no 



Channel. In the summer the temperature to the North (say, 

 in St. George's Channel) rises and approaches that which suits 

 the fish. Its genera] restlessness then leads to it enlarging the 

 region of its movements into the new zone of water of suitable 

 temperature, so that it migrates up into St. George's Channel 

 and the Irish Sea. 



" Feeding migrations " are a consequence of external 

 causes. Plaice, for instance, are always "on the move" 

 during spring, summer, and autumn. Let there be a certain 

 region of rough ground on which there is no " feed " : plaice 

 doubtless cross this in various directions, but don't stay there. 

 If, however, there is a " strike " of small mussels on the region 

 the plaice which cross it will tend to stay there to eat the 

 shellfish, and so there will appear to have been a migration 

 on to this region from the surrounding ones. Old fishermen 

 know these conditions very well and expect migrations when 

 they foresee a strike of mussels. 



" Spawning migrations " are the results of the development 

 of internal secretions. In some way or other these cause the 

 fish to " prefer " water of a certain depth (pressure) and 

 temperature. It is unlikely that a plaice which is ripening 

 sexually for the first time knows M^here to find these conditions, 

 but its general restlessness leads to its exploration of the 

 whole region in which it lives, and having found the agreeable 

 conditions it stays among them. 



" Shoaling migrations." The causes that lead one fisli 

 to seek certain conditions also impel others, thus we get shoals. 

 Probably fish are not " gregarious " in the sense that they 

 want each other's society : the shoal is brought about by the 

 occurrence of external conditions that affect all mature plaice 

 at the same time. 



Plaice-marking Experiments. 



Much has been made out as to the movements of plaice, 

 and other fishes, by studying their sizes and density on different 



