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quantities in the colder water more or less mixed with polar water off the east and north 

 coasts and often in great depths, collect together in the spring to spawn on the shallow 

 banks off the south coast in the warm Atlantic water. Here also we have migrations 

 of considerable extent for the purpose of spawning, and something similar also occurs 

 with the herring at Iceland. In the Danish waters we also have the garfish [Be/one) 

 and the lumpsucker [Cyclopterus) migrating inwards in the spring or early summer 

 to perform the function of spawning, but when this is completed they disappear again more 

 or less quickly etc. In these cases it is the migrations which are specially conspicuous. 

 On the other hand, we also know many examples, when the fish for longer or shorter 

 periods of their life remain in certain waters without propagating there. The witch 

 (Pleuronectes cynogiosszis) and the angler {Lophius) for example live in the northern 

 part of the Kattegat, but do not propagate there. At Iceland I have found striking 

 examples of such cases. It is true especially of the cod. This is spawned on the south 

 coast and is found here in the warm Atlantic water as tiny pelagic fry, whilst no or 

 practically no cod are spawned off the colder east and north coasts. Nevertheless the 

 earliest annual groups of the cod occur in far larger numbers at the east and north 

 coasts, where quite different natural conditions prevail, than at the south coast, where 

 they are spawned. Somewhat similar holds good at Iceland, for example, for the plaice 

 and dab, partly also the haddock, and it is my belief, which the investigations on board 

 the "Thor" have strengthened year by year, that the more the biology of the different 

 species is unravelled the more examples will be found of this. We have here evidently 

 a phenomenon of great generality in the biology of fishes, which may be expressed as 

 follows: Many fish at the spawning period are far more sensitive tp external 

 surroundings (depth, temperature, degree of salinity) than at other times, so that, when 

 about to spawn they often demand quite definite temperatures and depths, whilst otherwise 

 perhaps and at other periods of their life they are able to live under quite different con- 

 ditions ='. It seems therefore, that to obtain these definite outer conditions must be of 

 great importance for the fish, because for this reason they must often undertake widely 

 extended migrations. Wherein the reason of the greater sensitiveness of the fish to 

 external conditions at the spawning period lies, cannot be answered with certainty. For 

 many species it may possibly be, that the individuals, which otherwise live scattered over 

 wide regions of diverse character, are driven by this sensitiveness to collect on far smaller 

 areas, which is of the greatest importance for the fertilisation of the eggs. Perhaps the 

 reason is, that the definite outer conditions of life which differ for the different species, 

 and which are sought for by the spawning fish, are the most favourable for the devel- 

 opment of the eggs and the tiny fry, perhaps both these conditions or several others un- 

 known are of importance in this regard, but I shall not enter into the matter further so 

 as to avoid treading too much upon hypothetical ground. 



I have only brought forward these views, because they are perhaps not sufficiently 

 attended and further as showing, that it is by no means the eel alone of our important 



^ In my opinion therefore, if we wish to describe and define the distribution of one or other species of 

 fish according to the surroundings in which it lives, we must first of all investigate under what outer conditions 

 its spawning takes place, or what is the same, under what circumstances its tiny fry occur and in much less 

 degree note the place of discovery on as large an area as possible without regard to the year's group taken. 

 In this way we will surely reach to a much clearer understanding of the distribution. 



3o* 



