484 SOME RESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 



these in the past have lent tliemselves badly to investigations of this 

 kind. The official returns of fish landed were almost entirely useless, 

 and it was necessary for the fishery research organizations to so organize 

 returns of fish caught as to show the connexions we speak of. Such 

 investigations as those of Fulton and Henking were alone suitable. 



Nevertheless, the dependence of fish migrations on hydrographic 

 changes in the sea was always a priori probable. For a fish in the sea 

 the water is just such a medium as the atmosphere is for a migratory 

 bird ; atid if we recognize that climatic changes were the main factors 

 in determining the breeding seasons and migrations of birds, it was 

 surely probable that changes in temperature, etc., in the sea affected the 

 breeding seasons and migrations of fishes. We know how very 

 intimately the period of incubation of a fish egg is determined by the 

 temperature of the water in which it develops ; and how the spawning 

 periods themselves are variable with the temperature of the sea. It 

 was just as reasonable to assume that fisli migrations were also in- 

 fluenced by temperature at least. The International investigations are 

 slowly accumulating instances of these connexions. Some such 

 connexions were established before the beginning of these researches, 

 but others are coming to light. Long ago Mobius and Heincke divided 

 the fishes visiting the Baltic into " north fishes " and " south fishes." 

 The north fishes have their homes in the Norwegian sea and the waters 

 surrounding Iceland and Faeroe. The south fishes come from the 

 temperate Atlantic. Mobius and Heincke noticed that the north fishes 

 only visited the Cattegat during the first part of the year, and the 

 south fishes during the latter part. Afterwards, when the hydrographic 

 periodicity of the waters of the Skagerak, Cattegat, and Baltic was 

 demonstrated, these migrations were correlated with the ebb and flow of 

 the Atlantic stream. We have seen that during the latter part of the 

 year warm Atlantic water accumulates in the depths of the Skagerak 

 and sets up a warm undercurrent into the Baltic, which is at a 

 maximum about the end of the year. The south fishes appear and 

 travel with this undercurrent, which sets up changes in the fishery 

 biology of the Baltic. Thus in the German fishery cruises of December, 

 1903, plaice and other flat-fishes, many of them spawning, were found 

 in the southern Baltic in this warm undercurrent. On the other hand, 

 flat-fishes were hardly at all found in this part of the Baltic in June 

 and July, at which time the bottom was covered by the water of the 

 cold undercurrent from the Skagerak, which enters the Cattegat in 

 spring. The hake is a typical south fish, and we have seen that its 

 capture in the North Sea is very inconstant and indicates a definite 

 migration. In the latter area it arrives towards the end of summer 

 with the incoming Atlantic stream ; then it is relatively abundant. 

 In the winter it disappears again. 



