The fauna of an inter-tidal mud flat | 95 



the lowest station of Transects 2 and 4 was quite ddinitcly avoided— A/acowa 

 occurred only once there during the whole period of sampling. The density of Macoma, 

 even in comparatively rich zones, appears rarely to exceed 200-300 per m^ and North 

 Bay would therefore be quite a dense area. However, Thamdrup recorded up to 

 1,000 per m- at Skalling, and Fraser far higher densities in the Mersey Estuary 

 (Table XIV). 



In general the great majority of individuals in the higher counts in North Bay were 

 young (< 10 mm) ; a particularly striking example was the maximum count of 428 per 

 m^ of which only four exceeded 10 mm. Densities of individuals ,10 mm rarely 

 rose to 60 per m^ Wohlenberg (1937) found >6,000 0-Group Macoma at Sylt. 

 Similarly the 6,300 individuals per m- recorded at Skalling (Table XIV) were all 

 young. Smidt found even 16,000 0-Group Macoma per m- in the same area, though 

 he also showed that the stock was reduced to some hundreds per m- in a few months. 



Stephen (1931) believes that Macoma breeds about April-May, young individuals 

 appearing by June and reaching a length of some 5 mm by the end of their first autumn. 

 By the end of their second autumn, Stephen found an average length of 8-9 mm, and 

 he believes that a heavy mortality occurs after the third summer. The growth rate of 

 Macoma in North Bay would seem to parallel closely Stephen's results. Young spat, 

 2-4 mm, appeared in the July samplings and reached about 5-6 mm by the first 

 autumn (November) and some 9 mm by the following autumn. The conditions for 

 growth in North Bay would appear to be favourable. Brady (1943), for collections in 

 June, gives an average length of 6 mm for I-Group and 12 mm for II-Group Macoma. 



Thamdrup (1935) records Scrobiculaha at the lower levels at Skalling. Smidt and 

 Wohlenberg found it occasionally in soft deposits, and Stephen (1930) records it 

 in soft sticky mud in the Firth of Forth. In North Bay this bivalve was widespread 

 in the soft substratum, though it definitely avoided the lowest tidal levels. Thus it was 

 never taken at the lowest station in Transect 2, only once at the lowest station of 

 Transects 1 and 4, and only twice at the lowest station in Transect 3. Spooner and 

 Moore (1940) also indicate that Scrobicularia is commoner in the upper half of the 

 inter-tidal zone. Except for one extraordinarily dense patch of 1,000 per m- recorded 

 by Spooner and Moore (1940), the densities of Scrobicularia would not appear to 

 exceed 300 per m- (Table XIV). Spooner and Moore describe the Tamar as a rich 

 Scrobicularia area, and they regard this species as generally abundant in sheltered 

 inter-tidal areas in the South of England (cf. also Holme, 1949). It would appear that 

 North Bay was at least equally rich (Table XIV). The higher densities recorded by 

 Spooner and Moore showed a high proportion of young spat. This was also true of 

 the North Bay samples, but counts of 100-150 larger individuals per m- were quite 

 common. Spooner and Moore do not give details of growth rates, but they remark 

 that specimens <12 mm in length include both O-Group and I-Group. This would 

 agree reasonably with the suggestion (p. 192) that in North Bay Scrobicularia grew 

 to about 6 mm by the end of their first autumn, to about 8 mm by the following July, 

 and to some 16 mm by the end of the second autumn when some acceleration is 



postulated. 



Spooner and Moore have suggested that Macoma and Scrobicularia may compete, 

 since they seem to have similar feeding habits. The population of Scrobicularia in 

 North Bay, while not very large, was about as rich as that of the Tamar Estuary, and 

 possibly therefore Macoma was to some extent excluded from North Bay. 



