— 11 - LATER STAGES: PLEURONECTIDS 



the proportion of males is about 55" o. With ascending length it gradually decreases till 

 at 45 cms. and over, it is reduced to about 8"/o. At 25—29 cms. the numbers of the 

 two sexes are equal. 



In all the fish below 30 cms. taken together there is a preponderance of males which 

 is reversed in those above this length. If the North .Sea data above be taken, excluding 

 the Scottish bays, an equality is attained at 20—24 cms.. This feature requires to be 

 carefully compared with known facts regarding the differences in growth and longevity 

 between the two sexes. Although the connection between the three series of differences 

 is patent it is with present knowledge impossible to select any one of the three as the 

 primary cause. 



Two generalizations seem to be evident from the distribution according to the locality 

 and season. A high proportion of males is characteristic of the catch from offshore spaw- 

 ning areas during the spawning season, whereas a small or low proportion of males is 

 found to be typical of the catch from the inshore nursery grounds, and from feeding 

 grounds like the Dogger Bank. These facts are found in even greater degree of intensity 

 in the Flemish Bight from the data derived from Lowestoft sailing trawlers, in which the 

 state of maturity was also examined. Whatever the cause of the phenomenon, it seems 

 that it constitutes a useful means of locating spawning areas of the plaice. 



The author discusses the possible causes under three heads: — 



(a) that there is a larger number of males present in the area from which the cat- 

 ches are taken. 



(b) that in the spawning season the males are more liable to capture than usually. 



(c) that in the spawning season the females are less liable to capture than normally. 

 He is inclined to believe that all three are factors in the result, and to attach the 



greatest importance to the third, though more facts are required. 



The excessive destruction of males at the spawning season will commence its operation 

 from the average size at which the male attains maturity upon the particular ground. 

 Wallace's work, above referred to, has confirmed the marked differences in average-size- 

 at-first-maturity of males between plaice from the more northerly grounds of Flamborough 

 and Dogger on the one hand and from the southerly grounds in the Flemish Bight. The 

 curves of percentage proportion of males at different lengths (5) caught on these grounds 

 show a similar difference, the proportion falling rapidly at a higher average length in the 

 former than in the latter. It is here assumed that the fall in proportion of males is 

 largely if not entirely due to the gradual operation of trawlers, and it this can be sub- 

 stantiated it is possible that a new standard of the intensity of fishing may be devised by 

 a study of the proportion of the sexes in the catch of commercial vessels. 



Certain facts are also elicited with regard to the distinctive migrations of each sex. 

 In the central spawning area of the Flemish Bight it was found that of plaice over 30 

 cms. in length caught in the beam trawl 5CKX) were ripe males and only 582 were females 

 of which latter only 182 were mature. In contrast to this, the neighbouring inshore nurseries 

 show a preponderance of small females (immature) and the inference is drawn that there 

 is an early migration offshore, to the spawning grounds , of small males upon arrival at 

 first maturity. A certain amount of evidence is also given for, the view that a special 

 migration of females takes place on the first attainment of maturity. This interpretation 

 is also placed upon the results as regards sex-distribution given in a previuos report by 



