— 154 — 



are very diiferent for the different species. Each has so to speak its own field and 

 even if the differences between the species are naturally not everywhere equally great, 

 yet amongst all the 17 species we do not find two with precisely the same requirements. 

 A striking example in this direction is offered by the genus Molva, all three species 

 of which reproduce within the regions investigated. Referring to Chart VII we see 

 that the depth is of importance here , the species naturally dividing into two groups ac- 

 cording to this-: a deep-water group (consisting of the blue ling M. byrkelange and the 

 Mediterranean ling M. elongata) which requires depths of about 1000 meters for spawning, 

 and a shallow- water group (the common ling M. molva) which spawns chiefly within the 

 200-meters line. This Chart also shows however the importance of the salinity and 

 temperature (see further pp. 146 and 149). Thus, whilst the two deep-water lings 

 hyrhelange and elongata resemble one another in requiring deep-water and in not spawn- 

 ing in the Norwegian Sea, where both the salinity and the temperature are too low for 

 them, yet their requirements with regard to the two latter factors are quite different. M. 

 elongata known first from the very salt and warm Mediterranean requires considerably 

 higher salinity and temperature (min. ca. 35'50 **/oo and 8° — 9°) than the blue ling, such 

 high numbers indeed that they only occur, at the depth in question here, in the waters 

 to the south of the Large Outer Bank west of Ireland. The result of the different re- 

 quirements of the 3 lings in regard to depth, temperature and salinity is that they par- 

 tition the whole region from north to south among themselves, that is, they 

 mutually exclude one another, as can be seen from Chart VII. Within the genus 

 Molva we thus have a striking example of how nearly related species can exclude one an- 

 other ("vicarious" species). Other examples are found in the case of the coalfish — pollack 

 (see Chart II), Norway pout— poor cod (Chart V) and the polar cod— cod (Charts I, VI), 

 all of which are pairs of related species, where the one owing to its requirements for 

 higher temperatures has a more southerly distribution than the other, with the result that 

 as "vicarious" species they are in the main mutually exclusive^ For the rest, we 

 notice that it is not only in regard to the areas (or depths) dwelt in that such species appear 

 "vicarious" or replace one another, but often also in regard to the spawning time^. Thus 

 we find for example in the cases of the blue ling— ling, coalfish— pollack, Norway pout — 

 poor cod, that the first-named species has an earlier spawning time than the second, 

 which, at least in several cases for example the two last-mentioned, seems also just to be 

 in connection with the fact, that the first "northern" species spawns at lower temperatures 

 than the second "southern" species (see also later, p. 155). 



' Cf. also torsk— hake, Chart VIII; cod— silvery pout, Chart I and whiting — Poutassou, Chart IV, 

 where however we have not to do with very nearly related species. 



^ During the translation of this paper Dr. H. M. Kyle has kindly drawn my attention to the fact, 

 that Fulton in his paper "Keport ou the Distribution and seasonal Abundance of Plat-fishes (Pleuronec- 

 tidse) in the North Sea"; (Report on Fishery and Hydrographioal investigations in the North Sea and 

 adjacent "Waters (Scotland), 1905), has already called attention to similar conditions among the flat-fishes 

 as those mentioned here. This appears, for example, from the following statement amongst others (p. 506): 

 "Thus, the spawning-seasons are complementary, and when the shoals of spring-spawners begin to disperse, 

 the summer-spawners begin to arrive or increase — ". 



For the rest, we find examples of this in all groups of fishes. Thus, at Iceland the arctic Agonus 

 decagonus spawns earlier than the common Agonus eataphractus and Gottus scorpius earlier than the 

 more southern C. bubalis, and the same applies to the spawning of the two Gottus species in the North 

 Sea and the Danish waters. 



