Lethal Values of Oxygen for Nehu and lao: Experiments 

 Using Single Fish 



Procedure. --For most of these tests Florence flasks of about 540 ml. capacity were 

 employed as experimental containers. Occasionally, depending on the size of fish used, larger 

 or smaller flasks were necessary. The flasks were surrounded by white cloth to prevent the 

 fish from becoming unduly excited, and a small piece of screening was placed in the neck to keep 

 the fish from entering this portion of the flask. 



After the fish was transferred to it, the flask was filled, stoppered, and immersed to 

 its neck in a water bath. Initial oxygen samples were obtained from flasks immersed in the water 

 bath but containing no fish. After all movement of the fish in the experimental jar had ceased, an 

 oxygen sample was quickly siphoned off, the water being allowed to circulate through the bottle 

 for a few seconds after it was filled. In some of the earlier experiments a 50-ml. sample was 

 then taken for pH determination, leaving about 50-70 ml. in the flask. The length of the fish, 

 from the tip of the snout to the end of the vertebral column, was taken to the nearest millimeter, 

 and with the exception of some of the earlier experiments, the fish were preserved in 10-percent 

 formalin to be weighed later on a RoUer-Snaith torsion bailance. 



As a check on the possibility of errors due to absorption of oxygen during siphoning, 

 water samples of low oxygen content were siphoned off from two flasks, one with a layer of min- 

 eral oil and the other without the layer. The difference in amount of dissolved oxygen between 

 these samples was within the error of the Winkler procedure. 



The experiments on iao were divided into two groups, a summer series and a winter 

 series, reported respectively in tables 8a and 8b. Eleven experiments were performed with nehu, 

 all in the late spring and summer of 1950, and these are reported in table 7. 



Results. --Considerable variability was evident in the results, as might be expected 

 in collecting data on individuals. The mean and standard deviation of all the summer lethal values 

 for iao (table 8a) was 1.10 - 0. 22 cc. /I. and that of the winter values 0. 96 - 0. 18 cc. /I. , with 

 mean temperatures of 25.6 - 0.9°C. and 21.9 - 0.6°C. respectively. The difference between 

 these mean values, when tested statistically by analysis of variance (table 8c), was found to be 

 significant at the P = 0. 05 level. The mean lethal value for nehu was 2. 02 cc. /I. with a standard 

 deviation of 0.35 cc. /I. , much larger than that for either series on iao. The great variability of 

 the nehu data was probably a consequence of the fish's behavior when confined to the experimental 

 flask. In contrast to iao, nehu generally remained greatly agitated throughout the experiment and 

 some undoubtedly became injured, thus causing an abnormally high lethal value of oxygen. 



Weights were not available for all the fish used, but the data on hand show no signifi- 

 cant relationship between weight of fish and the lethal value of oxygen. Regression lines for both 

 summer and winter data did not differ significantly from zero (P = 0. 1 in both cases). At best, a 

 slight trend towards greater resistance to oxygen deficiency in the larger fish was indicated. 



It seems fairly certain that the death of the fish in these tests was due to lack of 

 sufficient oxygen and was not affected by a lowering of the pH during the course of the experiment. 

 Hydrogen ion concentration was measured at the end of the earlier experiments and was never less 

 than 7. 80, averaging 7. 87. The water in the stock tanks often showed pH values of this magnitude. 



Some evidence is available (experiments 16, 17, and 18 of the winter series) to 

 indicate that lethal values of oxygen are about the same regardless of the initial oxygen content of 

 the water. This point, however, needs further investigating. 



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