Native Oysters

Evaluation: Some Action Needed/Closely Monitor

Issue Summary:

The Olympia oyster, the only oyster native to the West Coast, is present in much smaller numbers today than it was in the early 20th century. Oysters are important in regulating the health and diversity of the estuarine ecosystem. In an effort to encourage an Olympia oyster population in the South Slough estuary, researchers are re-introducing adult oysters in the slough and investigating the biology and ecology of naturally occurring Olympia oysters in Coos Bay.

What’s happening?

The Olympia oyster, Ostrea lurida, is the only oyster native to the west coast of the United States, and was once abundant in estuaries from Baja California to Sitka, Alaska. Interestingly, the oyster was not present in Coos Bay at the time Europeans settled in the area, but shells found in dredge spoils and shell middens indicate that they were present in the area historically, and were harvested by Native Americans. One hypothesis is that a tsunami and/or fire caused a huge input of sediment into the bay, smothering the oyster population.

Around the turn of the 20th century, Olympia oysters were heavily harvested along the west coast, mainly for the San Francisco market. This overharvesting, as well as the increased development of estuarine areas, loss of hard substrate, sedimentation, and pollution caused the Olympia oyster population to decline dramatically.

In the 1980’s, Olympia oysters were discovered growing in Coos Bay once again. Genetic tests indicate that these oysters are relatives of oysters from Willapa Bay, WA. It is possible likely that they arrived as juveniles attached to the shells of (non-native) Pacific oysters grown commercially in Willapa Bay and transported to Coos Bay for commercial culture. These juvenile Olympia oysters may have then spawned and their larvae settled elsewhere in the bay, setting up a new population.

In any case, the Olympia oyster population here appears to be stable and even increasing. A 2006 survey shows the oyster to be present mainly in the upper part of the bay, with particularly dense patches along the waterfront of Coos Bay, North Bend, and Eastside (see Fig 1).

Fig 1: 2009 Qualitative survey results. Groth and Rumrill (2009).

Recently, an increasing number of researchers have become interested in restoring Olympia oyster populations. Researchers at the South Slough Reserve are attempting to recreate an oyster population in the South Slough estuary. They are also partnering with the Oregon Institute of Marine Biology (OIMB) to conduct research into the biology and ecology of the oysters in Coos Bay (see below).

Why do we care?
Oysters provide a number of services to ecosystems. Because they feed by filtering phytoplankton and bacteria from seawater, a sizeable oyster population can reduce the amount of algae in the bay and help control water quality. They provide an organically rich fertilizer, which would contribute to the health and diversity of the bay.

Young oysters tend to settle near other oysters, forming large aggregations, or beds. These beds help stabilize the muddy bottom of the estuary and may improve habitat conditions for eelgrass, an important estuarine plant. The hard, complex surfaces provided by groups of oysters provide a unique habitat in which other estuarine animals can hide, settle, or lay eggs. In this way, a substantial oyster population could increase species diversity.

What’s being done?
South Slough spearheaded two main oyster restoration projects supported by NOAA’s Community-based Restoration Program (CRP), and the National Estuarine Research Reserve System (NERRS) Science Collaborative program.

The CRP supported several research projects investigating the biology and ecology of native oysters, many of which were led or assisted by community members and college student interns. One project involved collecting oyster juveniles, or spat, on shell bags in Coos Bay and then transferring these bags to South Slough (see Figure 2). Researchers then monitored the growth and survival of these juveniles for about a year. The juveniles survived well and grew, on average, about 10 mm between January and July. Although the CRP projects were completed in 2009, South Slough Reserve science staff members continue to monitor these shell bags, and are currently in the process of moving them from their current location at Younker Point to a more suitable area near Long Island Point. Monitoring living adults in South Slough will provide data on the feasibility of restoring oysters to this area; the adults may also serve as local sources of natural occurring Olympia oyster larvae for use in future restoration efforts, if needed.

Figure 2: Helpers unload bags of oyster shell.

The South Slough Reserve and the Oregon Institute of Marine Biology (OIMB) are partners in several Olympia oyster research projects supported by the NERRS Science Collaborative program. Graduate students at OIMB are currently investigating sexual development and timing of oyster larval brooding and release; mechanisms of oyster larval retention in the bay; oyster larval abundance vs. settlement throughout Coos Bay; and oyster growth and survival throughout the bay. The results of this original research should serve to inform the native oyster restoration efforts in South Slough and elsewhere.

Oysters are bivalves, a type of mollusk characterized by two opposing shells, or valves. They are related to clams, mussels, and other commonly known and often edible mollusks. They feed by filtering small particles from seawater. Many oysters, like other bivalves, release sperm and eggs separately in the water, where they meet and fertilize to form embryos outside the body of the mother. But Olympia oysters retain eggs within the mother’s shell. They “brood” their embryos for several weeks before releasing the young, now called larvae, into the water column (see Figure 3).

Figure 3: The Olympia oyster life cycle

All oysters and most bivalves produce larvae, which are generally less than a millimeter in length. The larvae swim, eat, and develop in the water for several weeks to several months. They then search for a hard surface on which to settle and metamorphose into a juvenile
oyster. Once settled, they are cemented to the substrate and remain attached to the substrate for the rest of their lives.

Baker, P., Richmond, N., and Terwilliger, N. 2000. Reestablishment of a native oyster, Ostrea conchaphila, following a natural local extinction. Marine Bioinvasions: Proceedings of the First National Conference MA MIT Sea Grant Program. 221-231.

Groth, S. and Rumrill, S. 2009. History of Olympia oysters (Ostrea lurida Carpenter 1864) in Oregon estuaries, and a description of recovering populations in Coos Bay. Journal of Shellfish Research 28(1): 51-58.

McGraw, K. 2009. The Olympia oyster, Ostrea lurida Carpenter 1864 along the west coast of North America. Journal of Shellfish Research 28(1): 5-10.

Polson, M.P., Hewson, W.E., Eernisse, D.J., Baker, P.K., and Zacherl, D.C. 2009. You say Conchaphila, I say Lurida: Molecular evidence for restricting the Olympia oyster (Ostrea lurida Carpenter 1864) to temperate western North America. Journal of Shellfish Research 28(1): 11-22.

Polson, M.P and Zacherl, D.C. 2009. Geographic distribution and intertidal population status for the Olympia oyster, Ostrea lurida Carpenter 1864, from Alaska to Baja. Journal of Shellfish Research 28(1): 69-78.

Rumrill, S. CRP Progress Report.

Strathmann, M. 1987. Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast: Data and Methods for the Study of Eggs, Embryos, and Larvae. University of Washington Press. Seattle and London.

Trimble, A.C., Ruesink, J.L., and Dumbauld, B.R. 2009. Factors preventing the recovery of a historically overexploited shellfish species, Ostrea lurida Carpenter 1864. Journal of Shellfish Research 28(1): 97-106.