FISHERY BULLETIN: VOL. 80, NO. 1 



over the range of shell sizes used to fit the func- 

 tions: however, estimated lengths at age con- 

 verge near the sizes of the smallest recaptured 

 specimens. Estimated lengths at age 20 were 

 64.49 and 64.29 mm, respectively. Correspond- 

 ing growth increments from age 20-21 were 1.06 

 and 0.84 mm, well within the range of ob- 

 served growth for those sizes (Fig. 2). If calcu- 

 lated lengths at age 20 are assumed to be the 

 starting points for the Ford-Walford equation 

 (SL,n = 2.0811 + 0.9802 SL t ), the two acceptable 

 exponential equations yield virtually identical 

 growth curves when the Ford-Walford relation- 

 ship is iterated. Additional growth analyses 

 were conducted using the regression equation 

 fitted to weighted mean back-calculated lengths 

 for all ages because the maximum amount of in- 

 formation was used and the equation's behavior 

 in the vicinity of marking data was consistent 

 with empirical observations. However, further 

 research on the growth patterns of small ocean 

 quahogs is indicated in order to resolve differ- 

 ences between various data subsets in Table 3 

 and thus to define a more appropriate growth 

 model for these sizes. 



A composite growth curve incorporating the 

 aged samples and mark-recapture data is given 

 in Figure 4. The Ford-Walford equation was 

 iterated to age 100 and a predicted shell length of 

 96.91 mm. Although ocean quahogs reach a size 

 of at least 117 mm in the vicinity of the marking 

 site (Table 1), ages substantially in excess of 100 

 are not necessarily implied because of the statis- 

 tical variability in the marking data used to fit 

 the predictor (Fig. 2). Annual growth in shell 

 length is rapid during the first 20 yr of life, but 

 declines significantly thereafter. Average yearly 

 shell growth is 6.3% at age 10, 0.5% at age 50, and 

 0.2% at age 100. 



Estimates of the von Bertalanffy parameter to 

 (age at zero length) were computed as -27.29 yr 

 and -27.62 yr for the BGC4 and annual incre- 

 ment equations respectively, with SL20 = 64.49 

 mm (Gulland 1969, equation 3.5). Although pre- 

 dicted lengths at ages >20 are similar to those in 

 Figure 4, a relatively poor fit to younger ages re- 

 sults from both von Bertalanffy equations. 



The validity of using the age-length functions 

 given in Figure 4 to describe ocean quahog 

 growth at the marking site can be assessed by 

 comparing predicted growth to that from modal 

 progressions in length-frequency samples. Fre- 

 quency distributions from 1976 to 1980 exhibit 

 inter-sample variability in the position of major 



modes but no progressive shifts are discernible 

 (Fig. 5). However, expected growth during the 5- 

 yr period (Fig. 4) was smaller than could prob- 

 ably be identified, given the precision of length- 

 frequency sampling (Table 1; Fig. 5). Length 

 modes can be used to compute growth at the site 

 between August 1970 and February 1980 (Fig. 

 6). Average growth of the smaller mode (52 mm 

 in 1970) was about 13 mm, and the larger mode 

 (87 mm in 1970) added about 3 mm shell length 

 during the 9Y 2 -yr interval (Figs. 5, 6). Ocean qua- 

 hogs 52 mm in length are about 12-yr-old and 

 average 21-yr-old at 65 mm; the estimated age of 

 87 mm individuals is 60 yr and 90 mm quahogs 

 average 70-yr-old (Figs. 3, 4). Thus, predicted 

 growth during the period 1970-80 is strikingly 

 similar to that inferred from length mode pro- 

 gressions, implying that age analyses and mark- 

 recapture data adequately describe historical 

 ocean quahog growth at the site. 



The age-length relationships presented herein 

 have been computed for shell sizes in excess of 95 

 mm and ages up to 100 yr. However, computed 

 relationships for large sizes (>65 mm) are based 

 on average growth rates from mark-recapture 

 results and not from aging of individual speci- 

 mens. It is likely, based on these analyses, that 

 ocean quahogs do reach 100 yr in age; however, 

 direct age determination of large individuals is 

 contingent upon development and validation of 

 suitable methodologies. Internal banding pat- 

 terns present in shell cross sections were useful 

 in aging small specimens since formation of the 

 bands apparently occurs once annually. Seasonal 

 shell formation patterns (Jones 1980) and age 

 analyses of large individuals based on internal 

 banding (Thompson et al. 1980; Jones 1980) are 

 generally consistent with our data. Analysis of 

 shell cross sections of large recaptured speci- 

 mens may be useful in determining the periodi- 

 city of internal banding and the validity of the 

 aging technique for large ocean quahogs; study 

 of this material continues. 



The regressions of shell length vs. drained 

 meat weight for marked and unmarked ocean 

 quahogs taken during August 1979 were not sig- 

 nificantly different in slope or adjusted mean 

 (Table 4). If in fact soft-tissue robustness is a 

 valid index of relative condition, then marked in- 

 dividuals apparently suffered no lasting effects 

 from the stress of dredging and handling. This 

 observation is supported by the conclusions that 

 incremental shell growth of marked specimens 

 was similar to that computed from progressive 



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