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seems very probable, at least provisionally with all our present experience in mind, that 

 these fluctuations are caused by migrations. 



There are two kinds of migration. 



(1) In the first place it may be accepted in general, that the older individuals 

 migrate into deeper water, towards the northernmost part of the North Sea and the 

 deep-sea margin of the Skager Rak. Considering the 2 charts showing the average catch, 

 in kg. per hour of fishing for the whole year, of the 2 different classes "large" and 

 "small" in the hauls in the Northern North Sea, we see that the average weight of the 

 class "small" is fairly uniform over the entire Northern North Sea; whereas in the class 

 ''large" the chart shows a sharp rise towards the deep water. The highest average 

 catches of the "large" class are from the north-west of Scotland. Since the largest indi- 

 viduals in spite of their smaller numbers greatly influence the total catch, we also find 

 the same rule in fig. 40 showing the average total catch of haddock of all sizes, namely, 

 the largest catches in the greatest depths. 



The question, at what size the haddock first undertake these migrations towards the 

 deeper parts of the water, is naturally very difficult to answer. If we may use the 

 catches of the research steamers as basis, we might indeed conclude that a principal 

 reason for the great decrease in number of all haddock above maturity is to be found in 

 the already begun migration. The catches of the research steamers were made, as can 

 be seen from the hauls themselves, chiefly in the areas where the largest sizes are not 

 very common. This shows, as will appear more clearly from the following section on 

 the cod, that only an investigation of the whole North Sea Bank as far as the deep-sea 

 margin will be able to give a satisfactory result. 



(2) The marked seasonal migrations shown in the curves of the catches are of a 

 different kind. Of these the winter and spring migrations are to be regarded as spawning 

 migrations to the spawning places, similar to the long-known migrations which the cod 

 undertake yearly to their definite spawning grounds. The summer and autumn migrations 

 have naturally a different cause, but we do not think we are warranted in discussing this 

 question in detail here. We may refer however to the frequent concurrence of these mi- 

 grations with the appearance of shoals of spawning herring. The practical fishermen 

 have many experiences of this, but we cannot here enter further into the matter. 



In the following chapter we shall briefly return to these conditions in discussing the 

 question, what light the results obtained throw upon some practical fisheries problems. 



II. Cod 



North Sea 

 In the trawl hauls, which yielded the just discussed material on the haddock, 1 1,440 

 cod among other fishes were also taken and all were measured. Helland-Hansen has 

 subjected these cod measurements to statistical analysis, and we may give here some of 

 the principal results. 



