PETERSON ETAL.: MARK-RECAPTl'KE TKSTOF HARD CLAMS 



marketable size at Johnson Creek is quite important 

 to managers of the commercially harvested and valu- 

 able M. mercenaria resource in North Carolina. For 

 instance, although gametes produced by a 1-yr-old 

 clam, even of this size, may be viable (Porter 1964; 

 pers. commun. 3 ), the mass of gametes produced is 

 almost negligible when compared with larger 

 individuals (Peterson 1983). 



Although the general shape of the estimated M. 

 mercenaria growth curve (Fig. 8) is logarithmic, as 

 expected, the variance in the relationship is substan- 

 tial. Among all clams older than 4'/2 yrof age collected 

 from Johnson Creek, age explained only 1% of the 

 variance in size. Consequently, aging of Johnson 

 Creek M. mercenaria by inference from size-class 

 frequency would fail. Only in a population dominated 

 by young clams in the fast growing sizes could North 

 Carolina M. mercenaria be adequately aged by size 

 information. Thus, the utilization of growth band 

 analysis is an important key to inference on popula- 

 tion parameters in North Carolina M. mercenaria. 

 Unfortunately, the annual bands are not unam- 

 biguously evident on the outer shell surface, where 

 disturbance checks and other growth breaks appear 

 (as reported for other species such as S. solidissima 

 (Jones et al. 1978)), so that shell sectioning is 

 necessary for accurate aging. 



If recruitment success (reproductive effort times 

 subsequent larval and early postlarval survivorship) 

 were to remain constant across years, frequencies of 

 age classes would decline progressively with age at a 

 rate corresponding to the age-specific mortality func- 

 tion. Yet, the age-frequency distribution for Johnson 

 Creek M. mercenaria in January- February 1980 (Fig. 

 7) is characterized by lower numbers in each of the 

 three most recent year classes (1977-78-79) than in 

 the three previous year classes (1974-75-76). Tests 

 of sampling efficiency (Appendix) and data on the 

 seasonal progression in the size distributions of 0- 

 year class recruits (Fig. 5) demonstrate that the 

 "gap" in M. mercenaria's age distribution (Fig. 7) is 

 not caused by a sampling artifact. The relatively low 

 numbers in the 1977-78-79 year classes are a conse- 

 quence of reduced reproductive success, relative to 

 at least the three previous years, either because of 

 reduced reproductive effort or increased mortality of 

 larvae and early postlarvae. Although we have no 

 unequivocal way of distinguishing between these two 

 explanations, the close match between the increase 

 in North Carolina's commercial harvest of M. mer- 



'H.J. Porter, Institute of Marine Sciences, University of North Car- 

 olina at Chapel Hill, Morehead City, NC 28557, pers. commun. 

 July 1982. 



cenaria (Fig. 9) and the 3-yr decline in recruitment 

 success suggests that future studies should inves- 

 tigate the possibility that a recent reduction in the 

 spawning population of M. mercenaria in North Car- 

 olina through increased harvest (mostly from Core 

 Sound) has had an impact on reproductive effort and 

 recruitment success. The persistent uncertainty 

 among invertebrate population biologists about the 

 strength and nature of spawner-recruit relationships 

 remains the single biggest barrier to effective 

 management of invertebrate fisheries. 



ACKNOWLEDGMENTS 



We received field and laboratory assistance from 

 W. G. Ambrose, Jr., B. F. Beal, M. E. Colby, S. R. 

 Fegley, C. Furman, L. A. Howie, S. A. Hughes, J. H. 

 Hunt, S. H. Larson, N. M. Peterson, K. C. Pierce, S. 

 Shipman, N. T. Sterman, J. Tucker, and M. C. Wat- 

 zin. The manuscript was improved by critical 

 readings by W. G. Ambrose, Jr., B. F. Beal, S. R. 

 Fegley, R. H. Green, and J. W. Haefner. V. Page and 

 N. M. Peterson drafted the figures and H. Page pro- 

 vided the photographic prints. This study was spon- 

 sored by the University of North Carolina Institute of 

 Marine Sciences and by the Office of Sea Grant, 

 NOAA, U.S. Department of Commerce under grant 

 # NA8 1 AA-D-00026, North Carolina Department of 

 Administration. 



LITERATURE CITED 



A.NSELL, A. D. 



1968. The rate of growth of the hard clam Mercenaria mer- 

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 Barker, R. M. 



1964. Microtextural variation in pelecypod shells. Mala- 

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Chestnut, a. F. 



1952. Growth rates and movements of hard clams. Venus 

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 Clark, G. R. II. 



1974. Growth lines in invertebrate skeletons. Annu. Rev. 

 Earth Planet. Sci. 2:77-99. 



1979. Seasonal growth variations in the shells of recent and 

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1982. Seasonal patterns in shell microstructure of Mer- 

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 Coutts, P. J. F. 



1970. Bivalve- growth patterning as a method for seasonal 

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