PETERSON ET AL.: MARK-RKCAPTl RK TEST OF HARD CLAMS 



the 34 quantitative samples was 1.59 ± 1.28 (SE) per 

 0.25 m 2 . The size-frequency distribution of all 124 

 clams collected (Fig. 6) was dominated by relatively 

 large clams in the 6-9 cm range. Figure 7 presents the 

 distribution of these same 124 clams among age 

 classes. This figure was constructed by counting the 

 number of annual growth bands on each clam and 

 subtracting one, under the assumption (tested 

 earlier) that new recruits lay down their first annual 

 growth band during their first fall. Because the 

 January-February sample occurred about l A yr after 

 settlement (assuming an average settlement time of 

 July), the ages of clams in years are estimated by their 

 year class plus one-half. Average age of the clams 

 collected at Johnson Creek was 8.59 + 0.50 or just 

 over 9 yr old. The oldest clam in the sample was 

 estimated to be 32 yr old (Fig. 7). This age distribu- 

 tion (Fig. 7) reveals that each of the three most recent 

 year classes (1977-78-79) at Johnson Creek con- 

 tributed less to the total sample than each of the 

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

 In Figure 8, the shell length of each clam collected is 



lO.OCh 



9.00 

 8.00 

 7.00^ 



E 6.00- 



o 



<S> 



5.00 



4.00 



3.00 



2.00- 



.00 



0.00 



5 10 15 20 25 



NUMBER OF ANNUAL BANDS 



30 35 



FIGURE 8. — The relationship between number of annual bands (= 

 age in years + one-half for clams collected in January-February) and 

 length for all 124 clams collected in. January- February 1980 from the 

 Johnson Creek site in Core Sound. N'.C. Also indicated on the graph 

 is the minimum legal size for harvest in North Carolina. + indicates 

 mean sizes in each age class up to the 10th. The growth line drawn in 

 is the best fitted (r = 0.673; P < 0.001) logarithmic curve [length 

 (cm) = 3.176 + 1.819 In (no. of annual bands)]. 



plotted against its total number of annual bands ( = 

 age + l A). This graph illustrates the generally 

 logarithmic form of growth and provides an estimate 

 of age- specific growth of M. mercenaria in the 

 Johnson Creek area of Core Sound. The best fitted 

 logarithmic growth curve through all points is 



length (in cm) = 3.176 + 1.819 In (no. of annual 



bands) 

 r 2 = 0.673, P< 0.001. 



On the graph, we plot the minimum length at which 

 hard clams can be legally harvested in North Car- 

 olina. This size was calculated by converting the 

 minimum legal width of 25.4 mm to length by the 

 regression equation 



length (in mm) = -1.73 + 1.83 width (in mm) 



derived from fitting all 124 Johnson Creek clams 

 (r 2 = 0.97; P < 0.001). Figure 8 implies that most 

 clams at Johnson Creek reach legal size by age VA, 

 sometime during their second winter. This graph also 

 reveals how extremely variable a clam's size is for any 

 given age older than about 4 l A yr. For instance, a clam 

 75 mm long can be anywhere from age 4 l A to at least 

 age 25%. For clams older than 4 l A yr of age, size is a 

 very poor predictor of age (r 2 = 0.07, n = 75,0.02 <P 

 < 0.05 in a linear regression). 



DISCUSSION 



The results of mark- recapture (Fig. 2) demonstrate 

 that the individuals of at least one population of hard 

 clams along the southeastern coast of the United 

 States can be accurately aged by counting macro- 

 scopic annual growth bands in sectioned shells. The 

 population that we examined exhibited little 

 ambiguity in what constituted an annual band and lit- 

 tle variation among individual clams. These results 

 held true across a wide range of clam ages (1-13 yr) 

 and the aging errors made were not a function of clam 

 age (Fig. 4). Attempts to alter local environment by 

 adding mesh cages of two differing structures also 

 failed to alter the clear pattern of annual band 

 deposition. Nevertheless, because growth rate 

 declines with age (Fig. 8), annual bands came at 

 closer intervals in older clams and were somewhat 

 difficult to resolve inM. mercenaria older than about 

 19 yr of age. An examination of all small M. mer- 

 cenaria collected in February- April 1980 from a loca- 

 tion in Back Sound revealed that only a small 

 percentage (6.7%) of the 0-year class lacked an 

 annual band. Since settlement had almost certainly 



775 



