Population Studies of the Native Eastern Oyster,Crassostrea virginica, (Gmelin, 1791) in the James River, Virginia, USA

Abstract

We describe oyster population trends in the James River, VA from 1993 through 2006 using quantitative fishery independent survey data collected using a stratified random design. The 23 reefs contained in the study area cover a total of 2.41 3 10 7 m 2 and vary in individual size from 1.26 3 10 4 m 2 to 4.98 3 10 6 m 2. There is a marked pattern in density of oysters among the reefs: during the study period a small group of reefs comprising 5.4% of the total area consistently contained between 25.7 and 55.5% by number and 35.8 and 54.8% by biomass of the total oyster population. The highest density reefs exhibit, with very few exceptions, mean densities well in excess of 200 oysters m-2 , typically between 300 and 500 m-2 , with a single maximum value of 773 oysters m-2 in 2002 coincident with the highest annual recruitment observed during the study period. Recruitment events were usually followed by very high mortality with very small percentages of the population reaching ages $3 y of age. A strong stockrecruit relationship is absent; rather population demographics appear to be dominated by periodic high recruitment events. Biomass maxima tended to lag one to two years after recruitment maxima. Standing stock for the total system varied between 1.07 3 10 8 g and 3.31 3 10 8 g (107 and 331 metric tonnes) in 2003 and 2005, respectively as the 2002 recruits grew and suffered mortality. Age-at-length relationships were estimated from demographics: using a July 1 birth date and a November 1 survey date giving lengths of 37.3 mm at 0.33 y, 58.9 mm at 1.33 y, 80.5 mm at 2.33 y, 102.1 mm at 3.33 y and 123.7 mm at 4.33 y Length demographics were recast as age demographics to estimate annual proportional mortality. Mean proportional mortality values for age 1 oysters range from a low of 0.2-0.4 to a high in excess of 0.7. Age 2 mean proportional mortality values range from a low of 0.41 to a high exceeding 0.75. The proportional mortality for age 3 and 4 y olds generally exceeded mean values of 0.6 with highest values approaching 0.95. In all cases, these values exceeded mortality estimates calculated using traditional box count methods by a considerable margin. The ability to accurately estimate age specific mortality allows the construction of shell (habitat) budgets for the individual reef systems. Shell half-life loss rate estimates in the most productive reefs is between 2 and 3 y, and the population is maintained by the continual and extraordinary recruitment in the face of high mortality-the latter driven by disease (predominantly Perkinsus marinus) epizootics. The shell resource, even on the most productive reefs, is modest, equivalent to little more than a monolayer several centimeters thick. Individual reefs demonstrate remarkable stability as either high shell density + high population density associations (high:high) or low shell density + low population density associations (low:low), even in the face of temporal population and demographic fluctuations associated with disease related mortality. The probabilit...

Reconstructing early 17th century estuarine drought conditions from Jamestown oysters

“…3A), we see that the shells did not record these coldest conditions; rather, the most positive shell δ 18 O calcite values are~1‰ lower than predicted, suggesting the oysters stopped calcifying at a slightly higher T of~8°C. This calculation agrees with previous determinations of winter growth cessation in James River oysters (22,34).…”

“…End member salinities are controlled by freshwater influx and tidal exchange. James River salinities are lower between March and May due to elevated rainfall runoff (32-34) than between June and January (33,34). In the modern James River, the 2-psu isohaline migrates seasonally from Mulberry Point (MP) during wet conditions, to just above Jamestown Island (JI) during dry conditions (Fig.…”

“…Salinity sets the upriver boundary for modern James River oyster populations at approximately Deep Water Shoal,~24 km downriver of JI (34)(35)(36) (Fig. 1).…”

“…1). Although oysters can tolerate salinities as low as 2 psu for short periods (36,37), salinities of 5-7 psu typically describe the low salinity threshold for James River oyster populations (22,34,36). Modern oyster distributions are also set by factors such as availability of oyster shell habitat, predators, diseases, and fishing.…”

Declining impact of an introduced pathogen: ­Haplosporidium nelsoni in the oyster Crassostrea virginica in Chesapeake Bay

“…When oyster re cruitment rates are low, there is an overall loss of shell (i.e. calcium carbonate) from even the healthiest Chesapeake Bay system, the James River (Mann et al 2009). Increasing the size of oysters in (and by means of) sanctuaries may slow rates of shell loss, making positive shell budgets more likely even without marked increases in oyster recruitment.…”