Oyster (Crassostrea virginica, Gmelin 1791) Population Dynamics on Public Reefs in the Great Wicomico River, Virginia, USA

Abstract

We describe oyster population trends in the Great Wicomico River, VA, from 2000 through 2009 using quantitative fishery independent survey data collected using a stratified random design. The seven public reefs examined cover a total of 2.8 3 10 5 m 2 and vary in individual size from 1.36 3 10 4 to 7.16 3 10 4 m 2. The river is functionally divided by a sand spit into upriver and downriver regions. Oyster densities on the upriver reefs were typically an order of magnitude higher than densities on the downriver reefs within the same time period. Throughout the system, the highest observed densities were coincident with high annual recruitment events (2002, 2006). Recruitment events were usually followed by high mortality, with small percentages of the population reaching $3 y of age. A predictive stock-recruit relationship is absent; rather, population demographics appear to be dominated by periodic high recruitment events. In the absence of seed removal, biomass maxima follow 1-2 y after recruitment maxima. Standing stock for the system varied between 1.56 3 10 6 g and 3.63 3 10 7 g in 2005 and 2006. Year-specific age-at-length relationships were estimated from demographics data. Length demographics were recast as age demographics to estimate mortality. Observed proportional mortality between young of the year and age 2 oysters was approximately 0.88 for the 2006-y class, which is slightly higher than the 0.62-0.71 observed for the 2007-y class. The ability to estimate age specific mortality accurately allows the construction of shell (habitat) budgets for the individual reef systems. The Great Wicomico oyster population appears to be maintained by episodic and extraordinary recruitment in the face of high mortality-the latter driven by disease (predominantly Perkinsus marinus) epizootics. The shell resource is modest, equivalent to little more than a monolayer several centimeters thick. Over short timescales (years), the available shell resource oscillates in concert with mortality. The shell accretion rate on upriver reefs is consistently 4-5 times greater than that observed on downriver reefs. Periodic modest shell planting has maintained the habitat base (the shell resource) throughout the system over decadal scales.

Overfishing, disease, habitat loss, and potential extirpation of oysters in upper Chesapeake Bay

“…The decline in habitat quality and quantity is likely due to a combination of direct removal from harvest, reduction in accretion rates from disease and lack of recruitment, and excessive siltation (Rothschild et al 1994, Hargis & Haven 1999, Mann et al 2009b). Current estimates suggest that average density of adult oysters in the Virginia portion of Chesapeake Bay is between 2 and 11 oysters m -2 (Mann et al 2009b, Southworth et al 2010. We found the average density of adults during 2004 to 2009 was 3.7 oysters m -2 habitat.…”

Oyster Demographics in Harvested Reefs vs. No-Take Reserves: Implications for Larval Spillover and Restoration Success

“…This is much more extreme than recruitment variability observed in Chesapeake Bay (∼45-175 m −2 ; Mann et al, 2009;Southworth et al, 2010). Conversely, in this study recruitment was generally higher on restored, cultch-planted reefs than natural reefs potentially due to slightly higher relief and settlement substrate available to potential settlers during restoration.…”

Quantitative Validation of a Habitat Suitability Index for Oyster Restoration

“…The GWR is located ∼10 km south of the Potomac River and 25 km north of the Rappahannock River, and has a small watershed consisting predominately of forested and agricultural lands (Southworth et al, 2010). The GWR is mesohaline and is considered a trap-type estuary with gyre-like water circulation patterns that has contributed to its history of significant natural oyster recruitment (Andrews, 1979;Southworth et al, 2010). The system is characterized by a single, central deep channel with an extensive sand shoal near the river mouth (Southworth et al, 2010).…”

“…The GWR is mesohaline and is considered a trap-type estuary with gyre-like water circulation patterns that has contributed to its history of significant natural oyster recruitment (Andrews, 1979;Southworth et al, 2010). The system is characterized by a single, central deep channel with an extensive sand shoal near the river mouth (Southworth et al, 2010). Oysters within the system exist on public oyster grounds, private lease areas, and no-harvest oyster sanctuaries (Schulte et al, 2009;Southworth et al, 2010).…”

“…The system is characterized by a single, central deep channel with an extensive sand shoal near the river mouth (Southworth et al, 2010). Oysters within the system exist on public oyster grounds, private lease areas, and no-harvest oyster sanctuaries (Schulte et al, 2009;Southworth et al, 2010).…”