The fauna of an inter-tidal mud flat 201 



successful stocking involving ca. 1,500,000 fry was made in the spring of 1946 (Gross, 

 1950). To what extent the young flatfish fed over the mud Hat at high tide it is impos- 

 sible to say; nevertheless a considerable increase in the amount of food taken by the 

 fish was inevitable, especially in view of the very high growth rate (cf. Gross, 1950). 



Possibly the drop in the fauna was also partly related to the severe frost in 

 February and March of that year. Thus an extract from the log book states that work 

 was impossible at the head of the loch (i.e. just off North Bay) on 28th February 

 1946, because of ice. The following day the whole of Kyle Scotnish was covered with 

 ice, and this cleared from the northern parts of the loch only on 5th March. The 

 general bottom fauna of Kyle Scotnish below low-tide mark was not severely checked 

 by these frosts (cf. Raymont, 1950) but over an inter-tidal mud flat the bottom animals, 

 except for those burrowing really deeply, would be exposed fully to the severe effects of 

 freezing. 



It is significant that almost all the fauna {Hydrobia, polychaetes, chironomids, 

 oligochaetes, etc.) showed the great reduction in numbers by July 1946, i.e. it was not 

 an unsuccessful spawning or a poor settlement by merely one important species of the 

 community. On the other hand, the population of large bivalves remained almost 

 stationary from November to July, and indeed the final population (125 large bivalves 

 per m^) was higher, except for November 1945, than at any other time (Tables IV, 

 XII, XIII). It is these larger bivalves which can burrow away from the surface, some 

 like Scrobicularia to a considerable depth, and thus escape, to some degree, adverse 

 conditions. By contrast, the youngest bivalves were decimated over the same period, 

 falling from more than 1,000 to 129 per m^ (Table IV). 



In the earher years few large bivalves occurred over North Bay (Tables XII and 

 XIII). Probably these molluscs are periodically greatly reduced in numbers by adverse 

 conditions in the area. The lack of the older bivalves is especially true of Mytilus 

 which is of course a surface dweller. By contrast, Scrobicularia, which burrows deeply, 

 and as older individuals may therefore escape the worst conditions, is not so poorly 

 represented by the older age groups (Table XI). 



The only marked exception to the general seasonal fluctuations, and to the large 

 increase in November 1945, was in the polychaete group (Fig. 4). These animals 

 exhibited a fairly steady increase in population from November 1943 onwards, and 

 they reached an average of 1,200 animals per m- in the summer of 1945, after which 

 there was a very sharp decline to the summer of 1946 (Table IV). These changes 

 were mainly due to Pygospio (Fig. 4). This polychaete was numerically dominant 

 over the whole period November 1943 to July 1946. It also appears to have reproduced 

 in the years 1944 and 1945 sufficiently early for there to be a sharp increase in the 

 polychaete populations in each summer. From November 1943 to July 1944, for 

 example, the population o^ Pygospio more than doubled, and from November 1944 

 to July 1945 there was an increase of 50%. 



Phyllodocidae also contributed to the sharp peak in the summer of 1945 (Fig. 4). 

 The only other polychaete that played a significant part in the overall population 

 changes was Heteromastus filiformis— the second most important polychaete. During 

 1944 there appears to have been a successful brood of Heteromasius, so that between 

 July and November of that year the population more than doubled, but the most 

 surprising change was the very sharp decline in Heteromastus from July 1945, so that 

 the population was virtually wiped out by the summer of 1946. This was also true of 



