of the desired test temperature and was continuously 

 recorded. 



During a test 10 fish were placed in each of the 

 four troughs receiving artificially supersaturated 

 water generated by injecting compressed air into 

 water under pressure in a receiver. The tanks 

 were covered with black plastic to minimize dis- 

 turbing effects of wet lab activity. Troughs were 

 checked at least daily for mortalities which were 

 removed. About one-third of the mortalities were 

 randomly selected for gross necropsy to determine 

 cause of death. At least three times during each 

 4 day test dissolved gas tension was measured using 

 the Weiss saturometer with a mechanical shaker. 

 Water temperature was also recorded as was baro- 

 metric pressure using a mercurial barometer. From 

 these data the percentage of equilibrium saturation 

 was computed for total dissolved gases. In addition, 

 dissolved oxygen was measured by Winkler 

 titration, and pH was measured once during each 

 test. At each temperature it was necessary to run 

 replicate tests varying the gas tensions somewhat 

 in order to obtain a sufficient data base for probit 

 analysis to determine TL 50 values. In most cases, 

 three tests were required at each temperature. 



Mean gas tensions and mortalities at the end of 

 96-hr were analyzed by computer probit analysis to 

 determine fiducial limits about the fitted curve of 

 proportional mortality versus total gas saturation. 



RESULTS AND DISCUSSION 



Gross necropsies demonstrated that in nearly 

 all cases examined the cause of death was clearly 

 gas bubble disease, manifest by massive cardiac 

 blockage by emboli. Emphysema and petechial 

 hemorrhaging were commonly observed as was the 

 presence of emboli in blood vessels, gills, and 

 organs. Cases of not clearly gas bubble disease 

 mortalities were due to indeterminant causes, but 

 were likely due to unobserved gas embolism. In no 

 case were there any mortalities in the control tank 

 and at the termination of the test control fish showed 

 no external signs of gas bubble disease while they 

 were common among survivors which were exposed 

 to supersaturated water. 



Plots of proportional mortality against total gas 

 tension for each of the test temperatures are shown 

 in Fig. 1 through 4. It is important to note that the 

 curves are quite steep, indicating a narrow range 

 between non-lethal and lethal levels of gas tension. 

 This factor could be significant in planning mitiga- 

 tion activities and in defining water quality standards 

 as even a relatively small decrease in dissolved gas 

 content might have a large effect on resulting mor- 

 tality, provided that the new, lower gas tension was 

 below the lethal threshold. Species differences in 

 tolerance and depth distribution as it relates to 

 hydrostatic pressure compensation for excess gas 



tension may, however, dictate that each potentially 

 affected area be independently assessed prior to 

 instigating mitigation plans. 



10 



0.8 



0.6 - 



a. 0.4 - 



0.2 - 



0.0 



105 110 115 120 125 130 135 



GAS SATURATION (MEAN TOTAL DISSOLVED GASI 



FIG. 1 Acute bioassays of dissolved gas tolerances of Ictalurus 

 melas (Black Bullhead) at 8 C. 



10 



< _ 



0.6 - 



E 0.4 - 



0.2 - 



00 



100 105 110 115 120 125 130 



GAS SATURATION I MEAN TOTAL DISSOLVED GASI 



135 



FIG. 2 Acute bioassays of dissolved gas tolerances of Ictalurus 

 melas (Black Bullhead) at 12 C. 



1.0 



!= 0.8 - 



0.6 



0.4 - 



0.2 - 



00 



100 105 110 115 120- 125 130 135 



GAS SATURATION (MEAN TOTAL DISSOLVED GASI 



FIG. 3 Acute bioassays of dissolved gas tolerances of Ictalurus 

 melas (Black Bullhead) at lb C. 



Effect of Temperature on Tolerance 73 



