134 THE SEAS 



forms a considerable part of modem investigations into the 

 resources of the sea. This chapter has been devoted 

 mainly to describing what the plankton is and where and 

 how it lives, but in a future chapter will be given further 

 information of the part played by these small organisms 

 in the watery world, in the light of recent research. 



We cannot conclude, however, without some mention of 

 recent application of plankton research to fishery investiga- 

 tions. It has been found that in some regions, such as the 

 southern North Sea the diatoms may develop into patches 

 of sufficient concentration to be avoided by certain pelagic 

 fishes such as the herring. Under these circumstances 

 the herring may be diverted from their normal course of 

 migration, and this has important effects on the fisheries. 

 It is now a part of fishery research to watch the develop- 

 ment and movement of these diatom patches in an attempt 

 to show the fisherman where he is likely to obtain the best 

 catches. 



The study of plankton organisms is also proving of 

 use in helping to show the movements of water masses. 

 Some species of plankton animals are restricted to waters of 

 certain types. The presence of oceanic water can, for 

 instance, be detected in coastal areas by the presence of 

 certain oceanic plankton organisms, which therefore give 

 an additional indication of the origin of the water to those 

 chemical and physical characteristics usually made use of 

 by hydrographers. The study of this interesting drifting 

 community thus now assumes practical importance. 



