]92 J. E. G. Raymont 



following November, since each autumn has a fairly clear peak at 9 mm. The numbers 

 of Macoma larger than 10 mm is very small indeed, but those between 1 1 and 15 mm 

 may be regarded as in their second year of growth. In summer 1946 there was no 

 successful spatfall. A very few Macoma of about 9 mm length (presumably 1 year old) 

 persisted, but all other age groups had disappeared (Table X). 



Scrobiculaha, though never very numerous, grew to a considerable size in North 

 Bay, a proportion reaching 40 mm (Table XI). In July 1944 and 1945 a new brood of 

 Scrobicularia appeared only a few mm long but (as for Macoma) no successful spatfall 

 occurred in summer 1946. A difficulty is that in July 1945 the smallest Scrobicularia 

 included many of 2-3 mm, but also a large number about 8 mm. When the smallest 

 bivalves (less than 10 mm) are separated into 1 mm groups, it appears that the 2-3 mm 

 individuals were O-Group Scrobicularia, and that the 8 mm individuals were probably 

 an older group (Table XI). By November 1944 O-Group Scrobicularia reached a 

 length of some 5-7 mm, and the group of 7-9 mm in the following July would pre- 

 sumably be one-year-old bivalves (Table XI). Thus the spat of 1944 reached about 

 7-9 mm by July 1945, and possibly this was the same year group which attained some 

 16 mm length by the following November, and about 20 mm by the next July (1946), 

 when they would have been two years old (Table XI). If this is a true interpretation, 

 there was some acceleration in growth rate late in 1945 and during 1946. This could 

 well be a result of fertilization of Kyle Scotnish; the general bottom fauna in the loch 

 did not begin to increase clearly until late 1945 (cf. Raymont, 1950). 



As regards Mytilus, only the lower stations showed an appreciable population 

 and even there mussels, especially large individuals, were extremely sparse until 

 November 1945, when a marked increase appeared. The population then consisted 

 mainly of small individuals, the spat of the previous summer. July 1945 figures, 

 however, showed that very little spat had settled by that time. Gauld (private 

 communication) observed very heavy spatfalls of Mytilus over the North Bay area 

 somewhat later in that summer. Some random samplings made in collaboration with 

 Dr. Gauld showed parts of North Bay where tremendously dense patches (even 

 > 100,000 per m^) of very small mussels were present. Measurements on some of 

 these collections in August and September showed the great majority of these mussels 

 were below 5 mm— clearly the summer's spat (cf. also Table VI). Although a very 

 large percentage of this spatfall died off very rapidly, by November 1945 a considerable 

 population of young mussels was still present. By the following July the population 

 had declined markedly (Table XII). 



Comparison of the three July samplings shows that little spat had settled by that 

 time each summer (Table XII). The somewhat higher figure for July 1944 may indicate 

 a slightly earlier spatfall in that year. The 1944 brood grew to about 8 mm by Novem- 

 ber, but thereafter they disappeared (Table XII). It is possible that as they grew larger 

 they became smothered in the soft mud substratum. Frost may also have killed them 

 off during the winter. 



The abundant 1945 spat which appeared in August and September, mostly <5 mm 

 length, attained a length by November 1945 of approximately 9 mm (Table XII). 

 By July 1946, although no new spat had settled, the mussels had definitely established 

 themselves over a considerable part of the shore. Their densities had declined with 

 winter mortality, but there was a fairly rich group ranging from 10 to 35 mm (Table 

 XII). The mode of this group appears to be about 23 mm, and this suggests that the 



