FISHERY BULLETIN: VOL. 80, NO. 1 



equidistant between grooves, at 

 recapture. 



Marginal growth in shell length was thus equiv- 

 alent to the bracketed term. 



Implicit in Equation (1) is the assumption that 

 ratios between the linear parameters SL and h! 

 did not change between marking and recapture 

 (isometric growth). The assumption is supported 

 by comparisons of various standard shell dimen- 

 sions (i.e., shell length, height, and width, Chan- 

 dler footnote 6; Northeast Fisheries Center 

 Woods Hole Laboratory unpubl. data), particu- 

 larly considering the relatively small percent 

 changes in shell size between marking and re- 

 capture (Table 2). 



60 



70 



80 



90 



100 



SHELL LENGTH ( MM ) L 



Table 2.— Growth of ocean quahogs marked during August 

 1978, and recaptured during August 1979 (n = 67), and Septem- 

 ber 1980 (n = 200), at lat. 40°25' N, long. 72°24' W, in the Middle 

 Atlantic Bight. 



Three methods were used to fit growth equa- 

 tions to mark-recapture data. For ocean quahogs 

 recovered 1 calendar year after marking, length 

 at recapture was related to length at marking 

 using Ford-Walford and linear annual increment 

 plots described by Gulland (1969; Fig. 2). Ad- 

 ditionally, a nonlinear exponential equation was 

 fit to increment data and results compared with 

 those assuming the von Bertalanffy model. The 

 von Bertalanffy parameters L^ and K were also 

 estimated using the BGC4 computer program 

 ( Abramson 1971 ). The program was designed for 

 determining growth parameters when lengths of 

 unaged individuals are known at two points in 

 time, based on the algorithm of Fabens (1965). 



Equations derived from mark-recapture data 

 can be used to describe relative growth from an 

 arbitrary point in time (i.e., SL M , SL t , 2 , ... 

 SL,.„), but without at least one independently 

 derived age-length observation, absolute growth 

 curves cannot be established. Accordingly, anal- 

 yses of external banding patterns of small ocean 

 quahogs were critical in "fixing" growth curves 

 from mark-recapture. 



Figure 2.— Relation between calculated increment of growth 

 in shell length (millimeters) and initial length for ocean qua- 

 hogs marked during July- August 1978 and recaptured during 

 August 1979 near lat. 40°25'N, long. 72°24'W, in the Middle 

 Atlantic Bight. 



Shell Banding 



Small ocean quahogs retained from the July- 

 August 1978 cruise were analyzed for external 

 and internal shell banding patterns. Sequential 

 growth of individual ocean quahogs was followed 

 by measuring the maximum dimension (shell 

 length) of exterior bands appearing on the perio- 

 stracum, using calipers (Fig. 1). Maximum shell 

 length beyond the last band was also recorded. 

 The opposite valve was sectioned from the umbo 

 to the ventral margin and polished (Saloman and 

 Taylor 1969; Jones et al. 1978). An acetate im- 

 pression of the polished surface was made and 

 mounted between glass slides. Images were en- 

 larged with a microprojector to reveal internal 

 banding patterns. 



Internal lines present in shell cross sections 

 correlated in number and position with external 

 bands when the latter were distinct. The perio- 

 stracum on some shells was eroded near the 

 umbo, obscuring external bands. In these cases 

 "annuli" nearest the umbo were located on the 

 peels, but measurements of shell size could not be 

 made (Table 3). External marks present near the 

 shell margins on some larger specimens also 

 could not be discerned; internal banding was 

 again used to estimate age. Shell length statistics 

 were computed for each age/annulus subclass, 

 weighted lengths at annuli for all ages and 



26 



