oj the Fisher;/ Board for Scotland. 359 



the average to about 2| inches, the apparent range of size in the 

 previous spawning season would be from 7^ toll| inches. I say the 

 apparent range, because the real range must be considerably greater, 

 owing to the overlapping of the groups, and it probably extends from 

 about 24 to 36 cm. or more. But it seems to me most likely that this 

 group forms the main bulk of the milters in the next spawning season, 

 when they would probably range from about eleven to fifteen inches 

 and be four years old. This view receives suppoi-t from a comparison of 

 the average sizes of the males and females of the group, which do not 

 differ to any considerable extent. It is [)robably after the first spawning 

 that growth becomes retai'ded, and Gi-oup Y., comprising fish which are 

 over foul' years of age, undoubtedly represents males which spawned in 

 the preceding spring, and whose summer's growth has been slight. 

 Their average size in Novembei', it will be seen, was about an inch less 

 than the size of the females of corresponding age, and the curve shows 

 that the maximum number lie intercalated between the females of Groups 

 IV. and V. 



It may, I think, be said with some certainty of the females of Group 

 IV. — three and a half years old — that none of these spawned in the 

 preceding spring. Their apparent range in November is much the 

 same as the males, namely from about ten to fourteen inches, and their 

 mean size closely corresponds. In the preceding spring they would be 

 less by over two and a half inches, and in the following spring scarcely 

 an inch longer — i.e., ranging from about eleven to about fifteen inches. 

 The latter size coincides with the minimum mature size given by Holt 

 for the female plaice of the North Sea, and I think it probable that the 

 group of females present in the November haul which would spawn for 

 the first time in the ensuing spring is Group V. The fish would then 

 be five years old, with a size varying from about fifteen to eighteen 

 inches. 



In the obsei'\'ations made by various observers it has been noticed 

 that the mature and immature specimens of either sex, collected at the 

 same time, overlap to a considerable extent — so that, for example, one 

 may find a few mature females at thirteen and fourteen inches, while 

 the great number about these sizes are immature — the proportion of 

 immature gTadually declining, while the proportion of mature increases. 

 This circumstance is consistent with the view that such immature and 

 mature individuals, although of corresponding size, belong to different 

 .series and differ in age by one year. The coalescence or intercalation 

 of the groups, as the curves show, is considerable in all except the 

 younger fishes, and it increases in the older generations. If it were 

 possible to separate out all the females belonging, for example, to 

 Group IV., from Group III. and Group V., it would be found that the 

 variation in size in the group is much greater than the limits of the 

 gi'oups as defined in my tables. Thus, the number of those w^hich can 

 be said with absolute certainty to belong to Group IV. — i.e., those 

 beyond the range of overlapping — is compaiatively small, comprising 

 those around the median ordinate. Females belonging to Group IV., 

 for example, might, as the prolongation of the curve will show, range 

 from about 23 or 24 to 39 or more centimetres (nine to fifteen and a 

 half inches). 



It seems to me probable that maturity is determined in the plaice 

 of any given region not by size, but by age, that the variation in size 

 of mature or immature specimens, and their overlapping, is due to 

 variation in growth, and that the plaice in all regions spawn at the 

 same age, difference in the average size at first-matui'ity being due to 

 differences in the rate of growth in the different regions. That such 



