D.— ZOOLOGY 97 



remarkable increase in the herring taken on the Murman coast since 1931. 

 The quantities taken in that year and in 1932 and 1933 were respectively 

 23 times, 29 times and 68 times as great as the largest catch in the ten 

 preceding years. 



It is clear that an increased sea-temperature, probably of the order of 

 i-o to 2-o° C, has allowed various species of fish to extend beyond the 

 normal limits of their distribution, with the result that it has been possible 

 to establish productive fisheries in areas which formerly would not have 

 yielded an adequate return. It is evident, I believe, that at some future 

 date conditions will revert to normal and that a time will come when these 

 lucrative fisheries will cease to exist. 



In the present state of our knowledge we can do little more than guess 

 at the reasons for the increased temperature in these areas ; but the only 

 source from which warm water can come is the Atlantic drift, and it 

 therefore appears that in recent years this drift must either have increased 

 in volume, or, if the volume remains constant, in the temperature of the 

 water it carries. 



As you will have seen, I have in this address tried to draw a distinction, 

 which I believe to be a real one, between two kinds of fluctuations, both 

 of which have a pronounced effect on the marine fauna. Normal annual 

 fluctuations are a constant feature. They form the basis of fishery pre- 

 diction and our information, such as it is, is that their incidence is 

 restricted : a fishery for a certain species in a particular place will be 

 affected, while other species in the same place, or the same species in 

 another place will be unaffected. And it is to be assumed that the causes 

 of such annual fluctuations, though of these we know but little, are also 

 restricted both in space and in time. 



In contrast are what I have called long-period fluctuations, which 

 extend over a term of years and involve much larger areas. Such 

 fluctuations as these are due to a widespread change in one or more of 

 the hydrographic factors in the environment, and large numbers of species, 

 if not all, are aff^ected simultaneously or within a short period. Long- 

 period fluctuations may mask the effects of the annual fluctuations and 

 at times they will render fishery prediction unreliable. 



In the illustrations I have given you I have spoken chiefly of fish, 

 because it is of fish that we have best knowledge ; but it will I think be 

 evident that invertebrates are influenced in the same way and I believe 

 it may truly be said that all marine animals show great variations in abund- 

 ance. You will also not fail to note that though these fluctuations are of 

 the greatest economic importance they are equally of very high scientific 

 interest. 



The evidence I have given you indicates that long-period fluctuations 

 may be brought about in entirely different ways. In the Channel, as it 

 appears, the change can be traced to a deficiency in phosphate, while in 

 more northerly areas it is due to an increase in sea-temperature. But, 

 though there is this wide difference, the two sets of circumstances have 

 this in common, that they originate in the open Atlantic, at the edge of the 

 continental slope or farther to the west. It is here, in oceanic waters, 

 that the causes of these large alterations in European fisheries must be 

 sought. 



