300 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Sea by studying the temperature gradient during the last 

 months of the year in the sea further to the North. 



This explanation of the migrations of plaice in the 

 Irish Sea was suggested by me in 1911, and I reproduce 

 here a chart which was then made for another purpose. 

 Chart V was based on the plaice-marking experiments 

 carried out in 1906-10, and shows the main results of the 

 latter; distinguishing between such adaptive migrations 

 as have been considered above, and real spawning migra- 

 tions — movements which, however, might be regarded 

 as having very much the same meaning. 



It is impossible to present here the detailed evidence 

 on which the above chart was constructed, since the 

 hydrographic investigations of the years in question were 

 not complete enough to allow of the construction of charts 

 showing the isothermal lines. In 1912, however, the 

 marked-plaice experiments were begun again, on, it was 

 hoped, a larger scale, and according to methods utilising 

 the experience attained in the earlier investigations. 

 These extended experiments have, unfortunately, not 

 been made, but two fairly large lots of plaice were marked 

 by me (1) in June, 1912, on the summer plaice fishing 

 grounds near Nelson Buoy, and (2) in October on the 

 same grounds. The summer experiment was made in 

 order to investigate the movements of the fish about the 

 time when the marked segregation on this ground, which 

 is always observed from year to year, had fairly been 

 established. In October, or thereabout, the plaice begin 

 to desert this ground. " Practical " men say that they 

 have been fished out, but this is certainly not the case, 

 and the disappearance of the fish is due to their migra- 

 tion into other fishing grounds. The experiment in 

 October was made in order to show the nature of these 

 migrations. In each case about 200 plaice of lengths 



