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Non-native Oysters

by the Fish Site Editor
05 May 2004, at 1:00am

By Maryland Sea Grant, University of Maryland - The eastern oyster Crassostrea virginica is native to the Chesapeake Bay and coastal waters from Canada to the Gulf of Mexico. Though a prolific species, C. virginica populations in the Bay have been subjected to overfishing, habitat loss and, for some years now, parasitic disease, especially Dermo and MSX, which has devastated oyster populations.

Non-native Oysters - By Maryland Sea Grant, University of Maryland - The eastern oyster Crassostrea virginica is native to the Chesapeake Bay and coastal waters from Canada to the Gulf of Mexico. Though a prolific species, C. virginica populations in the Bay have been subjected to overfishing, habitat loss and, for some years now, parasitic disease, especially Dermo and MSX, which has devastated oyster populations.

The resulting economic losses have severely impacted harvesters, processors, equipment suppliers and other support businesses. The ecological impacts have also been immense, though only over the last decade have we come to appreciate the importance of oysters and oyster reefs in helping to protect water quality (Newell 1997) and provide habitat for diverse species.

Though not harmful to human health, MSX and Dermo are so virulent and pervasive in the Chesapeake that large-scale efforts to rebuild sustainable populations have not had much success. This is especially so in Virginia's portion of the Bay where high salinities are conducive to disease.

While rehabilitation efforts to restore the native oysters are ongoing - there is evidence of success in hatchery-bred strains such as CROSBreed that are resistant to disease - some stakeholder interests have been arguing for the introduction of a non-native species that is inherently resistant to MSX and Dermo.

Researchers initially focused on Crassostrea gigas, a species native to Japan and also the primary commercial species grown on the west coast of the U.S. as well as in major oyster fisheries throughout the world. More recently, scientists at the Virginia Institute of Marine Science (VIMS) have been studying the potential of Crassostrea ariakensis, another Asian species. All research and field trials in Virginia have been limited to employing infertile (triploid or tetraploid) oysters in Bay waters.

CRASSOSTREA GIGAS - (Pacific or Japanese Oyster)

The susceptibility of the eastern oyster Crassostrea virginica in Chesapeake Bay to MSX and Dermo disease led some researchers in the late 1980s to advocate exploring the potential of introducing C. gigas, a species that appeared capable of tolerating MSX and Dermo.

C. gigas had been imported to the U.S. west coast early in the century and has been the basis of the industry there, which is largely dependent on hatchery production. It is also the most widely cultured oyster in the world, having been introduced in such countries as France, England, New Zealand and Australia (Synopsis of the Oyster Ecology Workshop 1991).

The prospects of such an introduction led to a great deal of controversy over the potential ecological risks in the Chesapeake; many felt that there was not enough information on the ecology of C. gigas to even address the risks. For this reason, a workshop was convened that brought together international experts who met with biologists, resource managers and others to determine what was known and not known about ecology of C. gigas (Leffler and Greer 1991).

But C. gigas turned out to be a poor candidate for Chesapeake Bay, largely because studies showed that it was undistinguished with regard to growth, tolerance to disease and taste acceptability, compared with the native oyster (Calvo et al. 1999).

CRASSOSTREA ARIAKENSIS - (Suminoe Oyster)

With the entrenchment of MSX and Dermo disease in Chesapeake Bay, particularly in Virginia's high salinity waters, researchers at the Virginia Institute of Marine Science (VIMS) began exploring the potential of another non-indigenous species, the Asian oyster Crassostrea ariakensis, that gave indications of disease resistance.

VIMS researchers undertook controlled studies by spawning larvae in the hatchery and rendering them infertile so that they could not reproduce on reaching adult size. Field trials were initially conducted at low, medium and high salinity sites between June 1998 and September 1999; researchers compared survival, growth and disease susceptibility between triploid C. ariakensis and diploid (fertile) C. virginica, the native oyster. At low salnity sites, for example, mortality of C. ariakensis was 14% compared with 81% for C. virginica. At medium and high salinity sites, mortaltiy of C. ariakensis was under 15% compared with the death of all C. virginica. For details see Calvo et al. 1999, Calvo et al. 2000, Calvo et al. 2001.

While the success of C. ariakensis in resisting disease offers prospects for rebuilding Bay oyster fisheries through aquaculture, there are significant concerns and objections over the introduction of a non-native species. To inform Bay stakeholders about current knowledge on C. ariakensis studies and to discuss critical issues of importing a non-native species, VIMS organized a symposium on the Aquaculture of Triploid Crassostrea ariakensis in Chesapeake Bay, October 18-19, 2002. Hallerman et al. 2002 is a summary of that symposium.

Since the symposium, a number of institutions and organizations have issued position statements on the introduction of C. ariakensis into Chesapeake Bay. These include the Virginia Institute of Marine Science (VIMS), the University of Maryland Center for Environmental Science (UMCES), the Chesapeake Bay Program (CBP) and Chesapeake Bay Foundation (CBF).

Selected References

Hallerman, E., M. Leffler, S. Mills and S. Allen. 2002. Aquaculture of Triploid C. ariakensis in Chesapeake Bay: A Symposium Report. Maryland Publication UM-SG-TS-2002-01, Virginia Publication VSG-02-03. [pdf]

Calvo, G.W., M.W. Luckenbach, S.K. Allen, Jr., and E.M. Burreson. 1999. Comparative field study of Crassostrea gigas and Crassostrea virginica in relation to salinity in Virginia. J. Shellfish. Res. 18: 465-473.

Calvo, G.W., M.W. Luckenbach, S.K. Allen, Jr. and E.M. Burreson. 2000. A Comparative Field Study of C. ariakensis and Crassostrea virginica in Relation to Salinity in Virginia. Special Report in Applied Marine Science and Ocean Engineering No. 360. Virginia Institute of Marine Science.

Calvo, G.W., M.W. Luckenbach, S.K. Allen, Jr. and E.M. Burreson. 2001. A Comparative Field Study of C. ariakensis and Crassostrea virginica in Relation to Salinity in Virginia. J. Shellfish. Res. 20: 221-229.

Dew, J., J. Berkson, E. Hallerman. 2001. A Risk Assessment Simulation Model for Culture of Triploid C. ariakensis in the Chesapeake Bay. Version CD-ROM. Virginia Sea Grant VSG-01-12.

Leffler, M. and J.R. Greer, eds. 1991. The Ecology of Crassostrea gigas in Australia, New Zealand, France and Washington. Maryland Sea Grant, College Park, Maryland. UM-SG-TS-92-07.

Mann, R., E.M. Burreson and P.K. Baker. 1991. The decline of the Virginia oyster fishery in Chesapeake Bay: considerations for introduction of a non-endemic species, C. gigas (Thunberg, 1793), Journal of Shellfish Research 10(2):379-388.

Source: Maryland Sea Grant - University of Maryland - May 2004.

the Fish Site Editor