FISHERY BULLETIN: VOL. 80, NO. 1 



and Dickie 1954; Merrill et al. 1966); and surf 

 clam (Ropes and Merrill 1970; Jones etal. 1978). 

 Accordingly, we marked ocean quahogs by cut- 

 ting shallow grooves from the ventral margin up 

 the shell surface using thin carborundum discs 

 mounted on an electric grinder (Ropes and Mer- 

 rill 1970). Two parallel grooves 2 mm apart were 

 cut into each shell to distinguish our marks from 

 shells scratched by natural processes or during 

 dredging (Fig. 1). 



Marking operations were conducted from 26 

 July to 5 August 1978 (Table 1). A total of 41,816 

 ocean quahogs was notched by the previously de- 

 scribed technique. Batches of 3,000-5,000 clams 

 were dredged from within 9 km of the planting 

 site, marked, and redistributed. The method of 

 marking and planting clams was rapid; about 

 1,600 clams were marked per hour. A grid sys- 

 tem based on loran-C coordinates, was used to 

 indicate the location of each batch. Length-fre- 

 quency samples were obtained during the mark- 

 ing phase (Table 1), and 134 small ocean quahogs 

 (19-60 mm) were retained for maturity studies 

 and analyses of exterior and cross-sectional 

 banding. 



An intensive effort to recapture marked indi- 

 viduals was undertaken, 1 calendar year after 

 planting, during 14-21 August 1979 (Table 1). 

 Forty-three hydraulic dredge tows, each of about 

 5-min duration, were completed at the site. A 

 Northstar 6000 11 loran-C set and an Epsco loran- 

 C plotter were used in the systematic search of a 

 20,000 m 2 area. A total of 14,043 ocean quahogs 

 was examined; 74 (0.5%) had been marked. Re- 

 captured specimens were photographed, mea- 

 sured, and frozen intact at sea. A random sample 

 of 126 unmarked ocean quahogs was frozen for 

 length-weight comparison with marked indi- 

 viduals. 



Marked individuals were again recaptured, 

 approximately 2 yr after planting, on 9 Septem- 

 ber 1980 (Table 1). Two dredge tows yielded 

 1,899 ocean quahogs; 249 individuals (13.1%) had 

 been marked. 



Length-frequency measurements were ob- 

 tained from the site during routine assessment 

 surveys in January 1979 and February 1980. 

 Sampling within 10 km of the site was historic- 

 ally serendipitous; catch data were available 

 from four surveys between 1970 and February 

 1978 (Table 1). Lengths of ocean quahogs taken 



near the site exhibited a consistent bimodal fre- 

 quency distribution throughout the time-series. 

 Growth rate information from the mark-recap- 

 ture and shell banding experiments was thus 

 compared with that generated from modal pro- 

 gression in sequential length frequencies. 



A random sample of 278 ocean quahogs taken 

 from the site during February 1980 was frozen 

 whole for length-weight comparison with the 

 August 1979 sample. Small ocean quahogs (<60 

 mm) were also frozen intact for analysis of the 

 timing of periodic band formation in the shells. 



LABORATORY STUDIES 

 M ark- Recapture 



Recaptured specimens were thawed but kept 

 moist during all phases of analysis to prevent 

 shell cracking and disintegration of the perio- 

 stracum. A total of 67 of the 74 specimens recap- 

 tured in 1979 and 200 of 249 specimens recap- 

 tured in 1980 were suitable for growth analysis; 

 the remaining samples were either shell frag- 

 ments or from quahogs obviously dead when re- 

 covered. Shells were measured to the nearest 

 0.01 mm, using calipers or dissecting microscope 

 equipped with an ocular micrometer. Perio- 

 stracum obscured the shell edge of most speci- 

 mens and was subsequently removed from the 

 vicinity of the mark prior to measurement. Shell 

 lengths were obtained by pressing the perio- 

 stracum against the valves with calipers. 



Growth increments of recaptured ocean qua- 

 hogs were determined as the linear increase in 

 shell dimension along an imaginary line passing 

 through the umbo and equidistant between 

 grooves that formed the mark (Fig. 1). The linear 

 distance between the umbo and shell edge at the 

 mark was designed as h'\ shell length at marking 

 was computed for each quahog by: 



Oi-/f — o/v(*l 



[ 





{h'm - h',) 



] 



(1) 



"Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



where SL, = shell length (longest linear dimen- 

 sion) at marking, 

 SLm = shell length at recapture, 



h\= linear measurement between 

 umbo and edge of the shell equi- 

 distance between grooves, at 

 marking, 

 h'tn = linear measurement between 

 umbo and edge of the shell 



24 



