Da KIN AND Latarche — The Plankton of Lough Neagh. 85 



extensive Lough Neagh drained through the now streamless valley at 

 Poyntzpass to Newry, and so into Carlingford Lough ; and at another stage 

 by Monaghan, Smithborough, and Clones into the valley of the Erne. Still 

 later, Lough Neagh, which was then continuous with the Lake Belfast of the 

 Geological Survey, drained through the Dundonald valley from Belfast to 

 Newtownards, and so into Strangford Lough." We may assume that at 

 the beginning of these times, Mysis oculata occurred in the seas connected 

 with the lake. Perhaps it was even there that Mysis oculata underwent its 

 modification into Mysis oculata var. relicta. 



One important point, however, is the connexion between Lough Neagh 

 and the valley of the Erne. This probably means one origin for the Mysis 

 relicta in both Lough Neagh and Lough Ei-ne ; and it may be that Lough 

 Erne derived the Schizopod from Lough Neagh, or that Lough Neagh derived 

 it from the sea, via Lough Erne. 



The PoUan. 



It is impossible to conclude this account of the plankton of Lough Neagh 

 without some reference to the fish fauna which, though forming no part of 

 the plankton, is so closely bound up with it. Whether the plankton is looked 

 upon as the direct or indirect food of the larger aquatic organisms, there is 

 no doubt that it forms an important and essential link in the chain. More- 

 over, it must be regarded in some cases as a source of danger to fishes. In 

 Lough Neagh we have seen there is present very much plankton, there is 

 much phytoplankton and correspondingly much zooplankton. There is 

 abundant evidence, too, that the water is well provided with organic 

 substances in solution. 



We should therefore expect a large quantity of fish. That this is the case 

 is well known, for there is a famous fishery of one fish alone which provides 

 work for many people, and as the following figures will show is of great value. 

 The most famous fish of the lake is the Pollan. This is a species of sahnonoid 

 fish of the genus Coregonus. It is rather like a herring in appearance, but 

 with the small dorsal adipose fin of the Salmon. The fish occurs in huge 

 numbers, and is caught with nets. The species Coregonus pullan of Lough 

 Neagh is peculiar to the lake. Two other Pollan occur in Ireland, the Lough 

 Erne Pollan {Coregonus altior) and the Shannon Pollan (Coregonus elegans). It 

 is rather curious that in Loch Lomond, where we have noted many plankton 

 resemblances to Lough Neagh, there is a fish, the Powan, Coregonus clupeoides, 

 very like the Longh Neagh Pollan, and also caught in large numbers. 



Now it seems rather curious that more work has not been carried out 

 on the Lough Neagh fish, which is of such great importance. The fishermen 



[M 2] 



