FISHERY BULLETIN: VOL. M. NO. 4 



occurred before the onset of winter (Chestnut 1952; 

 see also data in Figure 5), this implies that for a small 

 percentage of M. mercenaria age was underestimated 

 by 1 yr. This is not a large bias in view of M. mer- 

 cenaria s life span (Fig. 7), but may cause misinter- 

 pretation in studies where distinguishing small 

 differences is important. 



Prior to our mark-recapture results and recent 

 observations by Clark and Lutz (1982), most scien- 

 tists suspected that this aging technique, so success- 

 ful for northern populations of M. mercenaria 

 (Pannella and MacClintock 1968; Rhoads and Pan- 

 nella 1970; Kennish and Olsson 1975), could not be 

 used for hard clams in the southeastern United 

 States because M. mercenaria there lacked the win- 

 ter period of slow, almost negligible, growth that is 

 associated with annual band deposition in northern 

 populations (Ansell 1968). Our analysis of presumed 

 daily growth increments along with sectioning 

 evidence in Clark and Lutz (1982) implies that 

 southeastern M. mercenaria also deposit the annual 

 band during a period of slow growth, but that this 

 period usually occurs from about June to October. 

 The results of our mark- recapture study confirm that 

 annual band deposition in 1-year class and older 

 clams occurs sometime in that same period (Fig. 2). 

 This season of annual band deposition corresponds 

 both with maximum seasonal water temperature 

 (Sutherland and Karlson 1977) and with the spawn- 

 ing season for M. mercenaria in the Cape Lookout 

 region of North Carolina (Porter 1964). Because 

 water temperature serves as the usual proximate 

 cause of spawning in marine bivalves, separation of 

 these two factors is difficult. The deposition of an ini- 

 tial band in clams only a few months old (Fig. 5) does 

 not permit rejection of the spawning hypothesis 

 because this initial band differed in appearance from 

 all subsequent annual bands and seemed to occur 

 later in the fall season (Fig. 5), implying that this first 

 band is of a different nature from all subsequent 

 ones. Jones (1980) also demonstrated that both 

 Spisula solidissima and Artica islandica deposit 

 annual bands at times of spawning, analogous to our 

 results for M. mercenaria. Regardless of the 

 mechanism, results of our mark- recapture ex- 

 periments dispel the justifiable doubts expressed by 

 several scientists concerning the interpretation of 

 shell growth lines (Clark 1974; Gould 1979; Jones 

 1981) and permit paleontologists, archaeologists, 

 and environmental biologists to interpret banding 

 patterns in shells of M. mercenaria from the 

 southeastern coast of North America and to apply 

 this biological chronometer to a wide spectrum of 

 problems. 



Application of this aging technique to a population 

 of M. mercenaria in Johnson Creek off Core Sound 

 revealed a surprisingly large frequency of older clams 

 and a high average age (>9 yr old). In comparison, 

 Kennish (1980) demonstrated almost 100% mor- 

 tality of M. mercenaria in Barnegat Bay, N.J., by age 

 9. A relatively low rate of commercial fishing mor- 

 tality in North Carolina prior to 1977 may contribute 

 to this difference in population parameters. Figure 9 

 presents clam landing data illustrating the recent 

 increase in hard clam harvest in North Carolina. This 

 recent intense harvest is perhaps made possible by 

 the sudden utilization of the accumulation of several 

 years' reproduction, whereas northern populations 

 of M. mercenaria may have been subjected to con- 

 tinuous high fishing intensity for a long period and 

 therefore exhibit age-frequency distributions that 

 are shifted towards the younger age classes. Alter- 

 natively, the differences in age distributions between 

 areas could be the consequence of natural differ- 

 ences in factors affecting hard clam life histories. 



,500 



C/5 



O 

 Q. 



o 1,000 



to 



Q 



500- 



o 



i— 



1965 



— i 1 1 1 1 1 r— 



1970 1975 



YEAR 



-600 



-400! 



200 tj 

 o 



1980 



FIGURE 9. — Annual hard clam, Mercenaria mercenaria, catch by 

 commercial clammers in North Carolina from 1965 to 1981, as 

 estimated by the North Carolina Division of Marine Fisheries. 



The growth rate exhibited by M. mercenaria from 

 Johnson Creek (Fig. 8) is higher than that 

 demonstrated by Chestnut (1952) for a sand- bottom 

 area of Bogue Sound, and higher than many (but not 

 all) of the growth rates recorded from other areas 

 (Ansell 1968). In particular, our age-size plot (Fig. 8) 

 implies that the average M. mercenaria in Johnson 

 Creek reaches the legal minimum size for harvest 

 (4.46 cm long) by age \ x h, whereas Chestnut's (1952) 

 data implied that it usually required 3 yr for North 

 Carolina hard clams to enter the catchable popula- 

 tion Clearly there is a large degree of individual varia- 

 tion in growth rate and size at any given age (Fig. 8). 

 Nevertheless, this new estimate of average time to 



776 



