of the Fishery Board for Scotland. 



191 



mark, and from the economic point of view it is obviously of the greatest 

 importance. (3) The lowest size at which the plaice is mature. This 

 was laid stress on by Fulton* as being the most serviceable limit, from 

 the practical point of view, for legislative purposes. (4) The average 

 size of all the mature plaice. This was also defined by Fulton (I.e.), and 

 is of interest, because it shows by contrast with the size at which the 

 plaice become ripe for the first time that the great majority of mature 

 plaice elude capture during their first spawning season, and survive to 

 spaAvn several times. (5) The greatest size at which the plaice is im- 

 mature — i.e., the size at which all or nearly all have spawned. This was first 

 proposed by Holtf in the course of his North Sea investigations, and was 

 founded on the supposition that such a size- limit was necessary, because 

 the numbers of plaice in the North Sea were on the decrease. (6) The 

 average size at which the plaice become ripe for the first time — i.e., the 

 size at which as many plaice are immature as mature, below which more 

 are immature than mature, and above which more are mature than 

 immature. This size-limit has been spoken of by previous observers, but 

 no endeavours have ever been made to fix it definitely. 



From the biological point of view, the last is perhaps the most 

 interesting and most important of the above size-limits. It is the size- 

 limit that seems the most difficult to fix, and yet is in reality one of the 

 easiest. It is the most representative size when two groups of the same 

 species or two species have to be compared. The other size-limits refer 

 to only a portion of the range of variations in the sizes of mature plaice, 

 whereas the average-size when first-mature is the centre point of the 

 range. And again, if regulations are to be framed upon such limits dis- 

 covered by naturalists, then the average- size at which the plaice spawn 

 for the first time is the most serviceable, from the biological point of 

 view. 



Referring now to the earlier work on this subject, and taking up, firstly, 

 that done on the east coast of England, it is necessary to give a brief 

 description of the observations and conclusions of Holt (I.e.). These 

 observations, as already mentioned, were carried on with the object of 

 obtaining the upper size-limit above which all or nearly all fish had 

 spawned. Hence, a process of selection was carried on of the mature and 

 immature, which unfortunately renders his tables of little use for the 

 present purpose. Thus, the males were almost entirely disregarded, 

 because it was well known previously that they spawned at a much 

 smaller size than the females. Again, the females were also selected, 

 those which were mature below a certain length the upper size-limit, 

 being specially sought for, and likewise the immature above this limit, 

 which was found to be 17 inches (43cm.). The smallest female that he 

 found mature was 13 inches (33cm.), and very few mature specimens, 

 comparatively speaking, were found between this size and 17 inches. On 

 the other hand, ripe males were common as low as 9 inches (23cm.), and 

 an exceptional one was ripe at 6 inches (15cm.). 



In comparison with the numbers and sizes to be presently given, these 

 show a greater range of variation, but as Holt included the whole North 

 Sea — or rather the North Sea north from Grimsby — this greater range is 

 easily accounted for. The plaice of the northerly North Sea are mature 

 for the first time at a greater size than those from the English coast and 

 southerly portions of the North Sea, and this difference would cause 

 a greater range of variation. Otherwise there does not seem any 

 pronounced difference between the sizes given by Holt and those found 

 by myself. 



* Fulton, T t W. Eighth Rep., Fish. Board for Scotland, 1890. 

 f Holt, E. W. L. Jour. Mar., Biol. Ass., Vol. II., 1892. 



