294 BRITISH FISHERIES 



towards the fishing grounds offshore. The gener- 

 ally accepted theory of the migratory movements 

 of the plaice is that the young fish spends its 

 early life in shallow water, and migrates into 

 deeper water as it becomes older. This theory 

 has been deduced from observations on the dis- 

 tribution of plaice of different sizes in water of 

 varying depths. Generally speaking, large plaice 

 are more abundant in deep water, and small plaice 

 in shallow water. But, again, the evidence for this 

 is not very complete. Plaice may be found in 

 certain shallow inshore areas, where trawl-fishing 

 has been prohibited for a number of years ', and we 

 must remember that the apparent natural distribu- 

 tion of the plaice may be due to the fact that the 

 shallow-water areas have been more thoroughly 

 fished than the deeper water offshore, and that 

 it is generally agreed that the principal effect of 

 extensive fishing is to reduce the average size of 

 the fish on the areas fished on, for the larger fishes 

 are fewer and are more easily caught than the 

 smaller ones. Most theories of the distribution 

 and migratory movements of fishes have been 

 constructed during the time when modern fishing 

 has been so largely developed, and subsequent 

 to the initiation of the above changes in the 

 distribution of large and small fishes. 



Further, it is very difficult to forecast the 

 probable changes in the fish fauna of a particular 

 area, such as a shrimping ground, which would 



