TEMPERATURES AT THE SEA SURFACE 53 



estimates thus arrived at an average temperature f->r the whole 

 mass of the oreanic water of the three great oceans. The 

 averages given by Kriimmel are 



Atlantic. Indian. Pacific. Whole Oceans. 



4-020 C. 3-82 . 373 C. 3-83 C. 



The temperature of the great mass of oceanic water, 

 approximately 38-8 F., is thus low, and shows that the oceans 

 are in the main extremely cold. The main source of organic 

 material in the sea is fortunately met with in cold-water areas. 

 At the same time it must not be forgotten that the upper layers 

 in which the plankton is produced are by far the warmest, and 

 there is a general tendency for warm surface currents to flow 

 from the Equator to the Poles. Of these currents the " Gulf 

 Stream " is the leading example. 



This Gulf Stream varies in strength from year to year, and 

 according to many oceanographers, such as Pettersson, there 

 is a direct connection between the thermal conditions of the 

 Gulf Stream in North European waters and the climate of 

 the adjacent countries, and possibly also in the migration of 

 certain fish of economic importance, such as herring (p. 60). 



A detailed consideration of the distribution of temperature, 

 both horizontal and vertical, in the various oceans and seas of 

 the globe is beyond the scope of this work. 



That, however, it is not a question of purely theoretical 

 interest a few instances will prove. 



Most fish of economic importance are found in waters with 

 a certain range of temperature, and in waters above'or below 

 certain ranges no fish of commercial value are met with (see 



P- 35)- 



In depths of 100 metres (54'6 fathoms) the temperatures 



correspond to and change with the surface temperatures as a 

 general rule. At 200 metres (109 fathoms) there is a perceptible 

 general lowering of the temperature. At 400 metres (218 

 fathoms) there are still wide areas in the tropics where the 

 temperature is 10 C. (50 F.) or over, and the influence of 



