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 × 107 m2 and vary in individual size from 1.26 × 104 m2 to 4.98 × 106 m2. 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 × 108 g and 3.31 × 108 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 probability of manipulating either shell and/or live oyster density to effect the transition of a low:low reef to a high:high reef is considered bleak in the face of extant recruitment and mortality patterns. The primary impediment to population expansion or rebuilding is high and uncontrolled mortality rather than a lack of recruitment. Given the large numbers of oysters in low salinity refugia that have the ability to continually contribute to the larval pool, active selection against disease susceptible oysters on a system wide basis is unlikely.