496 REPORT ON TRAWLING AND OTHER INVESTIGATIONS 



has gradually spread during the past century from the English Channel 

 northwards, over the North Sea, and within the past twelve years on 

 to Iceland. The average size* at which the female plaice of the 

 southern portions of the North Sea reproduce for the first time is 

 13 to 14 inches, and the smallest mature female so far recorded is 

 9 inches, f The largest immature female has been 17 inches. In the 

 northern portions of the North Sea the average size is 15 to 16, 

 whilst the smallest mature and largest immature recorded are 12 and 

 17 inches respectively. The plaice of Iceland have not yet been 

 closely examined, but they seem to have even larger proportions. 

 The difference between the plaice of the southern and northern 

 portions of the North Sea is just one year's growth. Further, the 

 characters of the adults of the former group resemble those of the 

 younger stages of the latter. 



" These problems have already been discussed in a previous paper, and 

 it was left an open question as to whether man's influence, or simply 

 differences in the environment had brought about these differences 

 between the southern and northern North Sea plaice. Further observa- 

 tions and records incline me to lay more stress on the selection 

 exercised by man, whilst not forgetting the possibility that the en- 

 vironmental conditions may play some part. The new records]: re- 

 ferred to are those of a group of small spawning plaice discovered 

 during March, 1901, near the Borkum Eeef in the German Bight. 

 The sizes of the spawning females varied from 21*3 to 26*2 cm. (8'5 

 to 10"5 inches), and apparently a large number was obtained. If we 

 consider the average size to be 9*5 inches, the difference between this 

 average and that obtained for the southern portion of the North Sea 

 during 1898-1899, viz. 12-5 inches, is again one year's growth. 



" The observations are comparatively few in number, it must be 

 admitted, but they point to the conclusion that man's influence may 

 be causing a retrogression in the average size at which the plaice 

 spawn for the first time. This would seem to lead to the further 

 conclusion' that the numbers of the species will be maintained at their 

 former level by the earlier reproduction, even if the older and larger 

 fish have been removed by man. This idea has indeed been expressed, 

 though from a different standpoint, but unfortunately nature offers no 

 great compensation, for the large plaice have over ten times more 

 offspring than the small. 



" In a former paper the view has been expressed that if we could 



* Eifjhteenth Kcp. S. F. £., iii. p. 197. 



t J. T. Cunningham, Journ. M, B. A., vol. iv. pp. 17 and 99. 



+ I 'am indebted to my friend Prof. Ehrenbaum, of Heligoland, for permission to 

 mention these records. 



