304 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



no doubt that there will be a significant variation in the 

 shape of the isothermal lines during these three months. 

 It is also to be noted that the straight lines on the chart 

 do not necessarily represent the general direction 

 of the tracks followed by the migrants. They are the 

 only lines that may be drawn for this purpose, and it is 

 quite possible that the fish in many cases may have 

 travelled along a curved track, thus crossing the isotherms 

 more transversely than is indicated in the chart. The 

 absence of any recaptures in the in-shore waters of the 

 bays and estuaries during these months is very striking, 

 also the absence in the fishing grounds off the coasts of 

 North Wales and the Isle of Man. There can be no 

 doubt that plaice of the range of lengths represented by 

 the experiment are moving out from the shallow water 

 in-shore towards the deeper water off-shore during the 

 time of increasing and maximal sea-temperature. But 

 it is also clear that the migration is not necessarily from 

 shallow to deep water, that is, it is not increasing pressure 

 that the fish is attempting to attain, but rather a lower 

 temperature. It is moving from warm water into cold 

 water, at the time ivhen temperature of the latter at its 

 point of departure is rising. 



Chart YII represents the results of the experiment 

 made in October ; and also those recaptures of fish, 

 liberated in the June experiments, which were 

 made in the months of October, November, and 

 December. The thick lines are again the isotherms for 

 November 15th, 1912. The months of recapture are 

 indicated by the nature of the lines representing the 

 tracks of the fishes : the broken lines represent recaptures 

 in October ; continuous lines those in November, and lines 

 with dots those of December. It will be seen that a 

 number of plaice (10) have migrated into Morecambe 



