172 



BULLETIN OF THE BUREAU OP FISHERIES 



2. Observations were made 1 minute, 4 minutes, 15 minutes, 1 hour, 4 hours, 



and 24 hours, respectively, after the beginning of each test. 



3. At every observation the condition of each individual was recorded, and 

 the time of the last reading at which an individual was alive was recorded as its 

 survival time. 



Two reasons led to the choice of the method just described — (1) it is relatively 

 simple and is applicable to tests on a considerable number of individuals; (2) the 

 time periods into which the tests were divided (1 minute, 4 minutes, etc.), constitute, 

 roughly, a geometric progression. Therefore, the degrees of tolerance represented 

 by survival for the various lengths of time belong to different orders of magnitude, 

 and the method affords a quantitative basis for the comparison of the tolerance of 

 various individuals and species. 



Fishes were considered dead when respirator} 7 movements had ceased, as no 

 case of recovery following cessation of respiration was ever observed. In tadpoles 

 no great effort was made to distinguish between heat rigor and death. The dis- 

 tinction proved to be unimportant, for many observations showed that recovery 

 from rigor never occurred except during the first hour of the test. Failure to move 

 in response to repeated mechanical stimulation with a blunt glass rod was taken as 

 evidence that a specimen was dead or in rigor. 



In the early stages of the work it became apparent that, under the conditions 

 employed, the tolerance limits for the members of a given species that had received 

 a certain treatment lay within rather definite limits. For example, all normal 

 bluegills and sunfishes survived at 34° and none at 36°. In other cases the thermal 

 death points were less definite, but, on the average, an increase of 2° in the trial 

 temperatures was sufficient to bring about a change from 90 per cent survival to 

 90 per cent death. However, the tolerance limit of the majority of the individuals 

 was found not to be a satisfactory basis for comparisons, as the tolerance of the 

 average individual may fall above or below that of the mode. For this reason the 

 average tolerance limit was computed for each series, the following equation being 

 used : 



t P 1 + (t-2)P 2 +(t-4:)P, _ 



100 1 



o 



In this equation, t is the highest temperature tolerated for the given period 

 by any of the individuals tested; t-2 and £-4 are temperatures, respectively, 2 

 and 4° below t; P u P 2 , and P 3 are the percentages of individuals for which t, (t-2), 

 and (£-4) are, respectively, the maximum temperatures tolerated; and T is the 

 average of the tolerance limits of the individuals in the series. 



The manner of applying the foregoing formula may be illustrated by the case 

 of the normal bass tested at 30°, 32°, 34°, and 36°. The percentages of individuals 

 surviving at these temperatures were as follows: 



Degrees Per cent 



30 100 I 



32 86 



34 26 



36 0 



Hence, £ = 34°. From these results it seems fair to assume that the 14 per 

 cent that died at 32° would have survived at 30°; hence, P 3 = 14. Furthermore, the 



