Unprecedented Restoration of a Native Oyster Metapopulation in Chesapeake Bay
DM Schulte*, RP Burke and RN Lipcius
Introduction
Unrestored Bottom Image
In marine and estuarine habitats worldwide, native oyster species were once vital
ecosystem engineers whose populations have been sequentially depleted through
over fishing and oyster reef habitat destruction. Such ecosystems remain severely
degraded absent the oyster's provision of water filtration and habitat, leading to
numerous expensive and labor-intensive efforts to restore native oyster populations,
which have until now largely failed. Consequently, non-native oyster species such
as the Pacific oyster (Crassostrea gigas) have been purposely introduced in many
ecosystems, under the assumption that native oyster restoration is a failed
endeavor. In Chesapeake Bay, the Eastern oyster (Crassostrea virginica) has been
similarly depleted to approximately 1 % of its historical population, leading some to
conclude that native oyster restoration is unlikely and that a non-native oyster
species should be introduced. We report an unparalleled restoration of a native
oyster population in the Great Wicomico River, a sub-estuary of lower Chesapeake
Bay, which was selected for experimental restoration in 2004 by the US Army Corps
of Engineers. The field manipulation involved three experimental restoration
treatments (high-relief reefs, low-relief reefs, unrestored bottom) and is believed to
be the most spatially extensive oyster reef restoration to date. The original project
covered 39.8 hectares, of which 34.2 remain. The lost acreage was from the lowrelief reefs. The network of oyster reefs, comprised of nearly 200 million live
oysters of three year classes, represents the largest and densest native oyster
metapopulation in restored sanctuary reefs of Chesapeake Bay and worldwide.
Moreover, the oyster reefs have been accreting at a rate that promotes persistence
and long-term sustainability, and whose oysters have increased the local population
approximately 55-fold, greatly exceeding the Chesapeake Bay Program goal of a
10-fold increase by 2010. Many of the reefs are exhibiting both vertical and
cohesive growth, not following the pattern of decline and loss typically observed on
native oyster restoration projects. Preliminary data indicate an exceedingly strong
recruitment of a fourth year class in summer 2008, demonstrating continuing
persistence of the metapopulation. Thus, restoration of native oyster populations is
viable in Chesapeake Bay, if appropriate scale, better site selection and
construction methods are used.
Low Relief Reef Images
Discussion
Great Wicomico River
The results observed on the USACE restored reef habitat in the Great
Wicomico River (GWR) exceed the 2000 Chesapeake Bay Agreement
goal of “By 2010, achieve, at a minimum, a tenfold increase in native
oysters, based upon a 1994 baseline” by a significant margin, a factor
of 5.5. This restoration project is the first in the Bay that has even met
the goal, much less exceeded it, on a significant (tributary or subestuary) scale, and is roughly approximate to the entire oyster
population in all of Maryland’s waters at present (MDNR, 2008). As an
MPA, the project is very successful according to most accepted criteria,
and at this time (4 years post-construction) fulfilling the primary goals of
establishing a viable, self-replenishing reef system, supplying recruits to
nearby areas, acting as a fishing refugia, and significantly augmenting
oyster biomass in the river system. As this restored reef system is a
permanent sanctuary, these benefits should continue to accrue.
Additionally, nearby private leased grounds are already experiencing
enhanced recruitment, likely due to the densely aggregated population
of reproductive adults on the restored reefs. Economic benefits to
these private leaseholds via this enhanced recruitment should be
significant.
The key to the success of this restoration effort was placing significantly
more shell substrate per unit area of bottom than is typically used in
oyster repletion efforts. The high relief reefs closely mimic a natural,
undisturbed oyster bar and their higher relief allowed for enhanced
recruitment, lower sedimentation rates, and higher survival of recruits.
Oyster densities recorded on the High Relief Reefs exceed any
densities ever observed in the Chesapeake Bay in the modern (postMSX) era. Other findings of note were the positive relationship between
adult density and recruitment, with recruitment being significantly higher
on the High Relief Reefs compared to the Low Relief Reefs, and that
the oysters are growing significantly faster on the High Relief Reefs
(47.3 mm ± 1.2 SE) compared to the Low Relief Reefs (41.0 mm ± 1.1
SE). Recent findings (Carnegie, unpublished data) indicate that this
stock of native oysters has significant disease tolerance, perhaps equal
to the best selected strains currently available. Disease mortality will
occur, though it is expected that significant numbers of oysters will
survive and continue to supply recruits to these reefs as well as other
suitable oyster attachment substrate throughout this river. If the
restored reefs are left undisturbed, it appears highly likely they will
persist and many will grow over time.
Materials and Methods
The study was carried out in the Great Wicomico River during November 2007, in order to be
able to fully assess the recruitment and survival of the 2005, 2006, and 2007 oyster year
classes. Oyster densities on High and Low Relief Reefs, and on nearby un-restored bottom,
were assessed via patent tong and underwater video. An experienced commercial waterman
was the owner and operator of the boat and patent tong which was used to obtain bottom
samples. A patent tong provides a quantitative sampling technique as it samples a discrete
area of bottom, unlike an oyster dredge. When initially constructed, the intent was to cover the
public oyster ground (Baylor Ground) polygons with a monolayer of shells of a particular
thickness. It was clear, based on earlier diver and dredge observations in 2006 that this was
not the case and that High Relief and Low Relief Reefs existed as patches of varying size
within the much larger Baylor Ground polygons. Potential High Relief Reef and Low Relief
Reef areas were determined by consulting maps developed using side-scan sonar data taken
immediately post construction of the reefs in fall 2004. Shell placement during construction is
highly heterogeneous and it is essential for an accurate survey to determine where shells were
in fact placed. The reefs vary in depth placed from 8 to 22 feet MLW, so visual confirmation of
shell placement is not possible. Video was needed due to the margin of error in the side-scan
survey (±10cm). As a result, the side scan survey likely missed some shelled areas and
overestimated the amount of shells placed in others. A variety of techniques are needed in
order to get an accurate assessment of oyster populations and document reef development.
Video indicated the un-restored bottom consisted of mostly silt/clay areas with minimal to no
shell present on the surface, Low Relief Reefs had highly varying amounts of shells, and High
Relief Reefs consistently had a substantial layer of thick shells covering 100% of any bottom
areas they were found on. After the patent tong and video survey were completed, it confirmed
that the post construction side-scan survey was approximately 90% accurate.
Acknowledgments:
Commonwealth Pro Dive, Deltaville, Virginia, for underwater video
Mike Bristow, commercial oysterman, for use of his boat, navigational skills, and
patent tonging
VIMS Marine Conservation Biology Lab for all their helpful assistance
Colonel David Hansen, for supporting the US Army Corps of Engineers
change in restoration strategy and techniques
High Relief Reef Images