574 The Animal Kingdom 



The opening of the St. Lawrence Waterway has permitted this ver- 

 satile animal to spread its range to the upper Great Lakes. Here they 

 have attacked many of the commercially important fish such as lake 

 trout and cisco. Unless effective control measures are found, the 

 Great Lakes fisheries may be ruined. One method which has been tried 

 is based on the knowledge that the lampreys go up the small streams to 

 breed. Fences built across these streams have been suggested as bar- 

 riers. This, however, is very expensive and quite difficult when one 

 thinks of the millions of small streams that pour their waters into the 



Fig. 193. — Some representatives of the class Agnatha. a, Myxine, the hagfish; 

 b, Petromyzon, the sea lamprey; c, an ostracoderm. 



Great Lakes. A more recent development has been to place metal 

 poles at the tributaries' mouths. The lamprey will attach to any ob- 

 ject, so they cling temporarily to these poles where they may be killed 

 by an electrical current. This method is still not too effective due to the 

 expense involved. At the moment, the lampreys seem to be increas- 

 ing in number and even spreading into the rivers. So thick have they 

 become in some areas that there are reports of their attacking swim- 

 mers. 



