1912] on The North Sea and its Fisheries. 419 



The trawlers that seek the coasts of the Spanish Peninsula and of 

 Morocco find in these warm waters a fishery totally unlike that of 

 the North Sea ; while, on the other hand, our temperature curves 

 make it plain and easy for us to understand how the North Sea has 

 common attributes with regions so far off as the White Sea itself, 

 with Iceland and Newfoundland and the Eastern States, and how 

 our staple fishes, such as the cod, the haddock, the plaice and hali- 

 but, and the herring itself, find their extensive distribution in all 

 these remote, but not dissimilar seas. 



Lastly, ere we leave this matter of temperature, let me point out 

 to you that the ocean not only acts in this part of the world as a 

 warming influence, but also here and everywhere has a great steadying 

 influence upon the temperatures. In another chart I show, not the 

 mean temperatures, but the range of temperature, the difference be- 

 tween the summer heat of the sea and its winter cold. A little way 

 west of Ireland the annual range of temperature is but 4°, and in 

 Shetland it is but 6° ; but the further we go into the narrow seas the 

 more violent is the seasonal fluctuation, the greater is its range, till 

 down in the German bight you have a range of at least 12 or 14° C, 

 or 30 to 35° F. The water there is far colder in winter than in 

 other parts of our sea. But there comes a great compensatory 

 warmth in summer, which again has its influence in favouring this 

 region as a nursery for young fish. 



The problem of salinity, the distribution of the amount of salt 

 in the sea, is a laborious one to investigate, but, as far as the North 

 Sea goes, its main results are easy to understand. Some of you will 

 see at a glance from this chart of Isohalines, how beautifully simple 

 the arrangement is, and how perfectly it is in accord with the distri- 

 bution of the three gateways of the sea, the two inlets of salt water, 

 and the great Baltic source of fresh. But the further study of the 

 salinity of the North Sea is very complicated indeed, for the mean 

 condition which my chart represents is subject to change, and the 

 changes are partly regular or periodic, and partly irregular and 

 obscure. There is a constant battle, as it were, between the quan- 

 tities of fresh water, on the one hand, that the Baltic sends in, and 

 the rivers bring down (for the former source especially tends to be 

 dried up when the inland sea is frozen in wintertime), and the vary- 

 ing supply of salt water from the ocean, for even the great ocean 

 currents have their annual pulse, their ebb and flow. In the summer 

 time over great part of the North Sea, water of low salinity spreads 

 from the Baltic, and such changes as this have, we have every reason 

 to believe, their close and intimate bearing on the migrations of the 

 herring. 



Lastly, together with these physical phenomena of salinity, tem- 

 perature and current, we study the distribution of plankton, as it is 

 called nowadays, the floating Ufe of the sea. On his great voyage 

 across the ocean, Darwin himself spoke of it as a weary waste of 

 YOL. XX. (No. 106) 2 P 



