Agricultural & Resource Economics

Chesapeake Bay oyster depletion

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Oyster ships warring in Chesapeake Bay. 1886 engraving by F. Cresson Schell

Chesapeake Bay oyster depletion has occurred significantly in the period 1960 to 2010. This depletion is caused by a number of factors, including water quality, disease and over harvesting. The native oyster to the Chesapeake Bay and all Atlantic Coast regions is the American or Eastern Oyster Crassostrea virginica. The Bay's ecological conditions are ideal for  oysters, and the oyster fishery was at one time the Bay's most commercially viable enterprises. However, in the last fifty years the oyster population has been devastated. Maryland once had roughly 200,000 acres of oyster reefs; in 2010 it has about 36,000 such reefs. In pre-colonial times, oysters filtered the entirety of the Chesapeake Bay in approximately 3.3 days; by 1988 this time had increased to 325 days. The oyster harvest's gross value has decreased 88% from 1982 to 2007. Today there are fewer than 500 oystermen, whereas twenty years ago the Bay supported over six thousand, illustrating the significant effects of oyster depletion. 

Chesapeake Bay characteristics

caption The Chesapeake Bay; Source: Axis of Logic

Considered to be the largest estuary in the USA and among the largest in the world with approximately 16 million people living in its basin, the Chesapeake Bay is a valuable resource in multiple areas of everyday life. Its main stem is 332 km long, with tidal waters extending over 11,400 km2 and 12,870 km of shoreline. It is surrounded by the states of Maryland and Virginia, but the Chesapeake drainage basin covers 166,000 km2 in six states. Over 150 rivers and streams drain into the Bay. 

The Chesapeake Bay is well known for its commercial, military, and recreational benefits. It supports fisheries worth approximately $1,000,000,000, and was once known for its great seafood production, especially blue crabs, clams, and oysters. The ship building industry began in colonial times and continues to be a vital industry in the region, providing jobs and is strongly connected to the large military base of Norfolk. Recreationally, the Bay is used for fishing, boating, hunting, and camping, and is a major source of income for most coastal cities. Additionally, the tourism industry in the Chesapeake generates abundant revenue and jobs for the people who live there. The Bay is also used for domestic and industrial waste disposal. 

Starting out as a body of water abundant in animals, plants and nutrients, the Chesapeake Bay has since dwindled in quality, particularly since its colonization by Europeans about 400 years ago. During the time of agrarian expansion extending into the 19th century, the Bay’s food web and natural production were altered due to runoff plant nutrients being washed into the bay, particularly nitrogen and phosphorus. In the later 19th century, industrialization exacerbated the pollution problem.

There has been a dramatic increase in nutrient inputs in the form of wastes from the growing population. These wastes include the runoff of agricultural fertilizers and animal wastes, atmospheric deposition of nitrogen oxides resulting from fossil fuel combustion, and the loss of wetlands and riparian forests diminishing important sinks for nutrients and sediments within both the watershed and the estuary.  Furthermore, the severe depletion of the Bay’s oyster population has increased the quantities of impurities in the water, because of the population’s function as a natural filter. Due to all of these factors, the Bay has gradually been shifting from a clear water system with plentiful plant growth to a turbid ecosystem dominated by abundant microscopic plants in the water column and stressful low-oxygen conditions during the summer. 

 Importance of oysters

caption The native oyster, Crassostrea virginica; Source: DEQ

Depletion of Chesapeake Bay oysters has degraded the function of the Chesapeake Bay ecosystem. Oysters serve as natural water filters, and their decline has reduced the bay's water quality.  Native oysters and their filtering capacities are a significant part of the ecosystem in the bay, influencing the rate of nutrient cycling and the stability of the Bay ecosystem. Oysters filter large amounts of phytoplankton and detritus (small organic particles) from the water column, allowing for the cycling of carbon and other essential materials, which is vital for the continuity and stability of a any living system.  The oysters' filtering abilities also decrease the development of algal blooms, which in turn increases the availability of oxygen in the water.  Furthermore, it elevates the Bay's aesthetics; for example, water that was once clear for meters is now so turbid that a wader may lose sight of his feet before his knees are wet.  The capability of oysters to filter the water depends principally upon the biomass of the oysters, the concentration of phytoplankton, and the retention time of water in the Bay, among other factors.  Thus, with the depletion of the oyster population, the water quality of the Bay has also decreased.  

Oysters also provide income, employment, and tax revenue. The peak of production occurred in the late 1880's when the Chesapeake Bay was the largest oyster-producing region in the world, producing more than twice the oyster catch of all foreign countries combined. The full economic value of oysters does not only include the primary sales of the raw, unshucked product, but also from secondary products and services.  These include shucking and packing houses, transport, manufacture of prepared oyster products, and retail sales.  For example, oysters worth $1 million in dockside value in Chesapeake Bay generate an estimated $36.4 million in total sales, $21.8 million in income, and 932 person-years of employment in 2004.  

Oyster reef habitats provide a vital role in the ecosystem.  Oyster reefs are created by the constant adhesion of new larvae to existing shells.  As the new oysters grow, available settlement space allows for the attachment of other organisms and more oysters.  Since the structure is not solid, but contains interstitial spaces, it is critical to habitation by oysters and other organisms. All oyster reefs have a diversity of species on, within, and around them.  These species associated with oysters and their shells are usually fouling or encrusting organisms that attach directly to the shells, including sponges, hydroids, bryozoans, barnacles, mussels, limpets, and some types of clams.  Some small crustaceans, worms, gastropods, and fish live among the sessile oysters and depend on them for food from the algae growing on or between them.  In addition, the shell provides refuge from predation for multiple species, including small clams, grass shrimp, crabs, and worms.  Some species of fish, like the gobies, skillet fish, blennies, oyster toadfish, and pipe fish, use oyster reefs as nesting sites.  By attaching their eggs in clean, articulated oyster shells, they provide a measure of protection for the eggs until they hatch.   

Methods of harvest 

When the oyster industry started, harvesters used crude tools such as rakes and tongs to harvest the natural and commercial commodity. In 1865, the use of dredges was legalized and, by that same year, hydraulic patent tongs were invented. These harvesting techniques greatly increased the amount of oysters obtained over a period of time. Many developments have been discovered since the early days of oyster harvesting due to technological advancement. These new developments both helped and harmed the oyster industry, making harvesting much easier while simultaneously rapidly destroying the oyster habitat.

In today's world, oyster fisheries along the Atlantic coast are traditionally based on harvest from public and leased bottom.  Some states prefer a public fishery, while others prefer to allow private investments on leaseholds.  In the use of public fishery by planting shell and using aquaculture to grow oysters, they distribute the young oyster larvae to known oyster beds.  Similarly, private growers perform almost identical activities to cultivate their leased grounds. 

In some locations, techniques such as off-bottom aquaculture (e.g., oysters grown in bags hanging from floating surface structures), have been developed; however, the use of oyster beds remains the predominant production and harvesting method. 

Causes of oyster decline

Water quality

In the 1970s, Chesapeake Bay was discovered to contain one of Earth's first identified marine dead zones, where hypoxic waters were unable to support life, resulting in massive fish kills. Today the Bay's dead zones are estimated to kill 75,000 tons of bottom-dwelling clams and worms each year, weakening the base of the estuary's food chain and robbing the blue crab in particular of a primary food source. In addition, crabs are sometimes observed to amass on shore to escape the hypoxic water, a behavior known as a "crab jubilee". Hypoxia results in part from large algal blooms, which are nourished by the nitrogen and phosphorus resulting from runoff of farm and industrial waste throughout the watershed. The algae prevent sunlight from reaching the bottom of the Bay while alive and deoxygenate the Bay's water when they die and rot. One particularly harmful algae is Pfiesteria piscicida, which can affect both fish and humans.  The erosion and runoff of sediment into the Bay, exacerbated by de-vegetation, drastic and rapid development in coastal areas, and increasing boat traffic, blocks vital sunlight. The resulting loss of aquatic vegetation depleted the habitat for much of the Bay's animal life. Beds of eelgrass, the dominant variety in the southern Bay, have shrunk by more than half there since the early 1970s.  Another cause for pollution is the drastic increase in human population surrounding the Chesapeake, causing a sharp increase in pollution flowing into the Bay. This pollution of toxic chemicals and heavy metals dumped into the Bay immensely depleted the oyster population through its hindrance of larvae development. The decrease of the water quality and increase in pollution diminished the quality of the Chesapeake Bay, making it an unsustainable environment, not being able to assimilate wastes faster than they are accumulated.  This greatly influenced the depletion of the oyster population, and since the oysters play such a vital role in the water filtering of the Bay, this only continues in a positive feedback loop, causing the water quality to further decrease. 


The Bay's oyster industry has suffered from two diseases: MSX and Dermo. MSX, which first struck the Bay in 1959 from Japanese oysters, is caused by the protozoan parasite Haplosporidium nelsoni. It is caused by drier climatic years. It dramatically reduced oyster populations in high salinity environments, and remains the biggest factor in the decline of the oyster population. Dermo, which was first discovered in 1949, is caused by the protozoan parasite Perkinsus marinus. It did not become a problem until it struck high and low salinity environments in the mid-1980's, and is shown to be more destructive because of its increased range and tolerance of salinities when compared to the MSX disease. Within two to three years, MSX and Dermo can kill more than ninety percent of exposed oysters.


The chief driver of oyster depletion is over harvesting. Ineffective government regulations allow anyone with a license to remove oysters from state-owned beds, and although limits are established, they are not enforced. The over-harvesting of oysters makes it difficult for them to reproduce, which requires high densities of adult oysters. In addition, over-harvesting deteriorates the reef habitat of the oysters, making life difficult for the future generations.

The harvesting pressure placed on native oysters in the Chesapeake has increased exponentially over the years due to the development of more efficient equipment.  With these new, highly efficient harvesting technologies, oystermen are able to gather a greater quantity of oysters in less time, reducing the integrity of the oysters' reef habitats. In 2010, the oyster catches amount to 30-100 bushels/day, whereas they were only 8-25 bushels/day before the development of new harvesting technologies. 

Effects of depletion

Oyster depletion in the Chesapeake Bay has many significant effects both environmentally and economically. Because the decline of the oyster population decreased the filtration of the Chesapeake Bay, nitrogen and phosphorus accumulated and caused a significant nutrient imbalance. This resulted in an increase of algal blooms and an environment unsuited for aquatic organisms.

The commercial oyster industry has been devastated in recent years, as the number of oysters harvested by oystermen has decreased over time. Oystermen seek employment elsewhere in other related industries due to the personal loss of their steady income.  However, the oyster industries are not the only ones affected, as many commercial fishing industries were negatively impacted due to the fish kills caused by the increased frequency of algal blooms.  Because of the environmental impacts of the depletion, many local industries were, and continue to be, rendered unstable. Restaurants serving seafood, oystermen and fishermen, and those in the tourism industry were injured due to their dependence on the steady harvesting of oysters. Over time, the value of harvested oysters has decreased from $59 million in 1992 to less than $15 million by 2003. Oyster ports that at one point were thriving are now considered ghost towns. The general health of the entire Bay determines the future maintenance of these industries. 

Asian oyster controversy 

Stemming from the depletion of the native oyster Crassostrea virginica, an idea was introduced in 2004 and for five years was disputed over whether or not to introduce the non-native oyster Crassostrea ariakensis into the Bay ecosystem. Meant to restore the dying oyster population through their alleged resistance to the diseases which significantly depleted the native oysters, the introduction of said non-native Asian oysters was first proposed by state officials. Before the non-native oysters could be considered to be introduced, the Programmatic Environmental Impact Statement (PEIS) needed to be created in order to analyze the potential impact of both species living in the same environment.  

Research was performed extensively, evaluating all risks and benefits that could possibly result from the Asian Oysters. Scientists feared the unintentional introduction of disease into the Bay, which would further decrease the native oyster population. This fear was partially caused by a previous encounter in the Chesapeake region where MSX was unintentionally brought in by Japanese oysters and became one of the greatest causes of death in the native oyster population. Some officials believed that this risk was minimal because the Asian oysters would be coming from farms in Oregon rather than Asia. In addition, the scientists feared that the introduction of sterile oysters in the Bay for experimental purposes would be capable of reproducing, as it may be possible for sterile individuals to become fertile over time. This fear resulted from a regulation stating that no more than one in one thousand oysters introduced into the Chesapeake for research can be fertile. If this fear were to come true, the Asian oysters could potentially obliterate the native oysters.

Those affected by the native oyster depletion, particularly those in the oyster industries, supported the idea of the Asian oysters in order to stabilize the economy. Their hopes were that the population loss would be refilled to its original state. Those in the fishing industry were weary because of the potential harm of introducing an alien species with an accompanying disease that could harm other harvestable organisms.  However, most of these fishing industries felt that adequate research and experimentation would terminate those risks, and thus supported the Asian oyster introduction.   

The US Army Corps of Engineers, the Commonwealth of Virginia and the State of Maryland all agreed on August 13, 2009, following a five year study, that the Asian oyster posed too much of an environmental risk to be introduced.  Instead, they would focus on a restoration strategy for the native Eastern oyster (Crassostrea virginica).  

Government legislation regarding oysters 

caption Description

Historically, the oyster controversy in the Chesapeake Bay has played a significant role in the government. The first legislation regarding the oyster fishery in Chesapeake Bay was passed in 1820 by Maryland, which prohibited the use of dredging in all of Maryland's waters.  In 1830, Maryland passed the "One-Acre Planting Law" that allowed citizens to lease one acre of land in return for private cultivation; however, a violation of this law only resulted in the punishment of a misdemeanor crime. The first closed season, lasting from April 13 to September 1, was enacted by Worcester County in 1846. Fishermen were permitted to use small dredges with the purchase of a $15 license in 1854. Unfortunately, all oystering laws prior to 1865 were revoked. In addition, a statewide licensing system was established and large dredges were legalized for the first time in Chesapeake history. "Oyster police" were established in 1868 to patrol for poachers in leased land. From 1906 to 1914, Maryland passed two important and impacting bills affecting oysters in the Chesapeake Bay. The Haman Oyster bill allowed the leasing of bottom portions in the Bay, while the Price-Campbell bill (1912) increased the number of leases allowed. This marked the first time in which fisherman were able to lease plots of land for cultivation. Political pressure caused legislators to pass the Shepherd Act in 1914, destroying all the leasing legislation. In 1927, the 10 Percent Shell Tax Law was passed requiring oyster processors to make 10 percent of their shucked shell available for state use in planting, a method to create new oyster beds. In 1951, legislation was passed requiring half of all shucked oyster shells to be sold back to Maryland for replanting in barren areas. This was important in that it was part of the oyster restoration effort, but its effect was minimized by loopholes found by packers and processors looking for better prices on the market. In 1953, the 55 Percent Shell Tax was passed, causing the tax on oysters to increase by 50 percent. This would reduce to 25 percent in 1965, allowing processors the option of cash payment instead of shell. In 1972, the Moratorium on New Leases halted awards of oyster grounds so the over-harvested could recover. 

There continues to be legislation passed regarding oyster restoration, and many government groups working towards this goal. The US Environmental Protection Agency, NOAA, US Army Corps of Engineers, Maryland Department of Natural Resources, Maryland Environmental Service, Maryland Department of The Environment, Maryland Department of Transportation, and University of Maryland are just some of the numerous government organizations working with oyster restoration in the Chesapeake.

Oyster restoration effort

caption Oyster Harvests and Laws in Maryland; Source: Maryland Aquafarmer Online

One can see many varying results from the efforts of federal, state and local governments working together with the Chesapeake Bay Program, The Chesapeake Bay Foundation, and other nonprofit environmental groups to restore the population of the native oysters. They encountered obstacles including the distance lag in pollution effects, since polluting substances from the Bay are found far upstream in tributaries far outside the Bay's realm, in some cases in far away states. Although the state of Maryland has given over $100 million to the cause of oyster restoration, the situation has only gotten worse.  

The Oyster Recovery Partnership is a group which attempts to use hatcheries to repopulate the Bay with oysters. Recently, six million oysters were placed over eight acres of the Trent Hall sanctuary, and furthermore, experimental reefs were created in 2004 and now house 180 million native oysters. Since 1999, 1.5 billion native oysters have been planted and raised in the Bay using backyard docks. Although a few of the projects were not successful, overall the effort has been and continues to be making progress. Much of the federal money given towards the restoration effort will be used to raise millions of disease free oysters and another large portion of that money will help with the use of aquaculture. A new technique used in planting the oysters is to place them in three dimensional underwater mounds as opposed to the traditional flat bottom. Additionally, certain areas of the Bay are kept off limits to oyster harvesting. River sanctuaries are becoming more widely used in order to boost the new populations of native oysters. These river sanctuaries show triple the survival rate than what was expected.

In 2000, the Chesapeake 2000 Agreement (C2K) was signed by the state of Maryland. This agreement called for the achievement of over 100 specific actions designed to restore the Bay's health and living resources. One major component of the C2K is the plan to achieve a minimum tenfold increase in the native oysters by 2010. 

In 2004, the Oyster Management Plan (OMP) was established with the goals of improving current oyster management and rebuilding native oyster populations. Strategies included in this plan are the evaluation of the use of sanctuaries and harvest reserves to obtain optimum ecological and economic benefits, habitat reconstruction, increased hatchery production, disease-resistant oyster breeding, evaluation of impediments of aquaculture, harvest management, improved coordination between oyster partners, the development of a database to track oyster restoration, and the monitoring of results. 

The efficiency of native oyster restoration also depends upon the removal of the causes of oyster depletion, including water quality, disease, over-harvesting, and pollution.

This article was partially researched and written by a student at Boston University participating in the Encyclopedia of Earth's Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.


Further reading



Gwizd, S., & Ham, B. (2013). Chesapeake Bay oyster depletion. Retrieved from


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