196 



J. E. G. Raymont 



Cockles were not very common in North Bay; the density of oWer individuals 

 never exceeded 80 per m-. The much higher figures occasionally recorded showed 

 without exception an overwhelming dominance of young cockles (<10 mm). The 

 densities do not, however, compare unfavourably with those for other areas in Britain, 

 especially as the high populations recorded by Stephen and by Spooner and Moore 

 were composed mainly of O-Group cockles. By contrast, Thamdrup found very 

 much higher densities at Skalling (Table XIV) ; indeed Cardium was the most abundant 

 bivalve on the waddens. The maximum quoted even then (4,675) did not take account 

 of O-Group individuals. Thamdrup showed that several thousand O-Group per m^ 



Table XIV 



Comparison of the densities of some of the members of the fauna in North Bay and in 



other areas. Numbers are maximal densities per m^. Figures in brackets indicate 



extraordinarily high densities recorded only once 



* = Loch Gilp, Scotland. j = Isle of Barra, Scotland. 1: = " abundant "\ no density quoted. 



also occurred, but that the densities declined rapidly during autumn and winter. 

 Smidt and also Wohlenberg found even larger settlements of O-Group cockles: 

 Smidt found 20,000 to 30,000 per m^ quite frequently, and Wohlenberg recorded some 

 40,000 per m^ but he also showed how rapidly those numbers were reduced. 



Stephen (1931) concluded that Cardium bred in summer at Millport, the smallest 

 cockles appearing about August and reaching a length of <6 mm by the end of their 

 first autumn. By the following July these cockles reached 12-13 mm. Brady (1943) 

 states that in June one-year-old cockles had an average length of 11 mm, and Smidt 

 (1951) shows in July one-year-old cockles reaching a length of 12 mm, or about 

 16 mm in a favourable year. 



The results of the North Bay collections, and also the results from Loch Craiglin 

 (Raymont, 1947) strongly support Stephen's view that Cardium spawns in summer, 

 but the growth of the cockles up to November in North Bay was somewhat less than 

 Stephen found. However, the 1945 spat attained a length of about 15 mm in North 



