24G THE CONDITIONS OF LIFE IN THE SEA [PART III 



somewhere in the Arctic regions, and that as the season progressed 

 these fishes gradually migrated to the south. It is now fairly 

 certain, however, that the herring which appear off the east coast 

 of Britain are mostly local in their origin; and that they form 

 shoals at certain times in the year for the purpose of breeding. It 

 is only when these shoaling movements are taking place that the 

 herring are sufficiently crowded together to allow of the practice 

 of the methods of fishing. But it is now nearly certain that the 

 shoaling migrations are to be associated with changes in the salinity 

 and temperature of the sea water. If the reader will refer to a 

 chart of the positions of the great summer herring fisheries ^ and 

 then compare this with the chart of the temperature and salinity 

 of the sea during each successive month, he will just be able to see 

 that as the isotherms 13° C. to 15° C. pass to the south, that is as 

 the Atlantic water of the European stream passes to the south of 

 the North Sea from the north, so also does the position of the 

 maximum herring fishery change. But the correspondence is not 

 an exact one, and one must not conclude, on this account, that there 

 is no connection between the temperature and salinity of the sea 

 and the shoaling of the herring. It is far more probable that the 

 lack of exact correspondence is due to the defective fishery statistics, 

 on the one hand, and the infrequency of the hydrographic obser- 

 vations, on the other. 



(2) Winter herring in the Skagerak. The winter herring 

 is a fish which does not inhabit the Atlantic water proper (salinity 

 35 per 1000 and over) but a mixture of this with coast water, 

 which is called "bank-water," and which has a salinity of about 

 32 to 33 per 1000. When this layer of bank-water is abundant 

 in a sea-area then the winter herring is also abundant ; and when 

 the bank-water is scanty, or absent, then the winter herring is also 

 rare, or absent altogether. Such a scarcity of bank-water may be 

 due to the exceptional flooding of the sea-area with the Atlantic 

 water from surface to bottom, and when this occurs the winter 

 herring fishery is a failure^. 



^ See Kj'le, Bulletin Statistique, Cons. Perm. Internat. Explor. Mer, vol. i. 

 1906, where there are charts of the situation of the herring fishery, from month to 

 month, during the year. The hydrographical data may be found in the Bulletins des 

 Besultats, also published by the International Council for Fishery Investigations. 



2 Pettersson, Bappts. et Proc.-Verhaux, vol. iii. Appdx. A, 1905. 



