55 
Bottom Oxygen and Carbon Dioxide—Samples should be taken 
from the bottom. The oxygen content of water over breeding grounds 
should be about 4 c.c. per liter, and CO, probably not more than 1-5 c.c. 
per liter, the precise amount depending on the fish species present (Shel- 
ford, 14; Wells, 13, ’15,’15a). The effect of CO, on the eggs of fresh- 
water fishes has not been fully determined (Ransom,’66). (Milner, 74; 
Anthony, 08; Paige,’08; Birge and Juday, ’11; Shelford, ’11,’11a, ’11b; 
Forbes and Richardson, ’13; Wells, 13.) 
Conditions favoring Fungi.—lt is probable that acidity, that is, much 
CO,, and bad aeration favor fungus attack. Such attacks usually occur 
on eggs or adults; not on fry (Clinton, ’93; Dean, ’93; Richardson, '13). 
VII. If the supply of useful animals is depleted will recovery be 
rapid or slow? 
Petersen and Jensen found that if the flora and fauna were removed 
from marine bottoms useful animals such as oysters can not again live 
on them until a series or succession of plants and animals has prepared 
the way. (Petersen and Jensen, ’11; Mobius, ’83.) The same is true of 
fishes in fresh water. A body of water deprived of all its vegetation, 
with the associated animals, requires much time for recovery. It is not 
simply the useful animals that must be taken into consideration, but the 
entire association (Adams, ’09; Shelford, 710, 11, 11a, ’11b). 
VIII. Can correct decisions be reached without investigation of indi- 
vidual cases which arise? 
Decisions relative to all the preceding points must usually be reached 
on the ground. Waters differ in their capacity to neutralize the effects 
of effluents, in the maximum and minimum flow, and in their dissolved 
content. Samples of water should be taken with reference to the par- 
ticular animal-problem in hand. Carbon dioxide and oxygen determina- 
tions should be made on the ground, and at least a small amount of 
observation is indispensable. Mere office decisions, made at a distance 
and based upon report and inferences, are dangerous. The ‘“‘antagonisms” 
of poisons, discussed on page 33, make decision without experiments 
unwise. 
IX. What is the real value of the waste when the amount of the 
damage which it causes is added to its commercial value? 
One continually hears it said that the recovery of this or that waste 
product does not pay; that this is an all-sufficient reason for not recover- 
ing it—and the matter is usually dismissed forthwith. We need an 
entirely new view-point. The value of any waste product is its com- 
mercial value, when properly recovered, plus the amount of loss it 
occasions when unrecovered. Practically all kinds of waste may be made 
into something useful (Koller,’15; Weston,’16; Feilding,’17). The 
recovery of grease from sewage or garbage might not pay under market 
conditions, but if the grease so recovered will prevent the national supply 
from giving out it should be recovered by all means. It has been esti- 
