20O The Under-Water World 



The river offers obvious advantages as 

 a spawning ground compared to the sea, 

 being immune from many foes that 

 haunt the oceans, and it is evident that 

 most of our river fish are derived from 

 marine forms that have migrated to 

 spawn farther and farther from the 

 perils of salt water. Such forms as the 

 miller's thumb, stickleback and burbot 

 are scarcely distinguishable from their 

 marine relatives. 



Fish migration is no mere theme for 

 the dryasdust type of scientist, for it 

 affects the man in the street to an extent 

 which few realise. Human agencies have 

 aggravated what is at best an exceed- 

 ingly difficult problem. Possible breed- 

 ing grounds of food fishes are given little 

 rest, for day and night the steam trawlers 

 of Europe furrow the sea bed from Fal- 

 mouth to the west of Scotland, and from 

 Grimsby to the Dogger Bank. The 

 White Sea as far as the North Cape and 



