— 72 - 



their spawning places chiefly in the more southerly, warmer part of the region. All of 

 them also occur further south and disappear on the west coast of the British Isles. We 

 must conclude that the temperature of 8° prevailing here sets a limit to the distribution 

 of these species towards the north or into the North Sea. In favour of this also we 

 have the fact, that the spawning of these species in the North Sea and Skager Rak or 

 in the Norwegian fjords only occurs later in the summer (June — July), when the temp- 

 erature has reached the same height as in the Atlantic in the spring months. 



Whilst the west coasts of the British Isles form the northern boundary for these more 

 southerly species, these coasts belong to the most southern part of the spawning regions 

 of the remaining species which live in shallow water. 



These species, namely Gadiis merlangus^ callarias, aeglefinus and Eswiarkii, are 

 also not restricted to the Atlantic Ocean. They all go more or less far into the North 

 Sea and Norwegian Sea. As remarked above, in their vertical distribution they form a 

 series, in which the whiting spawns in shallowest water, the cod somewhat deeper and 

 the haddock and Gadns Esmaikii deepest. These conditions can be best seen in the 

 North Sea, as all the forms occur there in great quantity and the differences are so 

 well-marked that a view can readily be obtained over the relation of the species to depth, 

 salinity and temperature. 



The Charts figs. 20 and 21 show that the saltest Atlantic water (see curves of 35,25 

 and 35 °/oq) only occurs at the bottom in the northernmost part of the North Sea. We 

 notice here also that these isohalins coincide approximately with the boundary for 6°. In 

 the southern, shallower part of the North Sea we have salinities between 35 "l^^ and 

 30 7oc. äf^d temperatures of 5°, 4° and 3°, varying according to the depths and the geo- 

 graphical position (see also fig. 5, section for P'ebruary 1906). 



If we compare the distribution of the principal gadoids with these charts, we find 

 amongst other things, that the two species haddock and Gadus Esmarkii form a group 

 which spawns in deeper, warmer and salter water (over 35 °/o), whilst on the other hand 

 the cod and whiting both spawn in water of lower salinity and lower temperature. 



Redeke's paper shows very clearly how the cod and the whiting in the Southern 

 North Sea spawn just beyond the 35 °l^^ limit. See especially his figures 6 and 7. We 

 give here his fig. 6 as fig. 23 and refer for the rest to the explanation of the figures, 

 which describe the conditions well. 



On the other hand, in the deeper layers in which the cod spawns, the temperature 

 in spring in the Southern North Sea is the same as everywhere in the Norwegian Sea 

 right to Finmark. 



With regard to the haddock, its spawning terminates on the Norwegian coastal 

 banks much further to the south than should presumably be the case according to the 

 temperature conditions. On the banks north of the Lofotens the temperature conditions 

 are the same as in the Northern North Sea, but on the other hand the salinity is much 

 lower than that of the Atlantic water in the North Sea. 



The natural history of the coal fish seems also to indicate a similar dependency on 

 the salinity or a connection with this (see below). 



On the slopes of the Atlantic coastal banks towards the Atlantic deep-sea 

 margin the following gadoids spawn according to our observations: Meyluccius vulgaris. 



