BARENTS SEA IN AUGUST, 1907. 91 



constructed, I am indebted to Dr. Wallace, who has made the deter- 

 mination of the maturity of 895 female fish on the fishing grounds of 

 the Dogger Bank, Clay Deep, and off Flamborough Head. 



In this curve for the central North Sea we find that the immature 

 females, commencing at 18-19 cm., attain their greatest frequency at 

 36-3Y cm., and ultimately disappear after 45 cm. The smallest mature 

 female occurs at 25 cm., with the actual greatest frequency at 

 40-41 cm. This curve, however, representing the mature fish, 'rises 

 again at 46-47 cm., so that it seems that its real mode lies between 

 these groups. The largest fish in this collection was 66 cm. long. 



In the curve for the Barents Sea (Fig. 2), the smallest immature 

 female was at 24 cm., and then, as in the central part of the North 

 Sea, the greatest frequency occurs at 36-37 cm., and the largest 

 immature specimen at 45 cm. The smallest mature' female was 

 found to be 35 cm. in length, and the greatest frequency occurs at 

 46-47 cm. (thus coinciding with the second mode in the curve for the 

 mature North Sea females. Fig. 3). The largest specimen had a length 

 of 73 cm. 



The size at which the numbers of immature and mature are equal 

 (the average size at first maturity), represented by the line M, is for 

 each curve in the vicinity of 40 cm. 



Thus in the Barents Sea with its intact stock of plaice, the mode of 

 the length frequency curve (Fig. 2), viz., 46-47 cm,, is some 7 cm. higher 

 than the average size at first maturity, viz., 39-40 cm. On the other 

 hand, in the diagram (Fig. 3) representing the stock in the central 

 parts of the North Sea, we should naturally not expect to find any such 

 great accumulation of mature fish ; but it is somewhat alarming to find 

 that the mode here (36-37 cm.) fails by 3 cm. to reach the average 

 size at first maturity (39-40 cm.). That is to say, some factor is at 

 work which keeps down the plaice population to such an extent that 

 the greater 'proportion are not, at the j^i^esent time, once permitted to re- 

 produee their kind. If we compare this with Dr. Petersen's previous 

 discussion of these points in his paper, " What is over-fishing ? " 

 {Journ. M. B.A., Vol. vi., 1903, pp. 587-94), it would appear as though 

 the chief theoretical effects of over-fishing in the North Sea, suggested 

 in that paper, were now being substantiated.* 



It may be that this enormous contrast is partly due to the fact that 



* Kyle {Journ. M. B. A., A^ol. vi, p. 496) suggests that oue effect of fishing a plaice 

 population is to reduce the average size at first maturity. The Barents Sea will afford the 

 opportunity of testing this ; meanwhile, if we were to accept the suggestion that sucli a 

 reduction has taken place to the extent of, say, 5 cm. in home waters, the deterioration 

 of the stock would be still more evident, and even a gi'eater proportion of North Sea plaice 

 than my curves tend to show would be prevented from attaining maturity. 



