THE SEA-FISHERIES 305 



ments of fish, from outside were caused by an inflow of 

 Atlantic (Gulf Stream) water in autumn, and of more 

 northerly waters in spring. Since then it has been estab- 

 lished by the work of many investigators that these inflows 

 of outside water into the North Sea are only part of a wider 

 annual periodicity in the system of currents of the North 

 Atlantic, In summer there is a great increase in the amount 

 of Gulf Stream water flowing over the Wyville Thomson 

 ridge towards the Shetlands and the North Sea. Below 

 this warmer and more saline water lies the cold Arctic water 

 of the Norwegian Sea, and it is about the line of junction 

 of these two bodies of water, at about 100 fathoms or more, 

 that we have what Sir John Murray called the " mud-line," 

 where detritus accumulates and where fishes and Crustacea 

 (such as Calanus) are present in quantity. This region is 

 the feeding-ground of the cod and other fishes, and the site 

 of important spring and summer fisheries. In addition to 

 this annual periodicity, which floods the Norwegian Channel 

 and North Sea with Gulf Stream water, Otto Pettersson has 

 shown that there is also a secular periodicity, which after 

 an interval of years results in a diminution of the pulse of 

 the Gulf Stream, so that for some months the inflow of 

 Atlantic water becomes much less, and as a result there is 

 an increased flow in the autumn of northern water into the 

 Norwegian Channel, etc., causing changes in the spawning 

 of the herring and in the consequent fisheries. This was 

 notably the case, for example, in November 1893 (see Otto 

 Pettersson, Ur Svenska, vii, 1922). 



Again, take the case of an interesting oceanographic 

 observation which, if established, may be found to explain 

 the variations in time and amount of important fisheries. 

 Otto Pettersson in 1910 discovered by his observations in 

 the GuUmar Fjord the presence of periodic submarine waves 

 of deeper Salter water in the Kattegat and the fjords of 

 the west coast of Sweden, which draw in with them from the 

 Jutland banks vast shoals of the herrings which congregate 



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