— 11 - JOBS. SCHMIDT 



believe thai tlio niellioil used, namo)y (ho discovery of the quite young pelagic developmental 

 stages, is specially suited, as it is much easier to catch the fishes in these young pelagic 

 stages than in the older, amongst other reasons, because the difficulties as to the 

 bottom, ditterences in apparatus etc. hinder a comparison of the catches, when the older 

 flsh living on the bottom are dealt with (or the pelagic fish in the case of G. Poutassou). 

 It appeared also, as soon as the "Thor" started work at Iceland, that we were able by 

 means of the young-fish trawl to prove the presence of species from the catches of the fry, 

 which were not known to occur there.' And the method has stood the test even 

 in more southern waters much better investigated than Iceland. I need only refer to the 

 Chart given here (Ch. VII) of the Molva species, from which it appears that M. byrkelange, 

 which was so to speak not known in British waters, has on the contrary its centre of 

 distribution to the N. W. and W. of Scotland. 



It is thus apparent that the investigations on the distribution of the pelagic stages 

 can in a relatively easy manner extend our knowledge of the regions occupied by the 

 different species. 



It is possible to go even further. As I have mentioned on earlier occasions, the fishes, 

 at least many species, vary greatly in their sensitiveness towards external conditions 

 (temperature, salinity, depth) during the different stages of their life." They are most 

 sensitive at the spawning season, and we thus find that they often undertake long 

 wanderings in order to obtain certain definite external conditions whilst spawning. This 

 is specially striking in the common eel (Anguilla vulgaris)^ which migrates out from the 

 quite shallow, fresh or brackish water where it grows up, to the deep, salt and warm At- 

 lantic water to spawn; but several species also undertake long migrations in order to 

 reach certain depths and temperatures favourable to reproduction. The result is that the 

 spawning fish, on account of their much more definite cravings after certain external 

 factors, are brought together on a much smaller area than that over which the species is 

 ordinarily distributed, i. e. a congregation of individuals takes place, which is 

 presumably of importance for fertilisation and thus the reproduction of the species. 



In great contrast to this stands the dispersion of the individuals over much 

 greater, in physical regards often very dissimilar areas, which arises from the drift of the 

 pelagic fry from the spawning places, and which naturally amongst other things has the 

 advantage of procuring better conditions as to food for each individual during growth than 

 if they had all remained at the spawning places. 



The biology of the cod at Iceland is very instructive in this regard. As will be shown 

 in more detail later, the cod spawns at Iceland only in the warm Atlantic water on the 

 S. a,nd W. coasts, but a large portion of the fry is carried to the N. and E. coasts and come 

 to live there under quite different conditions from those the fry which grow up on the S. 

 and W. coasts live under. Thus, whilst the spawning cod at Iceland are only to be found 

 in quantities at places where the water has a temperature of from ca. 4° to ca. 7° C, 

 the different stages in growth may well be found in quantities in water of, on the one 



' .FoHS. Schmidt, Fiskeriundersegelser ved Island ug Fceroerne i Sommeren 1903, p. 53, 1904. 



* Contributions to tlie life-history of the Eel (Anguilla vulgaris), p. 235. Rapports et Procès-Ver- 

 baux, Vol. V, 1906 ; — and, Blarking experiments on Plaice and Cod in Icelandic waters, p. 22. Meddel. 

 Komm. Havunders., Ser. Fiskeri, Bind II, Nr. 6, 1907. 



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