188 



BULLETIN OF THE BUREAU OF FISHERIES 



The precise cause of death at high temperatures is an important question, to 

 which only a partial answer can be made at present. Mayer (1917) considers that 

 it is due mainly to accumulation of acid in the tissues. The work of a number of 

 investigators has shown that several factors, some external and some internal, may 

 be involved. Loeb and Wasteneys (1912) showed that slight changes in the salinity 

 of the surrounding water caused marked changes in the temperature tolerance of 

 Fundulus. The importance of dissolved oxygen and carbon dioxide as limiting 

 factors in the life and distribution of fishes has been stressed by several writers 

 (Juday and Wagner, 1908; Birge and Juday, 1911; Wells, 1913 and 1915; and 

 Pearse, 1918). That hydrogen-ion concentration is an important factor in the toler- 

 ance of low oxygen tensions by fishes is maintained by Wells (1913 and 1915). The 

 influence of pH on the distribution of fishes has been pointed out by Shelford (1923) 

 and Coker (1925); while the work of Shelford and Allee (1913), Shelford and Powers 

 (1915), and Powers (1921) indicates that many (but not all) fishes can discriminate 

 between different degrees of acidity and show definite positive and negative reactions 

 to pH gradients. It appears that there is a good deal of difference between the pH 

 preferences of various species, and the observations of Miss Jewell (1922) indicate 

 that the reactions of individual fishes to pH may be modified by acclimatization. . 



There is a considerable amount of evidence to the effect that changes in the 

 composition of blood of fishes may play a large part in survival under unfavorable 

 conditions. Packard (1905, 1907, and 1908) materially lengthened or shortened the 

 survival time of Fundulus in oxygen-free water by injections of sodium carbonate, 

 acetic acid, or other substances. Birge and Juday (1911) suggested that if a fish 

 were able to change the alkali reserve of its blood it would better be able to tolerate 

 oxygen deficiency; and this theory is supported by Powers (1922 and 1922a), who has 

 secured evidence indicating that the ability of fishes to absorb dissolved oxygen 

 varies with the pH of the water, but that the effect is relatively slight in cosmopolitan 

 fishes. Krogh and Leitch (1919) showed that the blood of carp, pike, and eels can 

 unload oxygen at lower oxygen tensions than can the blood of trout and some marine 

 fishes. 



In the present experiments the changes from low to high temperatures neces- 

 sarily involved changes in concentration of dissolved oxygen; and from the literature 

 cited it appears that in many cases the death of fishes at high temperatures must be 

 regarded as the result of the combined action of a number of factors, some internal 

 and some external. However, the strong aeration in these experiments was designed 

 to reduce to a minimum the fluctuations in oxygen and carbon dioxide tension during 

 the tests, and there are two reasons for beleiving that temperature was the limiting 

 environmental factor in the tolerance of the animals tested: 



1. While the death symptoms commonly seemed to indicate asphyxiation, this 

 appeared to be due to internal rather than external causes. Of two fishes in the 

 same jar, frequently one would be dying (apparently from asphyxiation), while a 

 similar individual, which had received the same preliminary treatment, would be 

 breathing easily and would survive in good condition. 



2. The results obtained in the 4-minute and 15-minute test periods, in which 

 there was very little opportunity for change in the dissolved content of the water, 

 ran almost perfectly parallel with the results obtained in the longer test periods, 



