FISHERY BULLETIN: VOL. 81. NO. 4 



cross section of each clam to determine whether any 

 repeating feature, like the winter growth bands of 

 northern M. mercenaria (Pannella and MacClintock 

 1968; Kennish 1980), served as an annual chrono- 

 meter. By comparing the inked lines marking size at 

 introduction and size at known dates of measure- 

 ment on the outer shell surface to the polished cross 

 section, each of three independent observers re- 

 corded 1) an estimate of the number of growth bands 

 deposited on each clam since introduction and 2) the 

 season of band deposition. For a subset of 

 experimental clams, acetate peels were prepared by 

 the standard procedures (Rhoads and Lutz 1980) 

 and examined under a Wild Mil microscope 

 equipped with ocular micrometers 1) to ascertain if 

 annual markers were more or less evident than in 

 macroscopic view and 2) to utilize presumed daily 

 growth lines to estimate the exact period of annual 

 band deposition and to determine by measuring the 

 daily increments whether the annual band represent- 

 ed a period of relatively slow or rapid growth 



-Year Class 



Because our mark-recapture test of the aging 

 technique did not include any clams in the 0-year 

 class (which was unavailable in June) and because 

 the first annual band might easily be overlooked, we 

 designed an independent test of our ability to 

 recognize the very first annual band in M. mer- 

 cenaria. From February to April 1980, we collected 

 allM. mercenaria from 432 samples that were taken 

 by hydraulic suction dredge (described below) from 

 0.25 m 2 sampling frames to a depth of 15 cm and 

 passed through a 3 mm mesh. This sampling process 

 collected all clams >0.5 cm long with high efficiency 

 and without size selectivity (see Appendix). Equal 

 numbers of samples were taken from a shallow, sub- 

 tidal eelgrass meadow and from nearby unvegetated 

 sandy bottom at similar depths (—1.0 m) along the 

 Shackleford Bank edge of Back Sound, about 8 km 

 northwest of Cape Lookout (Fig. 1). All M. mer- 

 cenaria were brought to the laboratory, killed by 

 steaming, measured with calipers, and sectioned to 

 expose growth bands. Here we compared the total 

 numbers of clams which lacked any growth band with 

 the numbers with a single band. Recruits sampled in 

 February- April lacked a growth band if 1) we failed to 

 recognize the initial annual band or 2) the recruits 

 settled too late in the season to be branded with that 

 season's band. Under the assumption that the winter 

 dredge sampling faithfully collected all surviving re- 

 cruits from the previous year's recruitment season, 

 the ratio of clams with zero bands to those with zero 



or one band represented an estimate of the frequency 

 of clams in each year class whose age is underes- 

 timated by 1 yr. 



We devised one further test of the accuracy of iden- 

 tification of the initial annual band in M. mercenaria 

 collected from the Cape Lookout vicinity of North 

 Carolina and of the assumption that by late winter 

 (February-April) M. mercenaria recruits had grown 

 sufficiently to be efficiently collected in our hydraulic 

 dredge sampling. In June 1978, we installed at Mid- 

 dle Marsh 36 1m 2 field enclosures of the identical 

 design described above and used in the topless treat- 

 ment for the mark-recapture test of the aging 

 methodology. All enclosures were located in shallow 

 subtidal areas (—0.5 m deep at low tide): 17 on a 

 muddy- sand flat in a protected embayment at the 

 western end of Middle Marsh and 19 in the Zostera 

 marina meadow adjacent to the site used for the 

 mark-recapture experiment (Fig. 1). After installa- 

 tion, all M. mercenaria >7 mm were removed from 

 each plot by twice systematically sieving the top 10 

 cm through a 6 mm mesh. Marked M. mercenaria 

 individuals were returned to these plots at densities 

 varying from to 28 per m 2 as a part of another 

 experiment not reported here. The plots were resam- 

 pled in September 1978, April 1979, October 1979, 

 and May 1980. At each sampling, any unmarked M. 

 mercenaria were collected by sieving, measured, and 

 removed. Because most of these were new recruits, 

 these data provided an indication of the size- 

 frequency distributions of 0-year classM. mercenaria 

 for both early fall and spring seasons in the Cape 

 Lookout region of North Carolina. In April 1979, 

 additional 1 m 2 enclosures were added at both sites, 

 such that total areas sampled in October 1979 and 

 May 1980 were 27 m 2 at the western Middle Marsh 

 site and 29 m 2 at the Z. marina meadow site. We com- 

 pared the seasonal size-frequency distributions of 

 these new recruits with the distribution of size 

 (length) at first band in a field collection of all age 

 classes of clams (methods described below) made 

 from nearby Johnson Creek in Core Sound, N.C. (Fig. 

 1). This comparison provides a further test of the 

 accuracy of our recognition of the initial annual band 

 of North Carolina M. mercenaria. 



Application of Aging Technique to a 

 Field Population 



We collected M. mercenaria on two occasions from 

 Johnson Creek to provide samples on which to apply 

 our aging technique. Johnson Creek is a tidal creek 

 on eastern Core Sound ~18 km northeast of Cape 

 Lookout (Fig. 1). Bottom type was soft, muddy sand. 



768 



