192 THE OCEAN RIVER 



other hand, is found only in less salty water, such as Channel 

 water. The kind of arrow worm found in the waters off Plym- 

 outh is an important clue as to how far the Atlantic water has 

 pushed its way eastward. 



Before 1931 the arrow worms off Plymouth were Sagitta 

 elegans and there was a valuable herring fishery there; in later 

 years the herring fishery ceased. The plankton was poorer, and 

 the water now contained Sagitta setosa instead of Sagitta 

 eleganSy thus suggesting that since 1931 Atlantic water had 

 failed to enter the Channel and that the water that replaced 

 it was too poor in phosphates to support the herring fishery. 

 At the other end of the Channel, in the North Sea, a similar 

 movement takes place at the northern boundary of the Chan- 

 nel water. Thus a study of arrow worms in the plankton off 

 Plymouth helps us to predict in a rather general way both the 

 Plymouth fishery and the fishery of the North Sea. 



Plankton indicators — as organisms like Sagitta are called — 

 are sometimes useful in tracing the path of ocean currents, 

 though they must be used with proper scientific caution. Their 

 tale of travel from far places is sometimes familiar to fisher- 

 men when they see in the cold Gulf of Maine waters or in the 

 waters off European shores the Portuguese man-of-war or trop- 

 ical kinds of jellyfish telling of their long drift from warmer 

 latitudes. At times subtropical plankton is found in the scien- 

 tist's silk nets as far north as the Labrador Sea. There are other 

 cases, though, where plankton is able to escape from the drag 

 of currents. If this were not so, many parts of the ocean might 

 lose their rich pasturage, due to its continual drifting away in 

 currents, without leaving behind a sufficient residue to breed 

 new stock. 



In the rich antarctic feeding grounds of whales there is a 

 continual northward surface drift of the diatoms and Crustacea 

 during the summer. This does not lead to depletion of the 

 plankton, however, because in winter the small organisms sink 

 through the water into warmer, deeper layers, which are return- 



