyster reefs are one of the most threatened habitats on Earth, despite being crucial marine structure.
Australia In the state of South Australia, oyster reefs mostly created by the Australian flat oyster (Ostrea angasi, or Southern mud oyster) once spread along of coastline, but by the 21st century, there were none left. Since the British colonisation of South Australia from 1836, the waters have been subject to overfishing (and in particular dredging the sea floor, which destroys the oyster beds), water pollution and disease. In addition to temperate reefs in southern Australia, significant efforts are also underway to restore tropical shellfish reefs in Queensland, where extensive reef systems built by species such as Saccostrea glomerata (Sydney rock oyster) and Isognomon ephippium (leaf oyster) once dominated intertidal zones along estuaries and mangrove-lined creeks. Much of this habitat was lost due to historical overharvesting, sedimentation, and coastal development. Community-led restoration programs, such as those coordinated by OzFish Unlimited, have pioneered tropical reef restoration techniques using recycled oyster shells and biodegradable or stainless steel reef units. Projects have been implemented in regions like Moreton Bay, the Burnett River, and Great Keppel Island, helping to rebuild habitat, improve water quality, and support marine biodiversity.
United States On the Atlantic coast of the United States, the eastern oyster (also known as Atlantic oyster and American oyster) acts as a keystone species by filtering estuaries and maintaining healthy coastal ecosystems. In addition to being a keystone species, oysters also serve as an indicator species—they are used to gather information on the overall health of the ecosystem.
Since the early Americas, oyster beds have been beneficial for the North American Indians, who harvested oysters for subsistence purposes, and for the European colonists in the region. Since the 19th century oyster beds have been vital for commercial harvests and have generated substantial income. However, the majority of oyster populations have greatly declined within the last century due to over-harvesting, dredging, increased sedimentation, invasive species, pollution and disease. It is estimated that more than 85% of oyster reefs are gone, making it the most threatened marine habitat in the world.
As a result, this has spawned a need for oyster restoration projects in order to revitalize the depleted natural resource of the oyster population.
Europe The native oyster species, the European flat oyster Ostrea edulis, historically ranged from the Norwegian Sea near North Ireland and the UK to Mediterranean Sea. Oysters were found in subtidal coastal waters and in the offshore deeper waters of the North Sea and Eastern Channel until the 19th century. Currently, the species is considered functionally extinct in the German and Belgian North Sea. Wild populations remain in along the western coast of Sweden and southern coast of Norway. One of the current challenges limited to Europe in restoring oyster reef is the presence of the invasive parasite Bonamia ostreae. Historically, the rapid decline in oyster reefs was the result of overfishing and exploitation of efficient fishery practices for centuries which eventually led to oyster fishing as an inviable and unprofitable source of economic value. Overall, the production of oysters in Europe has reduced by 60%.
China and Japan China, a region of high oyster diversity with 15-23 native species and a millennium of oyster cultivation, has experienced a dramatic decline in its coastal shallow sea of oyster reefs over the past three decades. Reefs in the Bohai Bay including the Hangu oyster reef experiencing a 70% loss due to algal farming rapidly expanding in the area, taking over and smothering the benthic populations. Similarly the Daijawa oyster reef in Laizhou Bay experiencing a 90% loss. Several factors have contributed to the overall oyster reef decline in China, including overfishing for the use of food and lime, habitat loss as city expansion increases, pollution from chemical industries, and hydrologic alterations reducing reproductive success.
Japan has a long history of oyster farming. Its history began when a Japanese fisherman found that oysters would attach to the rocks and bamboo branches of shallow waters. Japan is home to 13-30 species of oysters, including the commercially important Crassostrea gigas. However, in the recent years, Japan has faced an oyster reef decline, with mass oyster die-offs in once highly productive farms. The Momotori district of the Toshikima Island in Toba, Mie Prefecture, a usually major farming area, has faced a major decline in oyster success in the recent seasons. Die off's of 60-90% have been recorded due to higher summer sea temperatures. Fluctuating population numbers of the Crassostrea gigas has been recorded in the Seto Inland Sea of Western Japan. Environmental stressors such as low salinity, food availability, and pCO2 have all been linked to negatively impact oyster numbers in the Seto Inland Sea.
Africa Oyster reef ecosystems in Africa face significant degradation and multiple threats despite their ecological and economic importance. These threats have les to a substantial decline in oyster populations and their critical habitats they provide. A key factor in the degradation of oyster habitats in Africa is from the destruction of mangrove forest. Mangroves are vital for the health of oyster populations, as they provide shelter and breeding grounds for various marine species, including oysters. The loss of mangroves due to logging and over-harvesting directly affects inshore and offshore fisheries by reducing the food availability and eliminating crucial shelter and breeding areas, essential for oyster reef health. In Kenya, this destruction has led to significant economic losses, estimated at KES 582 million annually for inshore fisheries within the mangroves. Coupled with the increasing human population along the African coastlines causing even more overexploitation of the mangrove forests. Assessing the full extent of oyster reef loss in Africa is challenging due to a lack of comprehensive historical data. For several regions in Africa, including parts of South Africa, there is limited information on the past abundance and current condition of oyster reefs. This lack of data likely underestimates the severity of the losses and hinders effective conservation efforts information.
Techniques The first step in oyster restoration is to determine the locations for potential reefs. Expanding not only the population of oyster reefs but also the locality of them will help secure the extant of this species. Determining the location for potential reefs is often done by comprehensive surveys that measure multiple variables. The comprehensive survey consists of measuring salinity levels, water quality, previous existence of oyster reefs, substrate conditions, prevalence of diseases, tidal range, oxygen concentration, algae concentration, prevalence of predators, accessibility and security. Once the location is determined for new oyster reefs to form cultch—fossilized shell, coral or other similar materials produced by living organisms designed to provide points of attachment for oysters—is often obtained from sustainable recycling programs. Used oyster shells and clam shells are collected from farmers and restaurants and get disinfected by volunteers to then be used in oyster restoration. Once the used clam and oyster shells are returned to the water, these recycled shells provide substrate for oyster larval eggs to begin populating oyster beds that were laid out by volunteers. Cultching has been the most successful approach when it comes to oyster beds getting reformed, and was pioneered for oyster farming in the early 19th century. Cultching consists of collecting materials that would be suitable for spat to attach too, and would be considered as a "natural oyster reef." Cultch is processed and distributed in locations that LDWF consider acceptable for having a satisfactory outcome.