FISHERY BULLETIN: VOL. 80. NO. 1 



60 80 



SHELL LENGTH (MM) 



Figure 6.— Length-frequency distributions (5 mm intervals) 

 of ocean quahogs sampled near lat. 40°25'N, long. 72°24'W, in 

 the Middle Atlantic Bight, August 1970 and February 1980. 



tered from 1976 to 1980 (Fig. 5) and, considering 

 uniformity of modes over time, recruitment was 

 probably equally poor during 1971-76. Thus, cor- 

 responding modes in the 1970 and 1980 samples 

 were probably composed of the same year classes 

 (Fig. 6). Average size of the small mode in- 

 creased about 13 mm during the 9%-yr interval 

 between August 1970 and February 1980, while 

 the large group shifted about 3 mm (Figs. 5, 6; 

 Table 1). Size progression of modes was minimal 

 during 1976-80; intersample variation may be 

 primarily related to differential sample sizes 

 (Table 1). The effects of a sevenfold increase in 

 sampling intensity can be seen by comparing 

 August 1979 and February 1980 frequencies. 

 Modes are smoothed in the latter sample, yet re- 

 spective peaks are at precisely the same 1 mm in- 

 tervals in both (65 and 90 mm). Average shell 

 sizes ranged from 71 to 77 mm; however, trends 

 in shell length among samples were not apparent 

 (Table 1). 



The average lengths of recaptured ocean qua- 

 hogs (Table 2) were slightly greater than con- 

 current length-frequency samples (Table 1), 

 although length extremes of the marked indi- 

 viduals were not as great. Recaptured ocean qua- 

 hogs also exhibited the bimodal length-frequency 

 distribution (Fig. 2), indicating recaptured 

 specimens represented a relatively unbiased 

 sample of marked individuals and the ocean 

 quahog population in the immediate vicinity 

 of the study area. Calculated increments of 

 shell growth from ocean quahogs recaptured in 

 1979 ranged from 0.08 to 1.38 mm, and averaged 

 0.56 mm (Table 2). Those recaptured in 1980 



30 



grew an average of 1.17 mm (range 0.07-4.32 

 mm). Thus, incremental growth approximately 

 doubled between summer 1979 and summer 

 1980, implying growth rates were similar dur- 

 ing the 2 yr of the experiment and that marking 

 procedures probably did not significantly dis- 

 rupt growth patterns. Growth increments of 

 ocean quahogs at liberty 1 yr generally declined 

 with increasing shell length, although there was 

 substantial variation about a linear fit (Fig. 2). 

 The linear equation for predicting annual incre- 

 ment of growth from initial length is given in 

 Figure 2; the Ford-Walford equation is: SL m = 

 2.0811 + 0.9802 SL t , where SL is shell length (in 

 millimeters) at age t. An exponential equation 

 fitted to data in Figure 2(7= 14.1216 (exp 

 (— 0.0459X))) explained about 8% more of the re- 

 sidual variance about the predicted line than did 

 the linear equation. However, growth rates im- 

 plied from length-frequency analyses were sub- 

 stantially greater than those from the exponen- 

 tial fit, and were similar to rates computed from 

 the linear (von Bertalanffy) model. Thus, the 

 latter model was considered more valid. Esti- 

 mates of the asymptotic length (LJ and growth 

 coefficient (K) from two fitting methods are: 



BGCU Annual increment 



L^ (mm) 

 K 



107.06 

 0.0195 



104.95 

 0.0200 



Values of L^ from the two methods are >99.5% 

 (BGC4) and 98.5% (annual increment) of the 

 cumulative 1980 length-frequency distribution 

 at the study site. Estimates of K are relatively 

 low and characteristic of slow-growing, long- 

 lived species (Beverton and Holt 1959). 



Analyses of shell banding features present in 

 small specimens indicate both external and in- 

 ternal marks are produced once during the bio- 

 logical year in these sizes. Several of the small 

 recaptured ocean quahogs exhibited concentric 

 external rings, and these specimens formed one 

 such band during the interval between marking 

 and recapture (Fig. la). Studies of small un- 

 marked individuals retained from summer and 

 winter sampling demonstrate that external and 

 internal marks generally correspond in number 

 and position. Internal marks were particularly 

 useful in age determination when external 

 marks were eroded near the umbo or closely 

 spaced at the shell margin. Small ocean quahogs 

 captured during the summer exhibited wide 



