Field Data Collection Procedures

Hydraulic patent tongs are used to sample bottom. Photo by Missy SouthworthOysters are collected from the 43-ft long VMRC vessel J.B. Baylor with a hydraulic patent tong.  The open dimensions of the tong are such that it samples one square meter of bottom.  Upon retrieval of each sample (= patent tong grab), oysters are counted and measured (mm), and the volume of shell material (L) recorded. The recorded dimension for each oyster is the longest from the hinge to the shell margin. This is correctly termed shell height, although commonly described as shell length in most literature. We adopt the common convention and refer to shell length (SL) in this text.   A count of the number of oysters per tong is made in all years sampled (1993 through the most recent survey in 2013).  Prior to 1998 a representative subsample (n > 100) of oysters was pooled across individual samples for a given reef was measured and classified into 5 mm size bins. From 1998 to 2002, for each sample, all oysters were measured and classified into 5 mm size bins.  Beginning in 2003, for each sample, individual lengths were recorded to the nearest mm. Distinction is also made between Young of the Year (YOY) or spat, and older oysters. Spat, are generally closely attached to the substrate for their entire length when examined in the fall after recruitment to the benthos (the relationship has been described as annealed to the substrate) whereas older animals develop a cupped shape in the attached valve. Since 1998, samples with > 20 L of shell have been subsampled to facilitate processing.  When subsampling is necessary, an appropriate subsample factor is then applied to the resulting counts and length frequency distributions to estimate density and size distribution on a per m-2 basis.

Comparison of Brown and Black Shell. Photo by Missy Southworth

All articulated valves of dead oysters, commonly termed boxes, are similarly counted and measured.  Quantitative shell data (L volume) has also been collected at all samples sites for both brown (oxic exposed) and black (anoxic buried) shell since 2002 (since 2000 on a subset of reefs, in some cases back to 1993 for a whole shell measurement).

The Virginia oyster resource may have been considered as a series of contiguous sub populations in the rivers and bays in the time of Baylor, but these resources were fractured spatially by the epizootic actions of both MSX (Haplosporidium nesloni) and Dermo (Perkinsus marinus) with the resultant restriction of oysters to discrete populations in low salinity sanctuaries from disease. The definition of discrete versus metapopulations within the Bay, together with sources and sinks, remains a subject of investigation and debate. For the current report the population is considered as a series of adjacent stocks with no presumptions as to the levels of connectivity. These stocks are coincident with subdivision of the surveyed area in a north-south direction as follow: (A) Pocomoke and Tangier Sounds (P&T), (B) Great Wicomico River (GWR), (C) Rappahannock River (RR), (D) Piankatank River (PR), (E) Mobjack Bay and (F) York River (M&Y), (G) James River (JR), and (H) Elizabeth and Lafayette Rivers (E&L). In addition there are a modest number of stations in the Bay main stem adjacent to but not enclosed by various estuary mouths.

Table 1