184 



BULLETIN OF THE BUREAU OF FISHERIES 



Accordingly, bass, bluegills, and sunfishes that had been at 30° C. for one day 

 were exposed to 36° C. (96.8° F.), their tolerance limit, and then tested at higher 

 temperatures. It was found that one day at 36° enabled the bass to tolerate 38° C. 

 (100.4° F.) for 24 hours or more; while four days' acclimatization was required to 

 produce the same results in bluegills and sunfishes. These results indicate that the 

 tolerance of bluegills and sunfishes can be raised substantially if a high enough tem- 

 perature is used in acclimatization, but they also furnish confirmatory evidence of 

 of the still greater modifiability of the bass. When, in this connection, it is recalled 

 that the acclimatization temperature of 30° C. (86° F.) (which was the only one used 

 for the perch) was the normal tolerance limit of the species, the conclusion that the 

 perch is the least modifiable of the animals studied seems to be confirmed. 



MODIFICATIONS OF TOLERANCE IN TOAD TADPOLES 



The detailed results obtained with tadpoles are shown in Tables 4 and 5 and in 

 Figures 7 and 9, in which it appears that the changes in tolerance were clearly marked 

 but slow. Exposure to 30° C. (86° F.) for 24 hours produced no significant change 

 in resistance; while in 4 days at 30° C. the gain in tolerance was less than half that 

 shown by the bass. 



As in the case of bluegills and sunfishes, the slowness of the gain in tolerance 

 appears to be due, at least in part, to the fact that the acclimatization temperature 

 of 30° C. was too far below the normal tolerance limit to be very effective. This 

 view was supported by several tests. Fifteen individuals in five different lots were 

 exposed for one day to a temperature of 36° C. (96.8° F.) their tolerance limit. They 

 were then tried at 38° C. (100.4° F.), and all but one survived for 24 hours. On 

 being transferred to 40° C. (104° F.), however, all died. These results seem to 

 indicate that one day at 36° C. was as effective in raising the resistance of tadpoles 

 as four days at 30° C, and that the capacity of tadpoles for increase of tolerance is 

 not very different from that of bluegills and sunfishes. 



The most distinctive feature of the results obtained with tadpoles is the slowness 

 with which resistance was lost through exposure to low temperature. It will be 

 recalled that in those species of fishes having the highest normal resistance there was 

 a rapid lowering of the tolerance limit by exposure to 10° C. (50° F.), while in the 

 species having low normal resistance the decrease in tolerance was slow. Tadpoles, 

 on the other hand, with a normal resistance 2.3° higher than that of any of the fishes, 

 lost tolerance more slowly than any of the fishes except the perch. Table 6 brings 

 out this contrast. 



Table 6. — Relation between normal tolerance limits of different species and the number of degrees 

 centigrade by which the tolerance limit was lowered by exposure to 10° C. for four days 





Normal 



Amount of 





24-hour 



lowering of 



Species 



tolerance 



tolerance 





limit 



limit 





29.6 

 32.2 

 34.0 

 34.0 

 36.3 



0.6 

 2.2 

 3.4 

 4.0 

 1.9 



Bass - 



Bluegill. 









