RAPPORTS. XIII C4: MASTERMAN — 8 — 



regards the northern areas (A. B. & C.) the reverse was the case. So far as they go , 

 these results obtained "from different data and studied in a different manner completely 

 corroborate one another (see later). 



Certain of the data are also employed for determination of the time and duration of 

 the spawning period. The figures are meagre, and it need only be noted that such 

 evidence as there is points to an earlier spawning period in the English Channel and 

 Southern Bight of the North Sea than in the more Northern Grounds. 



The author also gives figures showing the average size-at-fîrst-maturity. By grouping 

 the samples, as before, into five natural groups, it is shown that the average-size-at-first- 

 maturity is as follows : — 



Males 



Smallest : Av. size-at-' ' Largest 

 mature. first-maturity. I immature. 



Females 



Smallest 

 mature. 



Av. size-at- ' 

 first-maturity. 



Largest 

 immature. 



Dogger 



Leman 



Flemish 

 Bight 



English 

 Channel 



-IS 



■13 



.'7 



51(32 



40 



29I30 



36 



(.8) 



3' 



22/25 



30 



25 

 23 

 18 



40 



35 



32/33 



32/34 



47 



41 



36 



37 



These facts seem to confirm and amplify the previous results of other investigators 

 that the plaice become mature at a smaller size in more southern waters, and to a great 

 extent the change is gradational. The sequence in the North Sea is complete, whether 

 the length of the smallest mature, largest immature or average-size-at-first-maturity be 

 taken. 



The question naturally arises at to the cause of this phenomenon. Is it merely the 

 result of a differing temperature upon each generation of resident plaice and closely pro- 

 portioned to the differences between each area, or is it due to a racial difference, the 

 gradations merely indicating a change in the proportion of the two races on the same 

 grounds? Some amount of light is thrown on this question by a study of the figures. 

 If they be plotted out in curves for each sex, as in the two selected here from the 

 Dogger Region (Fig. l), it is found that, whilst in case of the females the curves are of 

 a regular type not unlike the curve of error, in the males they are irregular and re-en- 

 trant, indicating a compound origin. Fifty per cent of mature males are found at 21 cm. 

 and again at 32 cm., whilst their proportion falls off in the intermediate lengths with a 

 maximum at 27 cms. The portion of the curve above 32 cms. resembles closely in 

 character that of the females but lies at a level of about 8 cms. below it. If it be con- 

 tinued from 31 cms. to 17 cms. in the same character it is then possible to resolve the 

 compound curve into a lower curve with an apex at a point between 18— 20 cm. (the 

 numbers are too few for exact determination) and a higher at 32 cm. The same feature 



