APPENDIX H' ; REDEKE — 8 — 



clusively on the distant trawling-grounds. This peculiar phenomenon, which has also 

 been similarly observed with regard to other fishes, is explainable on the hypothesis that 

 the fish die in great quantities at the spawning-time. How far this occurs also for our 

 plaice can only be determined by further investigations. At present, we still lack the 

 necessary material to be able to give certain data regarding the rate of growth and distri- 

 bution of these older plaice. 

 General The survey given here indicates only the main lines of the life-history of the plaice 



remar s ^^ ^^^ coast. It must bc admitted, that the conditions are not quite so simple as might 

 appear from the description above. Above all, it has only been incidently remarked, that 

 the plaice may also develop far from the coast; these consequently do not take part in 

 the general, periodic migrations or only slightly. So far as we can judge as yet, how- 

 ever, this occurs but exceptionally. 



On the other hand, a relatively large number of individuals of one generation remain 

 behind, when the others migrate. This has only been determined by a careful compari- 

 son of the results of the hauls. It is always the smallest specimens of a group which 

 are found together with the next younger, and the determination of the age from the 

 otoliths has especially enabled us to draw limits where the ordinary graphic method is 

 not sufficient to show the two diff"erent year-groups, just because the one year's group 

 is always but imperfectly represented in the hauls and is as it were fused with the other. 

 This method of determining the age, whose worth, it appears, is not generally recognized, 

 has shown itself to be quite indispensable and practically never a failure in investigations 

 under such conditions as prevail on our coast. 



Nevertheless, the picture just sketched of the distribution of the plaice is complete 

 and correct, so far as can be judged at present. 

 Uniformity of jf -^6 leave the small exceptions out of regard and consider by themselves the 



dominant groups on the diâ"erent grounds of our coast and their respective average lengths, 

 a surprising uniformity appears in their distribution. This becomes very apparent if we 

 draw lines on a chart connecting the places where plaice of the same average size were 

 caught. 



These lines, for which I would propose the name "isomegalins", show a course 

 which runs in general parallel to our coast. 



I have constructed such isomegahns for the condition as regards the plaice in spring 

 and autumn in the region under our survey (about JMay — ^June and November — December). 

 These give, as the accompanying figures show, a clear view over the dilferences between 

 the distribution of the plaice in the seasons referred to (see figs. p. 9). 



The dotted curves are the isomegalins, the Roman numerals indicate the respective 

 age-group, the Arabic the average length (rounded to 5 cm.) of the plaice living on the 

 grounds concerned. The contours outlined show the boundaries of the fishing-grounds' 

 in the region fished by the Dutch steam and sailing trawlers. 



These show how the I-group in spring keeps quite close to the coast and how the 

 Il-group also frequents only a narrow zone. The population on this littoral region is 



I A = Broad Fourteens and Brown Ridges. B = Texel Ground (or Haaks Ground). C = Terschel- 

 ling Flat. 



