— 13 — LATER STAGES: Pl.ia'HOM'K/llDS 



an indutinite type. It is known that, in the nurseries, the young Hsh may be temporarily 

 aggregated into shoals for purposes of feeding, and a similar aggregation seems necessary 

 for effective fertilisation during spawning. Hence arises the necessity for lish-marking 

 experiments and statistical records on a scale sufficiently large to eliminate the effects of 

 these aggregations. 



The cycle of distribution of a pelagic spawning fish may therefore be divided con- 

 veniently into (a) the pelagic drift from spawning ground to nursery; (b) growth move- 

 ment from nursery to general feeding ground ; and (c) spawning migrations from feeding 

 ground to spawning ground and vice versa. 



It is important that future research should concentrate itself upon the determination of 

 these migrations for each important species of flat fish. Johansen, in his recent report 

 distinguishes important seasonal migrations of the plaice, indications of which have been 

 found in the results of more than one author. He distinguishes the summer offshore 

 migration of the early age-groups of plaice. 



In the early summer months these fish move off into deeper water. How far this 

 migration is to be identified with, or distinguished from, the normal "growth-movement" 

 remains to be seen. Secondly, there is an inshore spring migration consisting of a mo- 

 vement shorewards of the fish of marketable size during the months of March — April. 

 He further distinguishes an inshore autumn migration of grown individuals (perhaps inclu- 

 ding more females than males) and lastly an offshore winter migration, comprising young 

 as well as old individuals. 



According to this author the summer and winter offshore migrations involve all the 

 age-groups and the spring and autumn inshore migrations are more or less confined to 

 the larger individuals. One is led to inquire whether the normal offshore growth move- 

 ment may not proceed throughout the year but become more or less masked in spring 

 and autumn by the feeding migration (?) of the larger sizes and apparently broken up 

 into the distinct movements (really part of one) in summer and winter. 



As regards migrations from one region to another not necessarily having a definite 

 relation to the shore he notes that in the Danish areas of A3, B4, and B5 the great pro- 

 portion of marked plaice have been re-caught within these areas. Further, these areas 

 do not appear to any important extent to include spawning grounds, which in the main 

 lie in the parts of the North Sea further to the West or North-West. It follows that the 

 Danish nursery-grounds are supplied with their annual crop of littoral fry by pelagic drift 

 from spawning grounds outside the areas indicated. The prevalent drift along the West 

 coast of Jutland is northerly so it is to be assumed that the littoral stages in A3 and 

 A4 arrive with this stream from spawning grounds southward. 



The young fish as they increase in size and age appear, according to the author, 

 to move offshore into deeper water, the majority in a North-westerly direction. Here is 

 shown a typical example of the life cycle with its pelagic drift and its growth-movement. 

 The spawning-migration also requires to be determined in order to complete the cycle. 



The account given, by the author, of the migrations into the Skagerak and Kattegat 

 and his distinction of a "Northern" and "Southern" race in the latter does not fall within 

 the scope of this review though the distinction of races has an important bearing on the 

 racial possibilities of the North Sea. 



