588 HOW DOES MAN CONSERVE HIS RESOURCES? 



If crude sewage is discharged into a river untreated, the organic 

 matter absorbs much or all of the dissolved oxygen in the water, 

 which is absolutely essential for plant and animal life. Oil wastes 

 poured out by oil-burning steamers are becoming a nuisance along 

 our shores and are responsible for the death of many food fish. 

 If our fish and other water animals are to be preserved, we must 

 stop the pollution of our national waterways. Dr. Henry B. Ward 

 is authority for the estimate that if rivers now polluted with 

 sewage and factory wastes were clean, they would put $100,000,000 

 a year into the pockets of taxpayers from the sale of marketable 

 fish. In addition to this thousands of persons could enjoy rivers 

 for fishing, boating, and bathing. 



Conservation of ocean fishes. We have learned that the re- 

 lation of fish life to their natural food is an important factor in 

 preserving our supply of fish, especially in inland waters. The 

 balance of life is equally important for fish in the ocean. It is 

 one of the greatest problems of our Bureau of Fisheries to discover 

 the relation of various fishes to their food supplies so as to aid in 

 the conservation of life in our lakes, rivers, and seas. 



Migration of fishes. Some fishes change their habitat at dif- 

 ferent times during the year, moving in vast schools northward in 

 the summer and southward in the winter. In a general way such 

 migrations follow the coast lines. Examples of such migratory 

 fish are the cod, menhaden, herring, and bluefish. The migrations 

 are due to temperature changes, to the quest of food, and to the 

 spawning instinct. Salmon and some other fish pass up rivers to 

 spawn ; the eel, on the contrary, leaves the rivers and spawns in 

 the ocean. Some fish migrate to more shallow water in the sum- 

 mer, and to deeper water in the winter ; here the reason for the 

 migration is doubtless the change in temperature. All of these 

 habits are studied by the fishermen, who are thus able to catch fish 

 where and when they are most plentiful. The cod and herring 

 fisheries are notable examples. 



The spawning habits of fish. The spawning habits of fish are of 

 great importance to us because the fish are caught in vast numbers 

 at the time of migration. Many of our most desirable food fishes, 

 notably the salmon, shad, sturgeon, and smelt, pass up rivers from 



