FISHERY Bl'LLKTIN: VOL. 81, NO. 4 



Pannella 1970; Crenshaw 1972; Kennish and Olsson 

 1975; Gordon and Carriker 1978; Kennish 1980). 

 Most published analyses of shell deposition patterns 

 which are directed towards estimating life history 

 parameters and growth rates in M. mercenaria have 

 been conducted on northern populations (but see 

 Clark 1979; Clark and Lutz 1982). Like many other 

 marine bivalves (e.g., Pecten maximus in Mason 

 1957; Scrobicularia plana in Green 1957; and Ma- 

 coma baltica in Segerstrale 1960), M. mercenaria 

 from New Jersey (Kennish 1980) to Massachusetts 

 (Pannella and MacClintock 1968) deposits a winter 

 band of slow- growth increments that can serve as an 

 annual marker. Because hard clams in southeastern 

 populations show a pattern of nearly constant monthly 

 growth year-round (Ansell 1968), we questioned 

 whether M. mercenaria in the southeast would 

 deposit a clear annual marker in its shell. Here we 

 report on mark-recapture tests of whether M. mer- 

 cenaria from the vicinity of Cape Lookout, N.C., 

 deposits any regularly periodic feature in its shell 

 that could be used to age the individual clams. A 

 rigorous experimental test of the aging technique is 

 of vital importance to the wide spectrum of scientists 

 who would like to utilize internal shell markers to age 

 M. mercenaria but cannot with confidence until test 

 data displace the doubts justifiably expressed by 

 Gould (1979) and Jones (1981). We also apply our 

 results to a southeastern population of hard clams to 

 demonstrate estimation of age-frequency distribu- 

 tion and to draw inferences about population 

 dynamics and growth. 



MATERIALS AND METHODS 



Tests of Aging Methodology 



1-Year Class and Older 



A mark-recapture study was designed to test 

 whether Mercenaria mercenaria in the vicinity of 

 Cape Lookout deposits any distinct annual marker 

 in its seasonal pattern of shell growth. On 2 1-22 June 

 1978, we placed 28 individually marked and 

 measured M. mercenaria into each of six 1 m 2 field 

 enclosures. This density is within the range occurring 

 naturally in this area but about four times the average 

 observed in a nearby Bogue Sound seagrass bed 

 (Peterson 1982). We employed a wide range of initial 

 sizes from 1.8 to 10.2 cm in length and kept size- 

 frequency distribution similar in each enclosure. 

 Marking was achieved by applying color- coded dots 

 of Mark-Tex Corp. 2 paints to the external shell sur- 

 face of each clam. Three perpendicular linear dimen- 



sions (length along the longest anteroposterior axis, 

 height, and width) were measured to the nearest 0.1 

 mm on each clam using vernier calipers. On four sub- 

 sequent occasions (17 October 1979; 22 May 1980; 8 

 October 1980; 9 October 1981), 5 to 11 (usually 7) 

 clams were removed from each enclosure and killed 

 by steaming in the laboratory to provide shells with 

 varying, but known, histories of terminal (marginal) 

 growth for macro- and microstructural analyses. 



Field enclosures were located in muddy-sand 

 sediments at a low- tide water depth of ss0.5 m within 



a protected embayment inside Middle Marsh in Back 

 Sound, N.C. (Fig. 1). Nelson (1979) and Homziak et 

 al. (1982) have described this site. Water tempera- 

 ture in Back Sound is seasonally variable; monthly 

 means at nearby Beaufort, N.C, vary from 4° to 29°C 

 (Sutherland and Karlson 1977). Salinities remain 

 high year-round, ordinarily above 32%o but with 

 lower values recorded after heavy rainstorms 

 (unpublished data for nearby Bogue Sound, by 

 H. J. Porter, University of North Carolina). All 

 six enclosures were located in an unvegetated 

 area within an eelgrass, Zostera marina, bed and were 

 protected on the north, east, and west by emergent 

 salt marshes {Spartina alterniflora) and on the south 

 by a sandbar which was exposed on spring low 

 tides. 



Enclosures were constructed from 4.2 m long by 13 

 cm high strips of 6 mm Dupont Vexar mesh, folded to 

 form aim 2 square, and forced vertically 10 cm into 

 the sediments. To anchor the enclosures, 0.6 m long 

 steel reinforcing rods were also pushed into the 

 sediments and were attached with nylon cable ties to 

 the Vexar mesh at each corner and at halfway points 

 along each side. The belowground mesh inhibited M. 

 mercenaria migration, while the aboveground mesh 

 served to identify the boundaries of the plots and 

 thereby aided recovery of the marked clams. To 

 induce locally differing sets of environmental con- 

 ditions, we added 1 m 2 tops made of 6 mm Vexar 

 mesh to two enclosures and partial tops made of two 

 parallel 0.25 m 2 Vexar strips to two other enclosures. 

 All complete and partial tops were fastened to the 

 enclosure walls with nylon cable ties at 5 cm intervals 

 and supported above the sediment surface with 

 wooden dowels implanted inside each enclosure. 

 Thus, our complete design consisted of replicate 

 clams inside two replicate enclosures of each of three 

 different caging treatments. 



Before introducing the experimentalM. mercenaria 

 into the field plots, we removed all large resident M. 



; Reference to trade names or commercial firms does not imply 

 endorsement bv the National Marine Fisheries Service, NOAA. 



766 



