— 12 - 



hand, from 0°— 1° and on the other, of from 10° — 11°; briefly, the cod is much more 

 sensitive towards external conditions at the spawning season than at any other time. 



From this consideration that the craving for definite external conditions is most 

 accentuated at the spawning time, it becomes of special importance to investigate, under 

 what external conditions the spawning of the various species taices place, 

 since we thus have a much better chance of clearly understanding what conditions the 

 distribution of the separate species. In order to do this, we must first determine the 

 regions within which each species spawns and then investigate the special conditions there 

 at the spawning time. 



To determine the spawning region of each species is therefore the first 

 task on the road towards an understanding of the geography and biology of the species. 

 Two ways of doing this are conceivable. (1) The various waters may be searched by means 

 of trawl, line, drift-net etc. in order to find where the adult spawning fish occur and 

 where they are lacking. To make such an investigation over so large a region as that 

 over which most species at one or other period in their life-history are distributed, is 

 however practically impossible, more especially when it is a question of species which live 

 in deep water or pelagically, so that they are never taken in our ordinary fishing 

 apparatus. Of several species we have as yet never seen any spawning specimens /'öac^Ms 

 Poutassou, argenteus, Molva hyrkelange and elongata etc.). 



(2) A second method of determining the spawning regions of the species is by means 

 of the pelagic net, to find where the pelagic eggs occur and where they are wanting. 

 From a technical standpoint there is nothing in the way, as it is easy to make a net 

 which will catch the eggs. But a further difficulty arises, namely, that for most species 

 we are unable to determine the eggs with certainty (see p. 10). And an 

 absolutely certain systematic basis is in my opinion the first condition for such a work 

 as this. 



For the present there is no other way than to go to the earliest stages in the 

 development which can be determined with certainty, namely, the early 

 postlarval pelagic stages. This I have done in the present work, knowing however 

 that it would be possible to make an even more thorough investigation if we only knew 

 how to determine the very earliest stages in development, namely, the eggs of all the 

 species. The present method of determining the spawning region of fishes from the distri- 

 bution of the pelagic fry is admittedly not free from error, since the oceanic currents may 

 carry the eggs and young from the spawning places, more or less according to the age, i. 

 e. according to the time they are under the influence of the currents. 



The method must therefore naturally be used with caution and critical attention. We 

 must especially lay chief stress on the distribution of the very earliest stages of the 

 pelagic fry, and further take the currents in account. In the few cases where the eggs 

 can be determined with certainty, we should naturally attach most importance to their 

 distribution in forming conclusions as to the spawning places of the species in question. 



What I have done therefore has been (1), with the help of the distribution of the earliest 

 determinable stages to map out the spawning regions of the various species within the 

 great expanse from the Polar Circle to the north coast of Spain; (2) by determining the 

 depth, temperature and salinity within and outside of the spawning region during the 



