HABITS OF YELLOW PERCH. 321 
appear to be particularly detrimental, provided enough oxygen is also present. Winter- 
stein (1908), however, thinks that fishes are affected by the presence of carbon dioxide, 
becatise some species succumb when its tension is only 8 to 12 per cent of the total 
pressure of gases; but he also found that in two instances as much as 144.7 and 204.6 
c. c. per liter were required to overcome Leuciscus. Shelford and Allee (1913) conclude 
that the narcotic effect of carbon dioxide is more important for fishes than its action 
as an acid. Various species tested by them were affected injuriously when the amount 
present was from 5 to 37.5 c. c. per liter. 
As a rule, four factors are of chief importance for the normal respiration of fishes: 
(1) Sufficient oxygen for metabolism, (2) lack of enough carbon dioxide to be injurious, 
(3) favorable temperature, and (4) proper reaction (salinity or acidity) of water. 
Though oxygen and carbon dioxide are the only gases which usually affect the respira- 
tory activities of fishes, others may be of some importance at times. Methane and 
ammonia sometimes occur in certain restricted localities, and are injurious; nitrogen 
may, if present in unusual amount, give rise to the gas disease (Marsh and Gorham, 
1905). But troubles from such gases are of rare occurrence. Gardner and Leetham 
(1914) have shown that a trout uses twice as much oxygen for respiration if the tem- 
perature of the water about it is raised from ro to 20° C. Wells (1913) found that 
fish died more quickly in alkaline than in acid water when gas conditions were poor. 
Marsh (1910) asserts that fish will not live in well-aerated distilled water and that they 
are very susceptible to dilute solutions of mineral acids. 
The respiration of fishes, then, requires reasonably pure water of proper chemical 
reaction and with a sufficient supply of oxygen. Tlie experiments of Shelford and Allee 
(1913, 1913@) have demonstrated that fishes respond to the conditions in their environ- 
ment in such a way as to spend most of their time in the optimum. Fishes are able to 
discriminate variations in the gas content of the water, and when placed in a graded 
series usually spend the most time where conditions are best. They are apparently more 
stimulated to turn away from unfavorable regions by the presence or carbon dioxide 
than by deficiency in oxygen, some turning back upon encountering 1.5 c. c. of carbon 
dioxide per liter. “‘We have in the experiments good evidence that fishes turn back 
from waters high in carbon dioxide and low in oxygen with precision and vigor. Also 
that if they enter such localities, they can not behave normally and may soon die.’ 
Wells (1913) also has shown that fishes are most active when in water containing a scanty 
supply of oxygen and has demonstrated (1915) that a number of fresh-water species 
select slightly acid water in preference to that which is alkaline. He also asserts (1913) 
that the future will show that the reactions of fishes are of more importance than their 
resistance to unfavorable conditions. Death after reaching the vital limit is unusual, 
but the avoidance of conditions which may mean death is of frequent occurrence. The 
behavior of fishes is such that it would usually keep them in optimum conditions; yet 
Juday and Wagner (1908) found that lake trout commonly entered deep waters which 
contained so little oxygen that they could not live in them for any length of time. Paton 
(1902) also observed that brook trout which were kept in water containing very little 
oxygen were able to survive for some time by remaining inactive on the bottom and 
thus reducing their metabolism to a minimum. ‘This brings us to the resistance of fishes 
to a marked deficiency in oxygen or to an unusually large amount of carbon dioxide. 
a Shelford and Allee, 1913, p. 251. 
