— 9 — 



APPENDIX H' : REDEKE 



thus very dense ". The Ill-group, which on the inner edge has an average length of 

 20 cm. and 25 cm. in the central part, is now distributed over the entire region from 

 about the 20 m. line outwards. (Compare the number on p. 7). 



In autumn, the position has quite changed. The I-group, which has now grown 

 from 10 to 15 cm., has become distributed out to sea, and the Il-group of about 20 to 

 25 cm. now predominates in the central part of the North Sea. 



The distribuât ion thus appears to be a function of the size and is so 

 uniform, that one can almost sa}' the plaice on our coast are so many 

 centimeters long when the depth in 'which they are taken is the same 

 number of meters. 



Spring-isoraegalins for the plaice 

 on the Dutch coast. 



Fig. 4 



Autumn-isomegaJins for the plaice 

 on the Dutch coast. 



Concerning the causes of the migrations of our plaice, we know as yet very little. 



In the first year, they probably depend chiefly on the food, to the extent that the 

 majority and certainly the larger plaice go away from the regions where the food is poor; 

 thus, the largest of all can travel the furthest and those left behind in consequence of 

 their slight nourishment grow but little. 



The salinity of the water seems to have no influence at all (except on the develop- 

 ment of the eggs), as the plaice (and other fishes also, as is well-known) can change with 

 the greatest ease from water of ca. 17 %o through all intermediate stages to water ot 

 double that salinity. They probably possess some regulating apparatus, by means of 

 which they are able to suit themselves to the altered salinity, somewhat as the homœo- 

 thermic animals can adapt themselves to temperature-changes. It is of more importance, 

 however, that the conditions of salinity in the region display very slight annual variations, 



Causes of the 

 migrations 



I In March 1904, for example, we caught over 10,000 plaice belonging mostly to the Il-group (mixed 

 with I-group) in a one hour's haul at Egmont. 



Appendix H' 2 



