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Part III. — Eighteenth Annual Report 



and reference has only been made to the literature directly bearing 

 upon the subject. 



The number of persons to whom I have been indebted during the pro- 

 gress of this work is large. To Dr. Hoek, the talented director of the 

 Netherlands Zoological Station at Helder, and his assistant, Dr. Redeke, 

 I am indebted for a very pleasant sojourn at Helder, as well as for many 

 suggestive criticisms of my work. Dr. M'Intosh, my own Professor at 

 St. Andrews University, has never failed me with his kindly advice and 

 assistance. Dr. Fulton, the Scientific Superintendent of the Scottish 

 Fishery Eoard, kindly granted me the use of the Marine Station at Nigg 

 for a short period, and gave me a portion of my material. But, above all, 

 am I indebted to the generous and friendly circle of Heligoland. Not 

 only had I the pleasure of good company and the use of their Biological 

 Station, including a considerable amount of material, but I had also the 

 benefit of advice and counsel from Professor Heincke and Dr. Ehrenbaum 

 over the initial, and perhaps the roughest, stages of the work. May 

 beginners always find such friendly and capable advisers. 



I have also to acknowledge the generous gift of <£20 from the 

 Worshipful Company of Fishmongers, London, which has aided me greatly 

 in defraying the costs of material and transport. 



Average Size at First-Maturity. 



The combination of words, " first-maturity," does not seem very 

 happy, but it is difficult to find any other short and compact expression 

 which would explain the meaning intended. A fish is said to be mature 

 once and for all when its reproductive organs have become ripe. There 

 are some difficulties in the way of accepting this definition as perfectly 

 general — e.g., the young of the salmon — but such is the usage of the 

 term, and this is the meaning employed here. " First-maturity " is then 

 taken to mean, the first time that the reproductive organs become ripe. 



A few years ago, when questions concerning the supposed decrease of 

 fish in the sea were more in evidence than they are perhaps at present, 

 attention was drawn to the great destruction of young immature fish that 

 was going on. It was believed by many that this was one of the chief 

 causes of the decrease, and it was therefore thought that if certain size- 

 limits were fixed to prevent the fish below these limits from being brought 

 to the market, then the young fish would have a better chance of becoming 

 mature, and thence of maintaining the species. 



The present investigations were not directly undertaken with the view 

 of deciding what this size-limit should be, and it was only when the 

 materials had been collected that they were seen to possess an importance 

 previously unsuspected concerning this very question. The number 

 of specimens is not so large as those of several naturalists who have pre- 

 viously investigated this subject, but large enough to illustrate a method 

 of grouping the mature and immature fish which displays better than has 

 previously been done the average-size at which the plaice spawn for the 

 first time. 



Before entering upon the discussion proper, it is advisable to distin- 

 guish between certain definite points — (1) The size-limit below which no 

 plaice are allowed to be sold. This size-limit differs in the various 

 countries where such limits exist, and has been fixed by the respective 

 Governments solely from practical expediency, and not from any con- 

 sideration of its biological aspect. (2) The size at which the plaice are 

 on the average in the best condition for the market — i.e., the best value- 

 size. This size was first suggested and defined by Petersen* of Den- 

 * Petersen, C. G. J. Report of the Danish Biol. Station, IV., 1893. 



