FISHF.KY BULLETIN: VOL. 81, NO. 4 



Clams inaccurately or inconsistently aged 



WMWAmrrrr^ 



Correctly" aged clams 



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 AGE AT INTRODUCTION (JUNE 1978) 



FliiURE 4. —The frequency distribution of age at introduc- 

 tion in June 1978 of all clams used in the mark- recapture 

 test of whether growth bands are annual in Mercenana mer- 

 cenaria from Cape Lookout, N.C. Two age distributions are 

 presented: The bottom distribution includes all 137 clams 

 possessing marginal growth that was correctly aged without 

 disagreement among three independent examiners and the 

 top distribution includes all 15 clams with either incorrectly- 

 aged marginal growth or with disagreement among age 

 readers. The two distributions do not differ significantly at 

 a = 0.05 in a x ; test of independence, pooling adjacent age 

 classes where necessary to maintain expectations above 

 unity. 



from the sections of 20 of the October 1980 and 

 October 1981 clams did not reveal any additional 

 repeating patterns in the shell deposition of M. mer- 

 cenaria that might be used as annual markers. 

 Furthermore, the growth band that was so evident in 

 macroscopic view of the polished section did not 

 retain its coloration and textural distinctions on the 

 acetate peels and was thus not as obvious. Numerous 

 finer growth breaks found in acetate peels were not 

 evident in macroscopic view. Some resembled dis- 

 turbance checks (Kennish and Olsson 1975; Kennish 

 1980), occurring only in the outer layer and, in mac- 

 roscopic view, appearing with slight brown dis- 

 colorations incorporated into the shell matrix. 



Many of these possible disturbance checks 

 appeared to be associated with the excavation and 

 measuring of the clams. To document this associa- 

 tion, we examined closely the polished sections of 23 

 clams retrieved in October 1981 and drawn approx- 

 imately equally from the three caging treatments. 

 On the outer surface of each shell, we marked the 

 position of the shell margin (the size) at each of the 

 known measurement dates. Because each clam in 

 this sample (except two that were missed during one 

 sampling) was excavated and measured on five 



occasions, 113 ( 5 X 23 — 2) disturbance lines would 

 be expected from sampling, if the sampling process 

 suffices to produce disturbance checks in the shell 

 matrix. Of these 113 positions on the shells, 96 con- 

 tained clear disturbance checks in the outer shell 

 layer. Only 35 additional disturbance checks were 

 evident in these shells during the period June 1978 to 

 October 1981, and 29 of those coincided with the initia- 

 tion of deposition of the annual band The six remaining 

 disturbance checks were not associated with our han- 

 dling or with annual band deposition, but their pre- 

 sence is not surprising given that natural disturbance 

 breaks have been reported for M. mercenaria else- 

 where (Kennish and Olsson 1975; Kennish 1980). 



Disturbance checks deposited at most times of 

 measuring provided several specific chronological 

 markers. We used these markers together with pre- 

 sumed daily growth lines to estimate the exact period 

 of annual band deposition and relative growth rates 

 within and outside of the period of annual band 

 deposition. Because we had no good test of the daily 

 nature of the presumed daily lines, we chose to carry 

 out these estimates only for shell growth increments 

 where we had an approximately year-long period of 

 growth bracketed by measurement growth checks 

 and containing the expected number (±20) of" daily" 

 lines. 



We examined six growth periods, one on each of 

 four clams and two on a fifth individual, which met 

 our criteria (Table 1). These clams exhibited great 

 variability in date of annual band initiation (June- 

 October) and termination (September-January). 

 However, the period of annual band deposition con- 

 sistently included summer or fall. Average daily 

 growth rate during the period of annual band deposi- 



Table 1. — The period of Annual band deposition and the average 

 daily growth 1 of hard clams during and outside the period of annual 

 band deposition, as estimated by using daily growth lines on acetate 

 peels or on thin sections. The six intervals examined (all but the first 

 two on separate clams) were bracketed by disturbance checks in the 

 outer shell layer that served as known chronological markers and 

 contained a number of daily lines equal to the number of days (±20) 

 between the known dates. 



'Growth was measured by calibrated ocular micrometer in the center of the shell 

 cross section along the axis of growth but convened geometrically to corres- 

 ponding lengths. 



: The annual band was still being deposited on 1 0/8/80, whereas for all other inter- 

 vals examined these dates mark actual initiation and termination dates for band 

 deposition. 



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