A MARK-RECAPTURE TEST OF ANNUAL PERIODICITY OF 

 INTERNAL GROWTH BAND DEPOSITION IN SHELLS OF 

 HARD CLAMS, MERCENARIA MERCENARIA, FROM A POPULATION 

 ALONG THE SOUTHEASTERN UNITED STATES 



C. H. Peterson, P. B. Duncan, H. C. Summerson, andG. W. Safrit, Jr. 1 



ABSTRACT 



Individually marked and measured Mercenaria mercenaria were placed at natural depths in the sediments 

 inside field enclosures of three types in an estuary near Cape Lookout, N.C., in June 1978. Subsets of hard 

 clams were collected and sacrificed on October 1979, May 1980, October 1980, and October 1981. Section- 

 ing one valve of each experimental clam along the axis of greatest growth revealed growth discontinuities 

 (both in texture and coloration) in the middle and outer shell layers. These growth bands were deposited 

 annually during the summer-early fall season. Enclosure type (microhabitat variation) did not alter the 

 regular annual pattern of band deposition: 937c of the experimental clams in October collections ( 1 15 of 123) 

 exhibited the predicted number of added growth bands in the increment of shell growth that had been 

 deposited since initial marking. Examination of presumed daily lines on acetate peels and thin sections sug- 

 gested that the annual band corresponds to a period of relatively slow growth. Only a few (=6.7%) of the M. 

 mercenaria recruits in spring samples failed to exhibit an identifiable growth band from their first summer- 

 fall period. A comparison of the size- frequency distribution at first band on older clams to the size dis- 

 tributions of new recruits in September-October and in April-May revealed that the first growth band on a 

 new M. mercenaria recruit is usually deposited soon after September-October during the clam's first fall. 

 Thus, southeastern M mercenaria near Cape Lookout can be aged by counting internal growth bands but, 

 unlike northern populations, exhibit slow growth and annual band deposition during summer-early fall rather 

 than in winter. 



Application of the aging technique to a January-February 1980 collection of M. mercenaria from Core 

 Sound, N.C., revealed a high proportion of older clams (up to 32 years of age) and a mean age of >9 years. 

 Growth rates, inferred from the relationship between size and estimated age, were high; on average a legally 

 harvestable size (4.46 cm in length) is reached by VA years. The age-frequency distribution from this collec- 

 tion revealed lower recruitment success of the 1977, 1978, and 1979 year classes than of previous year 

 classes. This partial year- class failure corresponds with the period of fourfold increase in commercial harvest 

 of M. mercenaria in North Carolina and suggests that further studies should test for a spawner-recruit 

 relationship among hard clams. 



The depositions! regularity of macro- and micro- 

 structural features in bivalve mollusc shells has been 

 exploited as a chronometer by scientists in several 

 disciplines (reviewed succinctly by Jones 1980). 

 Deposition patterns in bivalve shells have proved 

 useful to 1) paleontologists and environmental 

 biologists in reconstructing the local history of 

 environmental change (Pannella and MacClintock 

 1968; Clark 1974; Rosenberg and Runcorn 1975; 

 Pannella 1976; Rhoads and Lutz 1980), 2) 

 archaeologists in dating the seasons of prehistoric 

 site occupation (Coutts 1970; Koike 1973), and 3) 

 population biologists and fisheries managers in con- 

 structing quantitative life history and growth 

 schedules for the bivalves themselves (Rhoads and 

 Pannella 1970; Kennish and Olsson 1975; Jones et 



'Institute of Marine Sciences. University of North Carolina at 

 Chapel Hill. Moiehead City, NC 28557. 



al. 1978; Kennish 1980). Biologists and managers 

 should probably make more use of the historical 

 records preserved in bivalve shells to estimate sur- 

 vivorship curves by assessing age at death and 

 growth curves by measuring growth increments be- 

 tween successive chronological markers because 

 these integrative methods are more efficient than all 

 rigorous alternative methods, which require mea- 

 surements over at least a year's time. Concern over 

 the paucity of controlled tests of the regular 

 periodicity of repeating shell features (Clark 1974; 

 Gould 1979; Jones 1981) may be partly responsible 

 for the cautious use of shell information by inverte- 

 brate population biologists. 



The hard clam, Mercenaria mercenaria (L.), has 

 been used frequently by paleontologists, archaeo- 

 logists, and population biologists as a subject for 

 shell macro- and microstructural analysis (Barker 

 1964; Pannella and MacClintock 1968; Rhoads and 



Manuscript accepted June 19S:i. 



FISHERY BULLETIN: VOL. 81, NO. I. 1983 



'65 



