568 



placing a species of unknown resistance in an experiment with one of 

 the species given in Table III one may obtain results that will make 

 it possible for him to compare the resistance of the unknown species 

 with that of any of the species listed. It should be pointed out also, 

 that fishes of the same large taxonomic group have in general a similar 

 power of resisting detrimental factors. Thus, the darters are a group 

 possessing for the most part a low ability to resist untoward condi- 

 tions. The minnows (Cyprinidae) are fairly resistant as a group; the 

 sunfishes are more resistant than the minnows ; and the catfishes are 

 notably our most resistant group of fresh-water fishes. The place of 

 an untried species in the resistance table can be reckoned more or less 

 accurately by placing it with the listed representatives of the taxonomic 

 group to which it belongs. 



From column 3 (Table III) it will be seen that the resistance of 

 the fishes is rather closely correlated with the type of environment 

 which they inhabit. The more resistant species are found in ponds, 

 shallow, muddy-bottomed lakes, or in the stagnant pools of streams. 

 These are the fishes w^hich one sees in aquaria. They are able to with- 

 stand increased temperature and wide fluctuation in the oxygen and 

 carbon-dioxide content of the water, and to some extent are able to 

 live in the presence of the excretory products of their ow-n metabolism. 

 The stream fishes proper can not do this, and therefore die when placed 

 for any length of time in standing water. 



Summary 



1. The introduction of either carbon dioxide or carbon monoxide 

 into fish waters is certain to prove detrimental to the acjuatic organ- 

 isms, and especially to the fishes present in the water. 



2. Both carbon dioxide and carbon monoxide are poisonous to 

 fishes. Of the two gases, the monoxide is by far the more deadly. 



3. Fishes are very sensitive to small changes in the carbon-dioxide 

 content of the water, and tend to avoid detrimental concentrations of 

 this gas by a very definite turning back from them. Fishes do not 

 appear to detect the presence of carbon monoxide in the water, and 

 will swim into concentrations of this gas that kill them in a few 

 minutes. 



4. In general, the resistance of fishes is correlated w'ith the en- 

 vironment in which they are found. The more resistant species are 

 found in ponds and shallow lakes while the least resistant fishes occur 

 in the swift streams and in cold, deep lakes. 



