The fauna of an inter-tidal mud flat |93 



9 mm peak of November 1945 has now shifted to the 23 mm peak of July 1946. This 

 successful 1945 brood would therefore appear to have grown more than 20 mm in 

 approximately one year. The growth rate is higher than that suggested for Macoma 

 and for Scrobicularia, but an acceleration was suggested in the growth of Scrohicu/aria 

 over the same period. The success of the 1945 brood of Mytilus and the considerable 

 degree of colonization of the North Bay may be due in part to the general rise in 

 productivity with fertilization noted at that time (Raymont, 1950). 



For Cardium, an early spatfall occurred in summer 1944, the majority of cockles 

 in July being about 2 mm. As for Mytilus, the population of Cardium of more than 



10 mm length was generally extremely sparse over North Bay in 1944-45 (Table XIII). 

 It would appear therefore that 1944 was an early spat year but that the brood was 

 unsuccessful. 



In 1945, spat was more abundant in the November samples than in July, indicating 

 that the majority of the summer brood had not settled by mid-summer. Cardium 

 also followed Mytilus in that 1945 appeared to be a much more successful year; over 

 1,000 small cockles per m- had settled by November 1945. All these were less than 

 10 mm length, and the great majority were in the smallest length class (1-3 -5 mm) 

 (Table XIII). A few cockles occurred at this time in the 10-13 mm group, and these 

 may represent a few individuals of the I-Group, i.e. those which had settled in the 

 previous July. By July 1946 a very heavy mortality had occurred, and there was no 

 real spatfall. There was, however, for the first time, some indication of a spread of 

 the somewhat larger cockles (14-25 mm) which probably represented the spat of the 

 previous summer. The numbers, although small, suggest a fairly sharp break in the 

 size distribution between 10 and 13 mm (Table XIII). The numbers of cockles between 

 10 and 28 mm taken from all six stations added together were: 



10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 (Length (mm)) 

 0024 20 16 24 886822865233 (No. per m^) 



This suggests a modal length of about 14-16 mm, indicating a fairly successful brood 

 from the previous summer (1945) which has averaged 15 mm growth in a year. This 

 growth would be an improvement on that for Macoma and Scrobicularia. but it was 

 not as rapid as that of Mytilus. 



There is undoubtedly a considerable degree of overlap between this brood and an 

 older age group (20-28 mm). A second peak possibly occurs about 23-24 mm, and 

 this would therefore represent a two year old group of Cardium. The very low figures 

 for July 1944 might also indicate one group of cockles varying in size from 18-21 

 mm (II-Group) and another group of 10-13 mm (I-Group). An enhanced growth rate 

 in 1945 would agree with the results on other lamellibranchs. 



It seems clear that 1945 was a fairly successful year for both Cardium and Mytilus, 

 and that some spread of these bivalves occurred over the North Bay. Although part 

 of the success of the 1945 brood may be correlated with better feeding conditions due 

 to fertilization, the bivalves still experienced the difficulty of establishing themselves 

 for any length of time on a muddy shore, subject to prolonged exposure. 



DISCUSSION 

 The fauna of North Bay is a Macoma halthica community, and corresponds well 

 to that studied by Smidt (1951) in the Danish waddens. The polychaele and lamelli- 

 branch species of the infauna are very similar, with Hydrobia ulvae as the dominant 



H 



