THE ALLIES OF THE PERCH 329 



fish below certain lengths. In some locations the numbers 

 allowed each fisherman per day are limited. 



Power dams and pollution of the streams by sewage and 

 factory wastes are probably the greatest factors in reduc- 

 ing the fish population of our streams. Public sentiment 

 against, and knowledge of, these destructive conditions are 

 necessary before adequate laws may be passed and enforced 

 to protect the life of our streams. There are many very 

 worthy organizations whose chief object is the enlisting of 

 public sentiment in favor of conservation. Some states set 

 apart streams as fish refuges. In these refuges fishing is 

 either prohibited or very rigidly restricted. 



The lives of many thousands of fishes are saved annually 

 by the rescue crews operating in the backwaters after 

 streams have been flooded. The relation of this rescue 

 work to mussel propagation was mentioned on page 205. 



Indirect Value of Fishes. In judging the value or impor- 

 tance of the various kinds of fishes their use as food for man 

 is not the only relationship to be considered. The gizzard 

 shad (Doroso'ma) is rarely used for human food. Yet in 

 many streams it is very abundant, living very largely on 

 microscopic plants and animals. In turn it serves as one of 

 the most abundant food supplies of game fishes, which are 

 unable to use the food upon which the gizzard shad thrives. 

 Other species are noteworthy because of their destructive 

 habits. The gar pikes are outlaws, for they have voracious 

 appetites and live almost wholly on small fishes and the 

 young of species which are valuable as food for man. 



Mosquito Control by Fishes. Goldfishes and several species 

 of minnows feed very largely on insect larvae. In many 

 regions where ponds and swamps are the breeding places of 

 hordes of mosquitoes the mosquito problem has been solved 

 by introducing goldfishes, and minnows (especially of the 

 genus Gambu'sia), into the water. 



