in] THE FACTORS OF DISTRIBUTION 81 



Gulf Stream drift. If we study the migration paths 

 of some species of fishes we can see they move so as 

 generally to cross the isohalsine or isothermal lines in 

 the sea. If in any place the salinity and temperature 

 alter, the fish seem often to move so as to regain the 

 former conditions. Many fishes inhabit, by preference, 

 not the inflowing Atlantic water of high salinity, nor 

 the fresher coastal water, but rather the mixture of 

 the two. The herring which are caught during the 

 summer off the east coasts of Britain first appear off 

 the Western Hebrides and are then found further 

 and further to the south as the season advances. 

 This used to be explained by supposing that the fish 

 migrated from somewhere in the north and then 

 moved to the south in the course of the season. But 

 the herrings which appear at each station spawn, and 

 it can hardly be the case that they all belong to the 

 same shoals. It is far more probable that there are 

 local races of fish which inhabit the North Sea 

 throughout the year and that they shoal and spawn 

 when the conditions become optimal. This optimum 

 of conditions is the result of the inflowing Atlantic 

 water from the Gulf Stream drift, and this flows 

 further to the south as the year advances, while the 

 herring cling to the borders of the salt water where 

 it is mixing with the fresher water coming from 

 the land. Halibut and ling inhabit the bed of the 

 North Sea where it slopes down into the depths 



