FISHERY BULLETIN: VOL. 81. NO. 4 



small percentage of M. mercenaria in the Cape 

 Lookout region by using the technique of counting 

 internal growth bands. 



Figure 5 presents the size-frequency distributions 

 of all unmarked M. mercenaria collected from the 

 fixed 1 m 2 plots in Middle Marsh in both falls 

 (pooled) and both springs (pooled) along with the size 

 (length)-frequency distribution at first band for allM. 

 mercenaria collected in our samples from nearby 

 Johnson Creek. Because M. mercenaria > 0.1 cm long 

 could not easily invade our fixed 6 mm mesh 

 enclosures, virtually all unmarked clams collected in 

 the sampling enclosures are recruits. Recruitment 

 occurs during the summer months in North Carolina 

 (Chestnut 1952; Ansell 1968). Consequently, the fall 

 size- frequency distribution (Fig. 5) represents the 

 fall sizes of the 0-year class, truncated at 0.7 cm 

 because smaller clams are not efficiently retained on 

 our 6 mm sampling mesh and extended to larger size 

 classes by inclusion of some recruits from previous 

 year classes that were missing during sampling. The 

 spring size-frequency distribution (Fig. 5) contains 

 those 0-year class recruits that were missed and, 

 therefore, not removed during the previous fall's 

 sampling or that settled late (after September- 

 October) plus some larger recruits from other year 

 classes that were missed in previous years' 

 sampling. 



The spring size-frequency distribution given in 

 Figure 5 is biased towards smaller size classes, rela- 

 tive to the natural spring distribution of 0-year class 

 M. mercenaria near Cape Lookout, because the pre- 

 vious fall's sampling already removed the larger sizes 

 preferentially. Despite this bias, the sizes at which 



0-1 



1-2 2-3 3-4 4-5 5-6 6-7 7- 

 LENGTH (cm) 



8-9 9-10 



the first annual band was deposited in the Johnson 

 Creek clams resemble the fall size-frequency dis- 

 tribution of 0-year class recruits much more closely 

 than the spring size- frequency distribution (Fig. 5). 

 This helps confirm the accuracy of our recognition of 

 the initial growth band inM. mercenaria from Cape 

 Lookout. A comparison of the fall size-frequency dis- 

 tribution of 0-year class clams and the distribution of 

 size at first band (Fig. 5) also suggests that the first 

 annual band may be deposited somewhat later in the 

 season (perhaps October-November) than the sub- 

 sequent bands (June- October in our earlier mark- 

 recapture data). 



The size-frequency distribution of unmarked clams 

 in spring (Fig. 5) demonstrates that virtually all new 

 recruits in this system have grown sufficiently large 

 to have been efficiently sampled in our late winter 

 dredge sampling of Back Sound. Dredge sampling 

 efficiently captures clams down to 0.5 cm long (see 

 Appendix), and Figure 5 demonstrates that even in 

 this spring size-frequency distribution, which is 

 biased towards the smaller size classes, a very small 

 proportion of the 0-year class in the Cape Lookout 

 region is ±1.0 cm long. 



Application of Aging Technique to a 

 Field Population 



Because the size (length)-frequency distributions 

 of M. mercenaria collected on the two sampling dates 

 did not differ significantly (0.10 < P < 0.20 in a x 2 

 contingency test), we pooled all samples to form 

 estimates of the size- and age-frequency dis- 

 tributions of M. mercenaria at Johnson Creek in 

 January-February 1980. Average clam density from 



20 i 



s 

 <t 

 _l 

 o 



feio 



DC 



UJ 



CD 



s 



z 5-1 





— 



FlOliRK 6. — Size (length)-frequency distribution for all 124 Mer- 

 cenaria mercenaria collected during 2 d (16 January and 15 Feb- 

 ruary 1980) of sampling from Johnson Creek in Core Sound, N.C. 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 

 AGE CLASS 



FIGURE ".—The distribution of 124 clams collected in January- 

 February 1980 at Johnson Creek, Core Sound, N.C. into age 

 classes. Age class was estimated for each clam by counting the num- 

 ber of annual growth bands and subtracting one (assuming that each 

 new recruit laid down its first annual band in its first fall). Average 

 age class is 8.59 and, assuming that settlement occurred in July- 

 August, average age is 9.09 yr. 



774 



