304 FOUNDERS OF OCEANOGRAPHY 



dragged along the sea-bottom (the flat-fish). The methods 

 of fishing vary from place to place and from time to time 

 throughout the year. 



Many sea-fisheries are local and seasonal. This is due to 

 the movements or periodic migrations of the fish, and one 

 of the most important practical appHcations of oceanography 

 is to determine what causes these migrations in each parti- 

 cular case — why it is that one kind of fish is more abundant 

 in one locahty than in another, why the fish is present at 

 one season and absent at others, or is more plentiful one year 

 so as to give rise to a good fishery. We are beginning to 

 understand some of the causes of these movements of fish 

 and the variations in their abundance, but much has still 

 to be learned in regard to all. 



The movements may be classified into : — 



(1) Those caused by physical characters of the water 



(temperature, salinity, currents, etc.). 



(2) Those due to feeding needs. 



(3) Those explained by breeding or spawning habits. 

 As examples of the influence of the environment, we may 



take the case of the cod, which is a northern or cold water 

 fish, so in Norway it constitutes 80 per cent, of the total 

 fish-catch, and in our seas it is a winter fishery ; while its 

 relation, the hake, is a southern fish, frequenting warmer 

 water and making up 65 per cent, of the catch in the Bay 

 of Biscay. The case of the haddock, which is 50 per cent, 

 of the total catch in the North Sea and only 3 per cent, in 

 Norwegian seas, has been explained as due to the absence 

 of large areas of soft bottom at a suitable depth for that fish 

 on the coast of Norway. 



Nearly fifty years ago, Moebius and Heincke first showed, 

 from their investigations of 1877 and subsequent years, 

 that in the case of the Baltic and Kattegat there were annual 

 immigrations of northern fishes in spring and of southern 

 fishes in autumn ; and in their expedition of 1890, Otto 

 Pettersson and Ekman proved that these seasonal move- 



