APPENDIX H': REDEKE 



The plaice of 

 the fourth year 



place in the shallower regions being taken up by the next generation as described 

 above. 



From this it appears, that a great migration of plaice towards the deeper waters 

 takes place each year in summer and autumn, sometimes earlier, sometimes later, and 

 probably also not at the same time everywhere. The well-known fact, that the smallest 

 plaice are taken on our fishing-grounds in summer and autumn, can thus be explained. 

 The Il-group now mixes in the deeper waters with the Ill-group, which predominates 

 there in spring and is the objective of the energetic trawl-fishery carried on especially in 

 the months of March to May. In September 1904, we found the II- and Ill-group mixed 



together almost everywhere 

 in deeper water. Larger 

 shoals of the Il-group are 

 constantly arriving, however, 

 until this group finally predo- 

 minates everywhere in the 

 central part of the region 

 investigated during Novem- 

 ber, and the November hauls 

 on the true fishing-grounds 

 consisted almost entirely of 

 plaice of this group. 



The accompanying figure 

 2 (p. 7) which refers to hauls 

 made on the Brown Ridges, 

 one in June and one in No- 

 vember, is one example out 

 of many which may illu- 

 strate what has been said. 



In autumn, the average 

 size of these plaice varies 

 between 18 and 23 cm., and 

 is different on the diff'erent 

 fishing-grounds, in such a 

 way, that the largest speci- 

 mens on the average are always taken on the most distant grounds (e. g. on the Brown 

 Ridges), the smallest on the grounds nearest to the coast (e. g. at Terschelling lightship), 

 thus exactly as is the case with the preceding generations. 



In autumn and winter, the pkice at the end of their third year thus form the 

 chief component on our trawling grounds. 



As is well-known, the plaice are mature at this period. That is, for our region: 

 some of the males and exceptionally also some females become ripe in the next 

 spawning-season. These belong then, however, to the Ill-group, i. e. are in the begin- 

 ning of their fourth year. 



Most of the males of this group noticed to be mature had a length of about 20 cm. 

 In January, however, we fairly frequently took males of lé and 17 cm. with running milt. 



19 20 CenHm 



Fig. I 



Percentage frequency-polygons of two hauls of plaice, September 

 1904. — The continuous Une refers to a haul made in the Zuydersee 

 (240 specimens) consisting of the O- and I-groups, the broken line 

 to a haul made in the North Sea at VHeland (647 specimens) con- 

 sisting chiefly of the I-group. 



