RESISTANCE OF FISHES. 343 



To determine the efficiency of a given body of water as a 

 fish-producer requires the solution of a large number of problems 

 such as the above, and many of these problems demand that the 

 relative importance of resistance and reaction, in fish distribution 

 and existence, be known. In the discussion thus far, resistance 

 pf fishes has been shown to be of considerable importance in fish 

 survival, under certain conditions and at certain stages of 

 development. That fish reaction is the more important factor, 

 however, is indicated largely by one fact, namely, adverse condi- 

 tions in natural or artificial fish environments are seldom so general 

 or so sudden in appearance that the fishes cannot escape them, at 

 least to an extent sufficient for sitn'ii'al, by making the proper 

 reactions, and observation and experimentation indicate that fishes, 

 in the majority of cases, make such reactions. It seems pretty 

 well established that fishes regulate their own distribution, and 

 thus indirectly their existence, largely through their reactions 

 to the environmental factors which they encounter. In other 

 words, fishes seldom put their resistance powers to the test, so 

 long as there is a way out of the disturbing conditions. It follows 

 that resistance of fishes becomes a life and death matter, only 

 when adjustment cannot be made by a behavior reaction, and 

 such situations are rare in the life cycles of most fishes. 



On the other hand, fish reactions are an important factor 

 even in the smallest and most enclosed bodies of water, provided 

 the waters are able to support fishes at all. In ponds, for 

 instance, the fishes may be able to withstand the stagnancy of 

 the dry season, by coming to the surface, gulping the surface 

 film, burrowing into the bottom, etc. (Kendall, '10). In some 

 fish environments, vertical differences in conditions are most 

 important in their effect upon fish reactions. In small lakes 

 and streams horizontal conditions are important. In such 

 waters there are areas of vegetation, muddy bottom, sand}' or 

 gravelly bottom, ripples, pools, etc. Where such conditions 

 are present there is considerable opportunity for the selection 

 of habitat, upon the part of the fishes, and that they exercise 

 such choice has been demonstrated by Shelford ('no). As a 

 result of the choosing, we find the darters, for example, in the 

 swift-flowing rock-bottomed ripples, while the suckers in general 



