200 •'• ^- G- Raymont 



Sween showed fairly clear seasonal effects, even in the bottom fauna below low-tide 

 mark (Raymont, 1947, 1950). But in Loch Craiglin, and in the areas of Kyle Scotnish 

 below low-water mark, the maximum population occurred during the summer, 

 whereas in North Bay the populations in November were consistently larger than in 

 July. In Loch Craiglin, however, the actual months in which the highest densities were 

 experienced were August and September, and a few stations in the loch did show 

 higher densities even in November. In Kyle Scotnish, too, the high " summer " 

 peaks of the bottom fauna represented an average of the three months, July, August 

 and September. It seems clear, as indeed might be expected in a marine area, that the 

 high densities occurred in late summer or even afterwards. 



It is not surprising that the July samplings should give a lower population than the 

 November samples. Most of the young larvae produced during spring and summer 

 will not have settled and grown to a sufficient size to appear in July samplings. For 

 example, Smidt (1951) found that the vast majority of Hydrobia in June belonged to 

 O-Group, and measured only 0-5 mm. Since they grow only some 2 mm by the winter, 

 it is likely that sampling with a 1 mm sieve in July would miss the majority of the 

 brood. The strong seasonal fluctuation is in fact dominated by Hydrobia (Tables 

 IV and V), but the seasonal changes hold also for chironomids, bivalves, oligochaetes 

 and " other organisms ". Chironomids may be expected to oviposit mainly during the 

 summer, and therefore the new larvae will hardly appear in sievings before early 

 autumn. It has also been shown that the main spatfalls of Cardium and Mytilus occur 

 after July. During the ensuing winter and spring there will be heavy depredations on 

 the bottom fauna, and also the physical conditions (especially frost) over winter will 

 cause the fauna to decline. Little recruitment may be expected before July in the 

 following year. 



The results from November 1943 to July 1945 indicated then no real change in the 

 overall population, apart from these seasonal fluctuations. This is borne out by both 

 the average populations (Table IV) and by the results from individual stations (Table 

 VIII, Fig. 3). The rise in November 1945, however, was definite and widespread. The 

 average increase was 40%, and a large number of individual stations showed a rise 

 (Fig. 3, Table VIII). 



A rise of only 40% could easily result from a " natural " fluctuation, i.e. it might 

 be attributed to some favourable condition other than increased food supply. Wide 

 variation in the success of year-groups of bottom animals are well known (Boysen- 

 Jensen, 1919; Thamdrup, 1935; Wohlenberg, 1937; Stephen, 1931, 1932, 1938). 

 Nevertheless it is usual then for some species to show one particular year as a good 

 brood year, while for other species that same year may be very poor (cf. Boysen- 

 Jensen, 1919, for species of bivalves). The fact that so many of the main constituents 

 of the fauna over North Bay showed an increase at the same time (Table IV), lends 

 considerable support to the view that the rise in November 1945 was due to the 

 improved conditions resulting from fertilization. The accelerated growth rate seen 

 in some lamellibranchs (p. 192-193) would also agree with this conclusion. 



By July 1946 the average population over the North Bay had declined sharply from 

 the previous high autumn density to reach the lowest value for the period of sampling. 

 This marked fall may be largely explained as a seasonal change as in previous years. 

 It is not easy, however, to account for the especially severe drop in the density of the 

 fauna in 1946. Kyle Scotnish had been stocked with plaice fry in 1945, and a more 



