FISHKRY BULLETIN: VOL. 81. NO. 4 



RETRIEVAL DATE (INITIAL PLANTING O N JUNE 21-22,1978) 

 CAGING 



TREA TMENT OCT 1 7, 1 979 MAY 22, 1980 OCT 8, 1980 OCT 9, 1981 



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NUMBER OF GROWTH BANDS ADDED SINCE INITIAL PLANTING 



FIGURE 2.— The number of growth bands added per hard clam in the increments of marginal shell growth that 

 occurred since planting in the field on 21-22 June 1978. Numbers of bands added were determined for each 

 clam by majority vote of three independent observers, although disagreements in counts were rare (only 2 for 

 October 1979, 4 for May 1980, 3 for October 1980, and 4 for October 1981). All counts include an annual 

 growth band that had just begun to appear in all clams at the initiation of the experiment (2 1-22 June 1978). 

 Each histogram represents the pooled results from duplicate 1 m 2 field plots for each caging treatment. 



of agreement among clams collected on all October 

 dates. In the May 1980 collection, 25 of 29 clams 

 showed 2 additional growth bands. Although this 

 represents a slightly higher level of disagreement 

 among replicate clams, an examination of the 

 seasonal pattern of line deposition helps explain this 

 discrepancy. From the temporal data presented in 

 Figure 2, the band appears to be laid down annually 

 and predictably sometime during May to October. 

 Each year's band was evident (although not nec- 

 essarily completed) in virtually all clams collect- 

 ed on the three October dates. Furthermore, the 

 initiation of the band was evident in June 1978 for all 

 152 clams. Thus, the somewhat higher variance in 

 estimated numbers of growth bands added among 

 replicate clams retrieved in May 1980 may be a con- 

 sequence of variable dates of band initiation. All four 

 clams that deviated from the mode of 2 in the May 

 1980 collection had a third band just beginning at the 

 terminal margin of the shell, suggesting that late May 



was the approximate time of initiation of band 

 deposition in 1980. 



In June 1980, we counted annual bands to estimate 

 the age of each of the 152 experimental clams at the 

 time of their introduction into field enclosures. 

 Figure 4 shows the initial age- frequency distribution 

 1) for all those clams that laid down an additional 

 number of growth bands equal to that predicted by 

 the number of additional summer seasons and 2) for 

 all those clams for which additional age was either 

 inaccurately or inconsistently estimated by the three 

 age readers. Clams in this mark-recapture experi- 

 ment ranged in initial age from 1 to 1 0. Because some 

 clams planted as 10-yr -olds were not retrieved until 

 October 1981, we actually examined line deposition 

 in clams up to 13 yr of age. The two size- frequency 

 distributions do not differ significantly (P> 0.05 inx 2 

 tests of independence), implying that aging mistakes 

 did not vary as a function of absolute age. 



Microscopic examinations of acetate peels made 



770 



