131 



migrate on to tlie 8ol\vay ' Slaughter " area in March in order 

 to spawn. 



Temperature changes in the sea are closely associated 

 with the plaice migrations. The long contour lines on the 

 charts (Figs. 8 to 13) were drawn with the idea of elucidating 

 this relationship. They are called " Isoanomalous lines," and 

 indicate the rate at which the sea-temperature rises or falls 

 (according to the season) between the coast and the sea offshore. 

 But we have yet far too few data to allow of this subject being 

 pursued. One thing, however, may be noted : in their migra- 

 tions the small fish of the Irish Sea tend to move across the 

 isotherms. Obviously one impulse in the migrations is that 

 of the fish to tend to remain as much as possible in water of the 

 same temperature. 



The Groivth-rate of Marked Plaice. 



First we consider the general growth-rate of plaice as it 

 has been found by the methods indicated on pp. 102-7 and in 

 the data of Table XIV. (See Fig. 7.) These results show that 

 the mean rate of growth of Irish Sea plaice, males and females, 

 taken from all the regions is about 6 cms. (= 2| inch) per year. 

 That is fish up to five complete years in age. Taking the Age- 

 groups II, III, and IV, the mean increase in length per year 

 is 5-75 cms. (= 2| inches). 



How fast does a marked plaice grow ? This is easily 

 found by making measurements of plaice marked and liberated 

 before the beginning of the spring and then recaptured after 

 the end of autumn. Sometimes measurements can be made 

 on plaice that have undergone two or even three complete 

 seasons' growth. These results are fairly constant and they 

 show a ntean rate of growth of marked plaice of 3 inches. 

 Sometimes the yearly increase may be much more than this, 

 and it is generally greater in the case of marked plaice that 

 have made long migrations from Liverpool Bay into St. George's 

 Channel. 



