Pacific Oyster (Crassostrea gigas). Photo by Don Rothaus. Courtesy of the Washington Department of Fish and Wildlife.
Molluscs in the Class Bivalvia feed on phytoplankton and detrital particles suspended in the water column, serving as a key trophic link between microscopic primary producers and higher consumers. Epibenthic bivalves can function as ecosystem engineers through the provision of hard substrate and three-dimensional biogenic structure, while infaunal bivalves can function as engineers through physical alteration of soft substrate habitats. Numerous native and non-native species of bivalves occur in Puget Sound, including important aquaculture species such as Pacific oysters (Crassostrea gigas), non-native invasive species such as the purple varnish clam (Nutallia obscurata), and species targeted in recreational fisheries (e.g., native littleneck clams and non-native Manila clams). The native geoduck clam, Panopea generosa, is valued as a commercially-fished species and as an aquaculture species. The native Olympia oyster, Ostrea lurida (also known as Ostreola conchaphila) currently is a restoration target in Puget Sound, having been depleted through human activities in the last century.
Geoduck (Panopea generosa). Image courtesy of NOAA.
Geoducks are large Hiatellid clams distributed from Alaska to California. They can grow to shell lengths of 20 cm (Bureau et al. 2002), and are characterized by large fleshy siphons that can reach lengths of 1m. Geoducks are broadcast spawners with larval periods of 16 - 47 days (Goodwin and Pease 1989). After settlement, they exhibit limited mobility for 2-4 weeks, then burrow into the sand and begin feeding. Individuals are thought to reach maximum size within the first 10 years of life (Goodwin and Pease 1989), and can live for up to 168 years. Their longevity could render them particularly susceptible to over-exploitation (Orensanz et al. 2004).
In Puget Sound, geoducks occur primarily in low intertidal and subtidal habitats and are most abundant at depths of up to 20m, although observations of deeper individuals have been reported (Goodwin and Pease 1989). Found primarily in soft sediments consisting of sand and sand-mud, geoducks are contagiously distributed throughout the major basins of Puget Sound (Goodwin and Pease 1990). In a survey of 8,589 SCUBA transects, Goodwin and Pease (1990) found that geoduck abundance ranged from densities of 0 to 22.5 individuals/m2, with an average density of 1.7 individuals/m2. They found the highest densities in southern Puget Sound and in Hood Canal (Goodwin and Pease 1990).
Recreational and commercial fisheries for geoduck exist in Puget Sound. The recreational fishery typically occurs in intertidal habitats, while the commercial fishery occurs in subtidal habitats in areas leased from the State of Washington. Because the fishery is prosecuted in leased tracts, it is jointly managed by the Washington State Department of Natural Resources (WDNR) and the Washington Department of Fish and Game (WDFW). The current target for the commercial fishery in Puget Sound is 2.7% of the exploitable biomass based on a static value of 40% of the Maximum Sustainable Yield (MSY) (Bradbury et al. 2000). Recruitment of geoducks appears to be highly variable and driven by climatic forcing (Orensanz et al. 2004, Valero et al. 2004). Based on the combination of highly variable recruitment and long life span, Orensanz et al. (2004) caution that static exploitation targets may not be appropriate for this species. Geoduck abundance in Puget Sound is augmented through aquaculture, the ecological effects of which are not well understood (Feldmann et al. 2004, Straus et al. 2008).
As ecosystem engineers, oysters play an important role in the populations, communities and food webs where they occur (reviewed in Ruesink et al. 2005). Oyster beds provide structure and biogenic habitat for a suite of other invertebrates and fish (e.g., Lenihan et al. 2001). They also modify the physical and chemical properties of ambient water through feeding and excretion, maintaining high water clarity and conditions beneficial to macrophytes (Jackson et al. 2001, Ruesink et al. 2005).
The native Olympia oyster occurs from Alaska to Baja California, Mexico (Polson and Zacherl 2009). The size of the particles or phytoplankton ingested by oysters is determined by the size of their gills. Olympia oysters have larger gills and thus likely ingest larger particles than the common non-native Pacific oyster Crassostrea gigas (Couch and Hassler 1989). Olympia oysters are preyed upon by birds such as sea ducks and by crabs (Couch and Hassler 1989). They are relatively small, rarely reaching sizes greater than 5 cm, and have slow growth rates, typically reaching maturity after 4 years (Baker 1995, White et al. 2009b). Unlike many bivalves, fertilization is internal and larvae brood for 10-12 days within the mantle of females before spending 11-16 days as planktonic larvae (Dethier 2006). Olympia oyster spat have fairly narrow requirements for settlement, preferring hard, rugose substrates such as adult oyster shells (Trimble et al. 2009, White et al. 2009b). Beds of Olympia oysters are typically subtidal and individuals are known to be sensitive to extremes in temperature and desiccation stress (e.g., Baker 1995).
Status and Trends
Geoduck abundances for individual tracts throughout Puget Sound are estimated based on diver surveys conducted by WDFW according the methods described in Bradbury et al. (2000) and are posted online as part of the Geoduck Atlas , but abundances at the basin or sound-wide scales have not been summarized or published. Similarly, published fishery-independent population abundance data on trends in geoduck abundances are lacking.
Olympia oysters in Washington state have been heavily exploited (Kirby 2004) and currently exist at abundances far lower than were reported historically (White et al. 2009a) (Figure 1). In Puget Sound, abundance was greatly reduced in the early 1900s despite the implementation of reserves throughout the Sound. Industrial pollution from paper mills is thought is thought to have contributed to the lack of effectiveness of the reserves (White et al. 2009a). The continued lack of population recovery is thought to be driven by a combination of limitations in the amount of preferred settlement substrate (adult conspecifics), competition with non-native oysters, and predation from introduced predators such as the Japanese drill Ocinebrina inornata (Buhle and Ruesink 2009, Trimble et al. 2009, White et al. 2009b). Their sensitivity to environmental extremes further restricts the habitats they can occupy (Trimble et al. 2009). Because of their low abundance, Olympia oysters currently are listed as a Washington State Candidate Species by WDFW . A number of projects for restoration of Olympia oyster populations have been initiated in Puget Sound (e.g., Brumbaugh and Coen 2009, Dinnel et al. 2009, White et al. 2009b).
Figure 1. Olympia oyster harvest (1 sack is equal to approximately 4,000 individuals) in Willapa Bay (filled circles) and Puget Sound (open circles) from the mid 19th to mid 20th century based on Washington Marine Fish and Shellfish Landings (figure from White et al. 2009) (reprinted with permission from the Journal of Shellfish Research).
There are several aspects of the current understanding of geoduck and Olympia oyster populations that are lacking. Geoduck tracts are surveyed frequently by WDFW yet estimates of basin and Sound-wide population status or trends have not been conducted. As such, spatial and temporal trends in geoduck abundances are not known for Puget Sound. Further, while cultivation of geoducks augments population abundances, the ecological effects of geoduck aquaculture practices in Puget Sound are not well understood (Feldmann et al. 2004, Straus et al. 2008). The sensitivity of Olympia oyster populations to abiotic stress and to predation from non-native predators pose challenges to the undertaking of restoring them to their former abundances and such the outcome of such efforts remains uncertain.
Native bivalves are essential components of the Puget Sound ecosystem. Geoduck clams are extremely long-lived, rendering them potentially susceptible to overexploitation. While geoduck abundance is estimated at small scales (tracts), published accounts of Sound-wide estimates of population status and trends are lacking. Abundances of Olympia oysters have been very low in Puget Sound since the 1940s, despite the fact that they are no longer targeted by fisheries. The importance of native oysters to ecosystems has prompted restoration efforts throughout Puget Sound.
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About the Science Review
Puget Sound Science Review
- Ecosystem-Based Management: Understanding Future and Desired System States
- Ecosystem-Based Management: Incorporating Human Well-being
- Ecosystem-Based Management: Ecosystem Protection and Restoration Strategies
- The Biophysical Condition of Puget Sound: Biology
- The Biophysical Condition of Puget Sound: Chemistry
- The Biophysical Condition of Puget Sound: Physical Environment
- Threats: Impacts of Natural Events and Human Activities on the Ecosystem