PROPAGATION AND DISTRIBUTION OF FOOD FISHES, 19^2. 103 



anythino^ in return. The supply of fish in streams may be increased 

 also by the closing of small feeder branches that are suitable for 

 small fish and by the temporary closing of main streams in rotation. 



POLLUTION OF STREAMS. 



The following information on the pollution of streams, adapted 

 from the report of the New York Conservation Commission for 

 1922, will enable those interested in stocking public waters to decide 

 whether the streams in which they are interested are polluted to 

 the extent that fish could not survive therein : 



Pollution of a stream by sewage and other organic wastes may 

 injure fish by removal of gaseous oxygen dissolved in water. This 

 dissolved oxygen is as necessary to fish as is oxygen of the air to 

 the ordinary air-breathing animal; without it the fish suffocate. 



Certain common and easily recognized water plants and animals 

 show accurately the dissolved oxygen content of the water and can 

 be considered as indicators of the degree of pollution of a stream. 



Water in a state of chemical purity does not exist in nature. 

 Water flowing in any stream carries in solution certain salts, gases, 

 and other compounds and frequently various matter in suspension. 

 Only by the presence of these materials can aquatic life exist. 



Pollution may actually kill fish at some stages of development 

 and not at others. It may be harmful to some species and not to 

 others, and it may have effects in any degree of gravity. It may 

 affect a small area or many miles in the length of a stream. Intensity 

 of pollution, extent of the affected area, and the habits of the 

 fish normal to the stream must all be considered. 



Fish themselves are not always the best indicators of the condition 

 of a stream. Failure to catch fish does not prove that a condition 

 of serious pollution exists. Absence, real or apparent, may be ex- 

 plained by overfishing, invisibility, or failure to take the hook, or 

 the fish may have gone elsewhere on their own affairs and not because 

 of pollution. The small plants and animals present, which either 

 can not move at all or can not move far, show the average stream 

 condition better than do the fish. 



Plants growing in clean water are markedly green and frequently 

 of more highly organized types than those in polluted sections, and 

 animal life tends to be more active and highly organized. Typical 

 forms of animals and plants that will not inhabit polluted waters 

 are shown in the following list : 



Dobson or " Helgramite " (Corydalis cornuta, Linn). Larva. Three to four 

 inches long. Under or around stones in swift water. 



Stonefly {Perla sp.) Nymph. Under and around stones in swift water; very 

 flat body with two " tails." 



Caddis worm (Hydropsijche sp.) Larva of the caddis fly. Several species 

 with various habits and forms ; in swift waters mostly ; under stones in 

 loosely constructed pebble and webbing cases ; attached to stones on the 

 lee side of the current in compact stone cases, or in a funnel-like strainer 

 of webbing with a retreat. 



May fly (HeptOf/enia sp.) Nymph. Under stones in swift water; very flat 

 body, smaller usually than stonefly ; three " tails." 



Dragonfly {Anax Junius, Dru). Nymph. Extensible "lower jaw;" darts for- 

 ward by expelling water from behind ; found in dense growths of green 

 aquatic plants in still water. Damselfly nymphs are similar, but have three 

 long featherlike " tails." 



