Table 3. — Age structure and average length {mm.) at time of ring formation in sample of 351 sea scallop shells determined 



independently by 2 readers 



OBJECTIVITY OF THE CRITERIA 



To test whether our criteria for determining 

 which annuli were annual rings were sufficiently 

 objective, a summer assistant, William Evoy of 

 Reed College, was trained in the reading technique. 

 He was then given a sample of 351 shells to age 

 and measure, after which the same sample was 

 read by the senior author. 



The correspondence of results was very good 

 (table 3). There was disagreement on the num- 

 ber of rings, but never by more than one, on about 

 10 percent of the shells and a few additional dis- 

 agreements as to position of the annual ring. 

 The growth-rate equations calculated from the 

 two sets of data were virtually indistinguishable, 

 as shown in the following formulae: 



Z,+, = 37.49 + 0.7384Z, 



Z (+1 =36.75 + 0.7467Z, 



SUMMARY 



The shell of the sea scallop, in common with 

 many other mollusks, bears annual rings but they 

 are frequently weak and ill-defined or masked by 

 the presence of other annuli caused by nonannual 

 phenomena. By using annual marks on the 

 resilium, changes in shell curvature, changes in 



color pattern, weight of the shell, and areas of 

 attack by boring organisms it is usually possible 

 to localize the position of the annual rings so that 

 they can be found by intensive examination. 



These methods have been validated by showing 

 that a growth rate calculated from the annual 

 rings was almost identical with a growth rate 

 calculated from the growth of tagged and recap- 

 tured animals from the same area. 



LITERATURE CITED 



Dickie, L. M. 



1955. Fluctuations in abundance of the giant scallop, 

 Placopecten magellanicus (Gmelin) in the Digby 

 area of the Bay of Fundy. Journal of the Fisheries 

 Research Board of Canada, vol. 12, No. 6, pp. 

 797-857. 

 Lindner, Milton J. 



1953. Estimation of growth rate in animals by 

 marking experiments. U.S. Fish and Wildlife 

 Service, Fishery Bulletin 78, vol. 54, pp. 65-69. 

 Posgay, J. A. 



1950. Investigations of the sea scallop, Pecten 

 grandis. Third report on investigations of methods 

 of improving the shellfish resources of Massachusetts. 

 Commonwealth of Massachusetts, Department of 

 Conservation, Division of Marine Fisheries, pp. 

 24-30. 



1953. Sea scallop investigations. Sixth report on 

 investigations of the shell fisheries of Massachusetts. 

 Commonwealth of Massachusetts, Department of 

 Conservation, Division of Marine Fisheries, pp. 

 9-24. 



310 



U.S. FISH AND WILDLIFE SERVICE 



