328 THE SEAS 



may be taken to mean that a quarter of the catchable 

 plaice are taken out each year — that is those fish which are 

 large enough to be retained by the meshes of the net. At 

 first sight one does not think this to be really possible. 

 We say, the sea is so vast that the efforts of man can have 

 no effect upon the fish population. But when we realize 

 that a fish like the plaice inhabits only the comparatively 

 shallow water, and that this water is of a very limited 

 extent compared with the deep oceans, and when we are 

 told that the area covered by the nets of all British steam 

 trawlers in the year 1926 was about 108,000 square miles 

 and the area of the North Sea itself being only about 130,000 

 square miles, we realize that the chances in favour of the 

 plaice are small indeed. Especially is this so since the 

 trawling area is not distributed all over the North Sea, 

 but is concentrated on certain grounds where the plaice 

 are most abundant. 



Let us now turn for a while from the problems of the 

 plaice and follow the study of the life-history of that 

 pelagic fish, the herring. Earlier in this chapter we have 

 described the general methods of locating the spawning 

 grounds of the herring, by noting the presence of ripe fish, 

 by searching for eggs on the bottom with the grab, and by 

 seeking for " spawny " haddock. 



A study of the distribution of these spawning grounds 

 shows that the herring choose out regions in the close vicinity 

 of the coast and especially of the estuaries of rivers. 



It is next necessary to follow the movement of the young 

 herring as soon as they have hatched from their demersal 

 eggs. The baby fish very soon after hatching make their 

 way up from the bottom into the water layers nearer the 

 surface, and here they may be caught by means of tow-nets 

 in the same way as the young plaice. 



But when the herring is about three or four months old 



