MURAWSKI ET AL.: GROWTH OF OCEAN QUAHOG, ARCTICA ISLANDICA 



Table 5.— Ocean quahog shell length-meat weight regression 

 equations, and analysis of covariance for August 1979 and Feb- 

 ruary 1980 samples taken near lat. 40°25' N, long. 72°24' W, in 

 the Middle Atlantic Bight. 



20 40 60 80 100 120 

 SHELL LENGTH ( MM ) 



"P<0.01; n.s. = P>0.05. 



ventral margin when the periostracum was re- 

 moved (Fig. 1). A growth interruption was pro- 

 duced at the previous shell edge of small speci- 

 mens; new material was formed slightly below 

 the earlier shell margin and was shinglelike in 

 appearance (Fig. 1). Growth in larger ocean 

 quahogs was less distinct and thus more diffi- 

 cult to measure. Where clear growth interrup- 

 tions were not present, a faint yellowish band 

 contrasting with white shell material was inter- 

 preted as a marking-induced check and growth 

 was measured from that point. Shell growth was 

 assessed midway between grooves that formed 

 the mark since, in the case of larger specimens, 

 the depth of the grooves was actually greater 

 than the amount of new shell deposited (Figs. 1, 

 2). 



A total of 11,658 ocean quahogs was measured 

 directly from dredge catches at the marking site 

 during 1970-80 (Table 1; Figs. 5, 6). Although 

 minimum spacing of bars or rings in the rear 

 portion of dredges varied somewhat (Table 1), 

 size selectivity was apparently not significantly 

 altered. Repeated tows were made at the mark- 

 ing site during August 1979 with 25 X 25 mm 

 and later 51 X 51 mm wire mesh in the after por- 

 tion of the dredge. Size distributions of ocean 

 quahogs were nearly identical before and after 

 the alteration. A possible explanation for the 

 lack of differential selectivity is that shell, sand, 

 and live invertebrates may have clogged the 

 dredge at the beginning of tows, negating fur- 

 ther filtering ability. 



Two discrete length-frequency modes were ex- 

 hibited in all sets of samples (Figs. 5, 6). Few 

 small ocean quahogs (<50 mm) were encoun- 



FlGURE 5. — Length-frequency distributions (1 mm intervals) 

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

 the Middle Atlantic Bight, April 1976-February 1980. 



29 



