52 ILLINOIS ACADEMY OF SCIENCE 



to keep the water cool. So low is the resistance at this time 

 that these species often die from the shock of being seined out 

 of the water and transferred to the fish bucket. Thus, if one 

 were to draw the seasonal resistance curve of such fishes, it 

 would be a rather regular curve, the highest point occurring in 

 March and April and the lowest point in the latter part of 

 June and first part of July, with the difference in the level 

 of these two points a considerable one. From the low point 

 the rise in resistance is very slow and gradual up to the latter 

 part of September when the curve begins to rise more rapidly 

 up to the high point in the spring. 



For the above reasons it has been found impossible to state 

 that any certain temperature is the maximum which can be en- 

 dured by a given species. There are, however, definite specific 

 differences in the resistance of fishes to temperature as well 

 as to other factors. Of the species used in the experiments, the 

 black bull-head (Ameiurus melas) was the most hardy, even 

 though none but young of this species was used. This species 

 could be raised to 35°-36° C. before death occurred. The 

 other species follow in the order of their increasing resistance. 

 Silver shiner (Notropis atherinoides) 27° -28° C. ; straw-col- 

 ored minnow (Notropis blennius), 28°-29° C. ; common shiner 

 (Notropis cornutus), 28°-30° C. (Temperatures are approx- 

 imate). Field and experimental observations have been made 

 upon a large number of other species, but will not be tab- 

 ulated here. 



In experiments where fishes were subjected to sudden 

 changes of temperature, the changes were made through a 

 large number of the possible combinations existing in a range 

 of temperature, the highest point of which is just below the 

 general maximum for the species and the lowest point, one or 

 two degrees above freezing. Thus one series of experiments 

 consisted in changing adults of Notropis blennius from 28° C. 

 to 3° C, from 25° C to 3° C. ,from 22° C. to 3° C, and so on 

 down to a last change of from 10° C. to 3° C. In all the ex- 

 periments the method employed was to arrange the pans of cold 

 and warm water along side each other. The temperatures of 

 these pans were kept constant for each test. The fish was 

 quickly lifted from one pan to the other in the direction in 

 which the change was to be made, i. e., from warm to cold or 

 vice versa. The general effect of change from colder to warm- 

 er is, as has been noted, to increase the activity of the fish; 

 the reverse change tends to diminish these activities. In gen- 

 eral this increase or decrease of activity proceeds regularly 



