566 



probable that the reactions of most fishes have more to do with their 

 persistence in natural environments than does their power of resis- 

 tance, for the appearance of adverse conditions in natural waters is 

 seldom so general or so sudden that fishes can not escape, by the 

 proper reactions, at least sufficiently for survival, and observation 

 and experiment indicate that most fishes will so react. 



Although the exact relation between reaction and resistance in 

 organisms is not clear, as a general rule, those organisms which show 

 but little power of resistance to adverse environmental changes are 

 for the most part quite sensitive and quick to react to such changes, 

 while the more resistant species frequently show little or no signs 

 of definite reaction to the detrimental factor. 



The resistance of fishes to hurtful conditions varies with the 

 species, with age (or size and weight), with the individual (that is 

 with physiological state), and with the season. Practically all of the 

 fishes worked with are least resistant just after the breeding season, 

 or in the months of June, July, and August (see Wells, 'i6). In 

 September the curve of resistance begins to run up, and it continues 

 to rise throughout the winter months, reaching its maximum in March, 

 April, and May — that is, at the beginning of the breeding season or 

 just before. The relative resistance of species does not seem to vary 

 greatly with the season. Just how much species vary in their rela- 

 tive resistance to different harmful factors is a matter for further 

 investigation. The work so far, however, indicates that if species i 

 is more resistant than species 2 to factor a, it is fairly safe to con- 

 clude th.'it it will show a greater resistance to factor b also. In 

 Table III an attempt has been made to arrange the more common 

 species of northern Illinois fishes according to their powers of resis- 

 tance to detrimental environmental factors in general. Such an ar- 

 rangement must at this time be considered more or less tentative be- 

 cause of the large number of unsolved questions concerning fish resis- 

 tance in general, but the list as given will prove suggestive. 



In the table the least resistant species is placed at the head of 

 the list and given an arbitrary resistance value of i. The succeeding 

 species show an increasing resistance to lethal factors and their rela- 

 tive resistance is indicated by the figure in the middle column. The 

 third column indicates the environments where each species is most 

 likely to be found. 



