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2-1-2011
Oyster Reefs at Risk and Recommendations for Conservation, Oyster Reefs at Risk and Recommendations for Conservation,
Restoration, and Management Restoration, and Management
Michael W. Beck
Robert D. Brumbaugh
Laura Airoldi
Alvar Carranza
Loren D. Coen
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Recommended Citation Recommended Citation
Beck, Michael W.; Brumbaugh, Robert D.; Airoldi, Laura; Carranza, Alvar; Coen, Loren D.; Crawford,
Christine; Defeo, Omar; Edgar, Graham J.; Handcock, Boze; Kay, Matthew C.; Lenihan, Hunter S.;
Luckenbach, Mark; Toropova, Caitlyn L.; Zhang, Guofan; and Guo, Ximing, Oyster Reefs at Risk and
Recommendations for Conservation, Restoration, and Management (2011).
BioScience
, 61(2), 107-116.
https://doi.org/10.1525/bio.2011.61.2.5
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Authors Authors
Michael W. Beck, Robert D. Brumbaugh, Laura Airoldi, Alvar Carranza, Loren D. Coen, Christine Crawford,
Omar Defeo, Graham J. Edgar, Boze Handcock, Matthew C. Kay, Hunter S. Lenihan, Mark Luckenbach,
Caitlyn L. Toropova, Guofan Zhang, and Ximing Guo
This article is available at W&M ScholarWorks: https://scholarworks.wm.edu/vimsarticles/653
Articles
www.biosciencemag.org February 2011 / Vol. 61 No. 2 • BioScience 107
Oyster Reefs at Risk and
Recommendations for Conservation,
Restoration, and Management
Michael W. Beck, RoBeRt D. BRuMBaugh, lauRa aiRolDi, alvaR caRRanza, loRen D. coen, chRistine
cRaWfoRD, oMaR Defeo, gRahaM J. eDgaR, Boze hancock, MattheW c. kay, hunteR s. lenihan, MaRk
W. luckenBach, caitlyn l. toRopova, guofan zhang, anD XiMing guo
Native oyster reefs once dominated many estuaries, ecologically and economically. Centuries of resource extraction exacerbated by coastal degra-
dation have pushed oyster reefs to the brink of functional extinction worldwide. We examined the condition of oyster reefs across 144 bays and 44
ecoregions; our comparisons of past with present abundances indicate that more than 90% of them have been lost in bays (70%) and ecoregions
(63%). In many bays, more than 99% of oyster reefs have been lost and are functionally extinct. Overall, we estimate that 85% of oyster reefs have
been lost globally. Most of the world’s remaining wild capture of native oysters (> 75%) comes from just five ecoregions in North America, yet
the condition of reefs in these ecoregions is poor at best, except in the Gulf of Mexico. We identify many cost-effective solutions for conservation,
restoration, and the management of fisheries and nonnative species that could reverse these oyster losses and restore reef ecosystem services.
Keywords: shellfish, oyster reef, marine conservation, fisheries, habitat restoration
marine ecosystems, mainly of those that are intertidal or
that exist in clear water and can be aerially assessed. Records
of the abundance and catch of oysters and the distribution
of the ecosystems that they create can span centuries and
millennia, though usually not as continuous data sets. The
condition of oyster ecosystems has been considered in part
by others (e.g., Jackson et al. 2001, Kirby 2004, NRC 2004,
Ruesink et al. 2005, Lotze et al. 2006, Airoldi and Beck 2007),
but these estimates of condition have used data from only
a limited number of bays. To expand on these efforts we
synthesize quantitative data on the condition of oyster reefs
in more than 140 bays, provide an overall estimate of oyster
reef condition, and use this extensive information to identify
areas and opportunities to improve the condition of oyster
reefs at a global scale.
Assessing condition
We identified native oyster reef condition primarily as a
function of oyster abundance; we calculated condition using
estimates of past and present abundances from the litera-
ture. Measures of total reef area and size were occasionally
available. Fishery statistics for native oysters were the most
commonly available information for assessing the changes
in oyster abundance and the condition of reefs, but land-
ings data were rarely the only information used to assess
condition. Ultimately, reef size is a function of the num-
ber of living oysters, and larger reefs positively influence
oyster growth and survival (Lenihan and Peterson 1998,
O
yster reefs and beds (hereafter reefs) were once a
dominant structural and ecological component of
estuaries around the globe, fueling coastal economies for
centuries. Oysters are ecosystem engineers; one or a few
species produce reef habitat for entire ecosystems (Lenihan
and Peterson 1998). They have supported civilizations for
millennia, from Romans to California railroad workers
(Mac-Kenzie et al. 1997a, 1997b). In 1864, 700 million
European flat oysters (Ostrea edulis) were consumed in
London, and nearly 120,000 workers were employed as oyster
dredgers in Britain. Shell piles from historical harvests in
the southwest of France contain more than 1 trillion shells
apiece, underscoring both the productivity of the species and
the scale of harvest (MacKenzie et al. 1997b). In the 1870s,
intertidal reefs of the eastern oyster Crassostrea virginica
extended for miles along the main axis of the James River
in the Chesapeake Bay; by the 1940s, these reefs had largely
disappeared (Woods et al. 2005). In many coastal areas,
including the Texas coast, roads were paved with oyster shells
(Doran 1965).
Oyster reefs are one of the few marine ecosystems for
which direct estimates of condition can be calculated,
because most underlying reef structures are created by just
one or a few oyster species. Most estimates of the condition
of marine ecosystems are indirect and are derived from the
distribution of threats such as trawling, sedimentation, and
pollution (e.g., Halpern et al. 2008). There have been only
a few direct assessments of the condition of coastal and
BioScience 61: 107–116. ISSN 0006-3568, electronic ISSN 1525-3244. © 2011 by American Institute of Biological Sciences. All rights reserved. Request
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reprintinfo.asp. doi:10.1525/bio.2011.61.2.5
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Lenihan 1999). Water quality also affects oysters but is less
reliable as an indicator of oyster condition, as oysters can
thrive well past the point at which human health concerns
become an issue.
We considered the condition of native oyster reefs at two
different spatial scales: bays and ecoregions. For our pur-
poses, “bays” refers to bays, estuaries, embayments, coastal
counties, and portions of coastlines (e.g., Mobile Bay, Wad-
den Sea, Venice Lagoon). “Bays” was the most consistently
used term to describe this common ecological unit in
reports of oyster science, management, and conservation.
Ecoregions are regional, biogeographic units with coherence
in their species and ecosystems; we used the boundaries
identified by Spalding and colleagues (2007).
We established four categories of condition on the basis
of comparisons of current with historical oyster abundance
indicators (e.g., surveys, landings, catch per unit effort)
or aerially measured reef extents: (a) less than 50% lost
(good), (b) 50% to 89 % lost (fair), (c) 90% to 99% lost
(poor), and (d) more than 99% lost (functionally extinct).
We looked at records from between 20 and 130 years before
present to estimate historical abundances and extents. We
based the date range on the availability and reliability of
the data. Interestingly, surveys from a century ago were
frequently better than records from decades ago or even
the present.
We used practical and conservative rules for assigning
condition. When sources indicated that it was difficult to
find reefs, or that no reefs remained in bays where annual
catch records were high (usually > 10,000 metric tons) but
historical observations indicated that reefs had once been
extensive, we estimated that more than 99% of the habitat
was lost, and classified the condition as functionally extinct
(Jackson 2001). Such was the case, for example, in the
Wadden Sea (European Union), Narragansett Bay (United
States), Southport (Australia), and Ciénaga Grande de
Santa Marta (Colombia). The condition was classified as
poor (90% to 99% habitat lost) when evidence indicated
that fisheries were collapsing (or collapsed) but there was
evidence that reefs remained, even if long-term viability
was questionable (e.g., Chesapeake Bay [United States],
Bohai Bay [China]). There is abundant evidence that
shellfisheries continue well past the point at which 90%
of the habitat has been lost (MacKenzie et al. 1997b, Kirby
2004). We classified the reef condition as fair (50% to 89
% habitat lost) when abundance indicators were below
50% of historical figures or records indicated greater than
50% loss in reefs and there was evidence of significant
remaining reefs (e.g., Apalachicola Bay [United States],
Golfo San Matías [Argentina]). We considered the con-
dition good (< 50% lost) if fisheries were only lightly to
moderately exploited (or not at all exploited) and if many
areas of reefs remained relative to earlier abundances (e.g.,
Mobile Bay [United States], Nootka Sound [Canada]).
If there was any question, we discussed the data-driven
estimates with fishery and habitat managers and scientists
from each location. The only common debate in publica-
tions was not about the condition of the ecosystems but the
cause of the decline (e.g., Kirby 2004, Ogburn et al. 2007).
We identified the condition of oyster reefs across coastal
ecoregions using information from multiple bays within
ecoregions and national and regional publications on the
status of oyster populations (e.g., red lists) and fisheries (e.g.,
MacKenzie et al. 1997b, Gillespie 2009). We identified the
condition of oyster reefs in an ecoregion if there were one or
more references that characterized regional condition or if
the condition was firmly documented in three or more bays
within the ecoregion. When there were several bays in an
ecoregion and no other regional sources of status informa-
tion, the condition estimates were averaged for all bays in the
ecoregion and rounded to the nearest integer.
Oyster reef condition
The overall condition of native oyster reefs is poor in most
of the 144 bays in 40 ecoregions we evaluated (figure 1; see
supplementary material table S1 at http://conserveonline.org/
workspaces/Shellfish%20Reefs%20At%20Risk/documents/
oyster-reefs-at-risk-supplementary-table). Although indi-
vidual oysters are still present in most places, records of
historical (past 20 to approximately 130 years) and recent
abundances show that many reefs that were once common
are now rare or extinct as ecosystems. Oyster reefs are at
less than 10% of their prior abundance in most bays (70%)
and ecoregions (63%). They are functionally extinct—in
that they lack any significant ecosystem role and remain
at less than 1% of prior abundances in many bays (37%)
and ecoregions (28%)—particularly in North America,
Australia, and Europe. Very few bays and ecoregions are
rated as being in good condition (> 50% of reefs remain-
ing). Our results most likely underestimate losses because of
the lack of historical abundance records, which particularly
affects assessments in South America, temperate Asia, and
South Africa.
Our estimates of reef conditions are conservative because
(a) where there was question of status, we applied the higher
ranking; (b) for most bays and ecoregions it was clear that
abundances were usually at the lower end of their condition
ranking; and (c) the estimates were usually based on only
part of the historical loss, as reefs were probably more abun-
dant before the recording of fishery catches began.
Overall, we estimate an 85% loss of oyster reef ecosys-
tems globally (figure 1). We calculated this by using the
midpoint value for each condition category of oyster reefs
lost in ecoregions (e.g., 95% of habitat lost for ecoregions
in poor condition), and then averaged the loss among all
ecoregions.
Prior records from many bays indicated that oyster
reefs were abundant and supported large fisheries—up to
hundreds of thousands of metric tons of recorded catch—
but those reefs and fisheries are now greatly reduced or
gone (MacKenzie et al. 1997a, 1997b, Kirby 2004, NRC
2004, Ruesink et al. 2005, Lotze et al. 2006). We found
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frequent evidence that restrictions had been placed on
harvests and of concerns about the incidence of disease
and environmental degradation, but in the great majority
of cases, harvests continued until oysters could no longer
be fished commercially (MacKenzie et al. 1997a, 1997b,
NRC 2004).
The decline of oyster fisheries follows a common
sequence of events in many places globally (MacKenzie
et al. 1997a, 1997b, NRC 2004). Typically, the extensive
harvest of wild oyster populations results in the loss of
reef structure. Most declines start with the loss of verti-
cal relief and complexity, often as a result of dredging
and trawling, which exacerbates the impact of additional
stresses from anoxia, sedimentation, disease, and nonna-
tive species (Lenihan and Peterson 1998, 2004, Lenihan
1999, Lenihan et al. 1999). In many cases, years of declin-
ing harvest are followed by introductions of nonnative
oysters that are released directly into the wild or that
escape from nearby aquaculture (Ruesink et al. 2005).
Overharvest and disease often lead to a population crash.
Although oyster diseases occur in native populations, in
many places the incidence of disease is associated with
transfers of nonnative oysters for aquaculture and from
ballast waters (NRC 2004). Other anthropogenic factors
such as alterations of shorelines; changes in freshwater
inflows; and increased loadings of sediments, nutrients,
and toxins also contribute to declines (NRC 2004). There
are few if any bays where only one stressor has affected
oyster reefs.
Wild fisheries and remaining reefs
To compare present-day levels of wild oyster harvest among
ecoregions, we used global commercial catch data developed
by the Sea Around Us Project (Watson et al. 2004). These
catch data are primarily based on the national catch statis-
tics compiled by the Food and Agriculture Organization,
allocated to half-degree cells of latitude and longitude, and
then summed by ecoregion. To account for annual variation
in catches, we used the average catch in metric tons of native
oysters per ecoregion from 1995 to 2004. The biomass esti-
mates are based on numbers of oysters in their shells.
Most of the world’s remaining wild capture of native oysters
comes from just five ecoregions on the East and Gulf coasts of
North America, which together account for more than 75%
of the global catch (figure 2). Only 10 ecoregions in the world
reported wild oyster capture rates of more than 1000 metric
tons per year from 1995 to 2004; only six ecoregions have
average captures above 5500 metric tons, and five of these
are in eastern North America (Virginian to Southern Gulf
of Mexico ecoregions). Although there is catch remaining in
these six ecoregions, it is much lower than in the past. Indeed,
in most of the bays (20 of 34) in these six ecoregions, there
has been at least a 90% loss in oyster reefs; in some cases the
loss has been more than 99%. Therefore, the condition of the
oyster reefs is poor or functionally extinct in these bays and
regions, but oysters continue to be harvested. Contemporary
native oyster catches in the Gulf of Mexico are the highest in
the world, despite significant declines in abundance and reefs
in numerous bays (e.g., figure 1, table S1).
Figure 1. The global condition of oyster reefs in bays and ecoregions. The condition ratings of good, fair, poor, and
functionally extinct are based on the percentage of current to historical abundance of oyster reefs remaining: less than 50%
lost (good), 50% to 89% lost (fair), 90% to 99% lost (poor), more than 99% lost (functionally extinct). Ecoregion boundaries
are from Spalding and colleagues (2007). Not all regions with oysters could be assessed because of a lack of data (see text).
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