of the Fishery Boar xl for Scotland. 



197 



for the specimens from Heligoland is, therefore, about 20mm., and for 

 those from H elder 10mm. A slight advantage is given to the immature, 

 as they probably grow faster than the mature at this period. 



The values are plotted out in the curves shown on Plate IX. — the light 

 lines representing the observed percentages, the dark lines the corrected 

 numbers. The first two curves (A) have been sketched from the data in 

 Table I. given by Cunningham, and in these the abscissal axis is marked 

 out in inches. In the following curves this axis is marked in centi- 

 metres. In all cases the ordinate axis represents the percentage values as 

 given in Tables I. and II. An inspection of the first set of curves — those 

 formed from the data given by Cunningham — shows that both for the 

 males and the females the courses of the curves are irregular. The points 

 which mark the percentages do not lie on one straight continuous curve, 

 but on one which rises and falls irregularly, and the irregularity is more 

 marked in the case of the males. The reasons for this irregularity may 

 lie in the insufficient number of specimens at certain sizes or in their 

 non-representative nature. The irregularity is restricted, however, to 

 certain definite points, and the general trend of the curves seems suffici- 

 ently evident to permit free-hand outlines of the probably correct courses 

 of the curves over the points of irregularity. This outline is represented 

 by the dotted lines. 



It will be remarked that in all the pairs of curves, the curve of the 

 mature specimens begins between 8 and lOin. (20*3 and 25*4cm.) at or 

 near 00 / 00 , and mounts up gradually until between 14 and 17in. (35'0- 

 43'2crn.) it approaches 100 °/ 0(r The curve of the immature specimens, 

 on the other hand, follows the reverse course, beginning about 100 °/ 00 

 and ending at 0 °/ o0 . The two curves cross on the 50 °/ 00 line, and 

 the point of crossing is taken to represent the average of the combined 

 curves. The abscissa of this point is then considered to represent the 

 average size at which plaice are ripe for the first time. In the case of 

 the curves formed from the data given by Cunningham, this length is 

 13-7in. (34-8cm.) for the females and 10'5in. (26 - 6cm.) for the males. 

 If a slight correction, similar to that already described, be made for 

 growth, these averages become 14in. (35"6cm.) for the females and 

 10'8in. (27'4cm.) for the males. In the case of the curves formed from 

 my own observations, the corrected averages are 33'2cm. (13*lin.) for 

 the females and 26'6cm. (10*5in.) for the males. 



It appears, therefore, that the average size at first-maturity for the plaice 

 of the southerly North Sea lies between 13 and 14in. for the females and 

 10-1 lin. for the males. The close agreement between the two sets of 

 observations is somewhat remarkable when the comparatively small 

 number of specimens is considered, and shows that these averages must 

 be close approximations to the real averages in nature. It has to be 

 remarked also that the males are mature at a size 3 inches smaller on an 

 average than the females. This difference may correspond to a year's 

 growth — i.e., the males may be mature a year earlier than the females. 



If attention is again directed to the second set of curves (B), it will be 

 noticed that the curves are open at both ends. This permits of the 

 possibility that mature specimens may be found below the lowest limits 

 recorded, and immature specimens above the highest limits. In accord- 

 ance with this possibility, Cunningham has found mature female 

 specimens at 9in. (22'9cm.) and mature males at Gin. (15'2cm.) There 

 is, therefore, again a difference of 3in. between the earliest mature male 

 and female, and this may correspond, as above stated, to a year's differ- 

 ence in age. The general slope of the curves for the female specimens 

 probably represents better than any of the others what actually occurs. 



