Table 1. — Locations and sedimentary characteristics of the one 

 previous (IVIiddie Marsh) and five new study sites of Mercenaria 

 mercenaria. Sedimentary data came from seiving and pipetting 

 (Folk 1974) duplicate surface (0-5 cm) sediment cores taken in sum- 

 mer 1981 at each site. Percent silt-clay is percent of total sediment 

 dry weight in fine (>40) size classes. 



water temperature and salinity data are not available 

 for all sites, but records from a variety of sources 

 (Brett 1963; Thayer 1971; Williams et al. 1973; 

 Sutherland and Karlson 1977; H. J. Porter, Univer- 

 sity of North Carolina, Chapel Hill, unpubl. data; W. 

 Kirby-Smith, Duke University, unpubl. data) suggest 

 that 1) water temperature patterns probably do not 

 differ greatly across sites, with monthly averages 

 ranging from winter minima of 2°-4°C to summer 

 maxima of 29°-30°C, and 2) that salinities are slightly 

 more variable across sites. Localities close to Atlantic 

 Ocean inlets (Bogue Sound sand and seagrass sites 

 and the previous Back Sound site at Middle Marsh) 

 experience uniformly high salinities (30-36°/oo), ex- 

 cept after severe rainstorms (Brett 1963; H. J. 

 Porter, unpubl. data). Salinities in the upper portion 

 of North River are only slightly lower because there 

 is little freshwater inflow into that system (Thayer 

 1971). The lowest (22-28o/oo) and probably most 

 variable salinities on a week-to-week scale occur at 

 the two Core Sound sites, where exchange with the 

 ocean is reduced and where any persistent north 

 winds bring intrusions of low-salinity waters from 

 Pamlico Sound (Williams et al. 1973). 



At each of the five new study sites, we placed 

 groups of 80 Mercenaria mercenaria in 1 m^ field 

 plots in late summer 1980, excavated them by hand 

 in late summer 1981 to estimate growth and mor- 

 tality and to replace missing and dead clams, and 

 then finally recovered all living clams present in late 

 summer 1982 (Ikble 2). All M. mercenaria used in 

 these experiments were individually marked on the 

 external shell surface with color-coded dots of Mark- 



Ibx Corporation paints and measured initially and 

 at both yearly samplings by calipers to the nearest 

 0.1 mm in each of three mutually perpendicular 

 dimensions (length, height, thickness). Clams used 

 in these mark-recovery experiments were chosen to 

 reflect a size range from 1 to 10 cm in length. Before 

 placing the marked and measured clams into the 

 field plots, we first installed fences of 6 mm mesh 

 plastic (VEXARi) around the 1 m^ plots. These 

 fences were identical to those used and described 

 previously (Peterson et al. 1983) and were designed 

 to inhibit emigration and to mark off bottom plots 

 to improve our ability to recover the marked clams. 

 At the three unvegetated sites, we removed all ini- 

 tially present M. mercenaria and other large macro- 

 fauna before adding marked clams by first using 

 fingers to plow systematically the top 10 cm of 

 sediments and then twice systematically sieving in 

 situ through 6 mm mesh the entire 1 m^ surface to 

 that same 10 cm depth. This procedure was not used 

 at the initiation of the experiment at the two sea- 

 grass sites because it would have removed the sea- 

 grass itself. This same procedure was employed, 

 although using a 3.2 mm mesh, at both yearly sam- 

 plings to recover all marked clams from all 1 m^ 

 plots at each unvegetated site. At the two seagrass 

 sites, marked clams were recovered by using a 

 hydraulic suction dredge and collecting the contents 

 of the top 15 cm on a 3 mm nylon mesh bag (see 

 Peterson et al. 1983 for data on sampling efficienqr 

 of this device). Because of the removal of seagrasses, 

 the locations of all seagrass plots were then shifted 

 slightly (<3 m) to new, undisturbed positions for the 

 second and final year. 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Table 2. — Dates of experiment initiation and subsequent sampling 

 of Mercenaria mercenaria for each of the five study sites. 



'Each site held 1 m ~2 enclosures of Mercenaria mercenaria at 80 m ~^: one 

 enclosure contained clams derived from a common Back Sound site (sup- 

 plemented by 10-20 mm clams from other Back Sound origins), and at least 

 one other enclosure contained only locally derived clams. 



673 



