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Part III. — Thirteenth Annual Report 



is quite possible that the size attained in this series during four months may 

 be less than that in the other series, for a fish developing from January 

 to May had obviously more adverse conditions with which to contend 

 than another developing from May to September. The higher mean 

 temperature, and greater abundance of food in the latter case, are amongst 

 the factors which at once suggest themselves. With these difficulties, it 

 is hard to assign the age of the lowest of the series, but the difficulty 

 may be approached in another way. 



This series appears to date approximately within the spawning period 

 of the plaice, so that if the plaice were hatched in situ one might expect 

 to obtain a complete series, even up to the unhatched egg. 



But the eggs of the plaice are shed, as a rule, in the case before us, 

 outside the territorial waters, and the larval and post-larval plaice have a 

 pelagic habit for a period of about five or six weeks ; so that, including 

 the hatching periods, we find that the smallest arrivals in the shallows 

 (*5 inch) should be reckoned as 2 months old. Thus the May series 

 (1*15 in., -54 in.) extend from 4 months old to 2 months old, and repre- 

 sent those spawned from mid- January to mid- March, those spawned 

 later than this date not yet having arrived from their pelagic life in mid- 

 water. If we again divide these into those columns, 1*15 to '8 in., and 

 •8 to *5, the number in each division increases with the spawning 

 intensity : — 



January, mid, 1*15— "95 2 



February, ....... '95 — '75 4 



March, 75— -55 7 



This arbitrary division is sufficient to bring out the point that, in the 

 September series, the maximum number of individuals is found at the 

 mean size, and in the May series the maximum is at the end of the series 

 containing the smaller sizes. These results are exactly what might be 

 expected, if the facts of diversity of size are to be interpreted by a diver- 

 sity in time of spawning. 



We thus see that a plaice in the first eight months of its life may 

 attain an approximate length of 3 to 3| inches, and that, considering 

 the next four months from September onwards are mostly winter months, 

 the plaice of a year old will not attain much more than 4J to 5 inches in 

 length. 



Thus by January of the second year there will be a series of plaice 

 gradating from 5 inches to 2J or 3 inches, corresponding to a gradation 

 in age from 12 months to 8 months. 



Again, a series will then commence for the second year's brood, so that 

 the plaice of any district should fall into a number of group-series, each 

 of which presents a maximum and a minimum. By a study of these 

 group-series, one may be able to work out the full growth-rate of any 

 given species. This has recently been done for the plaice of Denmark 

 by Petersen, and his graphic group-series show very perfectly the con- 

 tinual repetition of the annual broods, and, moreover, the predominating 

 mean in each group, with a diminishing number at the maxima and 

 minima. It will be noticed that the four months difference in the 

 sizes of the individuals of each year becomes less and less evident with 

 age, and also that as the growth curve becomes less marked, the rate 

 of growth in length becomes less and less, until at maturity it is nearly 

 or quite suspended. The annual curves will approach one another till 

 they intersect at the year when maturity is reached, so that from a study 

 of the growth-rate we derive independent evidence of the age and size at 

 which maturity is reached in the species under consideration. 



