SEA-FISHERIES LABORATORY. 307 



achieved in the summer ones : the fish, experiencing a 

 change of temperature moves in such a manner as to 

 make this change minimal. In the summer it moves out 

 of water rising in temperature into colder water ; and in 

 the winter it moves out of water falling in temperature 

 into warmer water. 



The majority of the migrations studied by the 

 marking experiments are not spawning ones. The 

 greater number of the plaice liberated are quite 

 immature : practically all the females are, and only a 

 small proportion of the males are sexually mature. The 

 plaice fisheries in Lancashire waters are essentially 

 fisheries for immature fish, and it is only in the late 

 summer and autumn months that a few larger mature 

 plaice appear in the shallow waters. The question of 

 migration as an adaptation to temperature change is not 

 therefore complicated by the simultaneous occurrence of 

 spawning migrations, at least not to such an extent as 

 seriously to affect the conclusions made here. Migra- 

 tions of sexually mature plaice, or plaice about to 

 become sexually mature, do indeed occur : thus, rela- 

 tively large plaice move from Luce Bay (on the South 

 coast of Scotland) to the shallow water banks off 

 Ramsey, in the Isle of Man. Some, also, of the plaice 

 inhabiting the in-shore waters off the Lancashire and 

 Cumberland coasts may migrate towards this area. 

 There is no doubt that the most important (perhaps the 

 only) spawning area in the northern part of the Irish 

 Sea is this area off the North-East coast of the Isle of 

 Man. It is a fishing-ground which would well repay 

 investigation, in view of the results of the marked-fish 

 experiments of this and former years ; but, unfortunately, 

 the few observations made on it are of no value at all 

 for the purposes of this discussion. 



