474 SOME RESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 



north of the grounds mentioned, as far to the west as Scotland ; and 

 (2) the southern North Sea area, which comprises the banks in the 

 German Bight. Then we have in addition (3) the region of the 

 Skagerak. The catches of the steamers are expressed as average 

 numbers of pounds of fish caught per steamer per day. These average 

 catches are studied from month to month. Curves are also given by 

 Henking which show clearly how the fishing varies from month to 

 month throughout the year. The result of such analyses of the catches 

 is the defining of " Fishery Periods," which are periods of time during 

 which some particular fish is abundant on a certain ground. Such fishery 

 periods are of course familiar things, and one need not give as instances 

 more than the well-known herring fishery periods of the east coast of 

 Britain or the cod fishery periods of the Norwegian Lofoten Banks. 

 A fishery period is caused by the immigration of great numbers of 

 fishes into relatively restricted areas. Then begins the commercial 

 fishery, which ends with the emigration of the fishes from the fishing 

 ground in question. When the regular variations in the amount of 

 fishes brought from the (known) fishing grounds are studied from month 

 to month, it is seen that the existence of such fishery periods applies to 

 almost every kind of edible fish studied. These fiuctuations in the 

 abundance of fish on the different grounds from time to time through- 

 out the year are due to real migrations of the fishes; either to their 

 migration to different fishing grounds or to their dispersal through the 

 upper layers of the sea, where they are without the reach of the trawl. 

 " In view of the considerable material which forms the basis of our 

 review, it can hardly be considered that the fiuctuations in the curve of 

 the catches is referable to the captains of the boats wishing to 

 avoid the capture of certain species at certain periods of the year ; there 

 remains no other possibility, therefore, than that the number of fish in 

 front of the trawl has actually varied."* 



Into Henking's details of the migrations of food fishes in the North 

 Sea we cannot of course enter, and the reader is referred to the charts of 

 results. These curves when plotted for different regions of the North 

 Sea and Skagerak show that extensive movements of fishes go on. 

 The hake, for instance, is such a migratory fish. For the first three or 

 four months it is hardly at all taken in the North Sea or Skagerak. 

 Then it appears " in ever-increasing numbers, perhaps with the inflow 

 of Atlantic water." The shoals spread over the northern North Sea 

 and appear in dense masses in the Skagerak in June. .In that month 

 the fish is relatively scarce in the southern North Sea. Then later in 

 the year, August to September, when the fish has become less abundant 

 in the Skagerak, it increases in abundance in the northern North Sea 



* Henking, loc. cit., p. 18. 



