DAWLEY and EBEL: EFFECTS OF DISSOLVED GASES ON SALMONIDS 



1966; Bouck 1972^; Rucker 1972). Most of the 

 comprehensive studies, however, have been 

 analyzed in terms of nitrogen concentration, as- 

 suming it to be the controlling influence upon the 

 effects of gas bubble disease (even greater than 

 indicated by the 80/20 ratio of the partial pres- 

 sures Ng /O2 ). This assumption was based upon 

 supposed biochemical decrease of the effective 

 oxygen partial pressure within the fish. 



In comparing data from this experiment with 

 that from past research we should acknowledge 

 that our primary criterion during planning and 

 set-up stages was dissolved nitrogen + argon 

 concentration. At the outset of these experiments, 

 oxygen levels were monitored primarily for 

 documentation of overall water quality rather 

 than for use in analysis of their effect upon the 

 test organisms. However, upon examination of 

 initial results derived from each of the tests 

 carried out to lethal exposures, we found that the 

 times for LEjq and LE50 were consistently less in 

 test section A than in Section B. Analyses of in- 

 dividual gas pressures in each of the two sections 

 of the tanks were made to determine whether 

 variations occurred among the component gases. 

 We found that nitrogen concentrations were con- 

 stant in both areas, but oxygen concentrations 

 remained consistently lower (5-10%) in section B 

 than in section A. The lower oxygen concentra- 

 tions-thus lower (1-2%) total dissolved gas (TDG) 

 saturations-appeared directly correlated with the 

 lower mortality rates in section B of the test tanks. 

 For example, when we examined mortality rates 

 of individual groups of steelhead from A and B 

 test sections at 115% Ng + Ar, we found: Ng + Ar 

 saturation (in section B) of 116.0% and 88.2% of Og 

 saturation (TDG at 110.0% of saturation) caused 

 no mortality in 35 days for one replicate of 30 fish, 

 whereas Ng + Ar saturation (in section A) of 

 116.0% and 98.8% O2 (TDG at 112.1%) caused 50% 

 mortality in an average of 20 days for two 

 replicates. 



Effect of Supersaturation Stress 

 on Growth 



Exposure to sublethal concentrations (concen- 

 trations at which no substantial mortality oc- 



'Bouck, G. R. 1972. Effects of gas supersaturation on salmon in 

 the Columbia River. West. Fish. Toxicol. Stn., Environ. Prot. 

 Agency, Corvallis, Oreg. Paper presented at Ecol. Soc. Am. Symp. 

 Aug. 1972, 29 p. ^ ^ 



curred within 35 days) of Na+Ar appeared to af- 

 fect growth of both juvenile chinook and 

 steelhead. Mean weights and lengths of test fishes 

 after 35 days in dissolved nitrogen concentrations 

 of 105, 110, and 115% of saturation (Figure 2) were 

 in each instance less than those of controls. 



« 10 



lesl 

 control 



STEELHEAD 



m 



E 



120 E 



no 115 105 



PERCENT SATURATION Nj + A( 



Figure 2.-Comparison of mean weights (W) and lengths (L) for 

 test and control groups of juvenile chinook salmon and steelhead 

 trout after 35 days at saturation levels of 100% (control), 105%, 

 110%, and 115%. 



A statistical test of the hypothesis-that the 

 slopes of the regression of mean weight of control 

 fish groups and mean weight of test fish groups 

 were equal -yielded a value of t = 4.938 (P<0.02 at 

 4 df ). The same statistical test of mean lengths of 

 control vs. test groups yielded t = 1.36 (P<0.25 at 4 

 df). The lower t value calculated from length data 

 is attributable to the duration of the test not being 

 long enough to significantly overcome the varia- 

 tion in lengths between individuals within groups. 

 We attribute the difference between size of test 

 and control lots to the effect of supersaturation on 

 the normal growth of the test fish. 



After 30 days of testing at the 115% level, feed- 

 ing response of the chinook fingerlings became 

 lethargic. Many of the test fish had spinal flexures, 

 exophthalmia, and large buccal cavity gas blisters 

 and were unable or unwilling to move and accept 

 food when made available. By contrast, control fish 

 exhibited aggressive feeding behavior throughout 

 the tests. Gross gas bubble disease signs and 

 behavioral changes were less evident at 110% Ng 

 -I- Ar and nonexistant at 105%. 



Testing for changes in the condition factor of 

 juvenile fall chinook and steelhead during long- 

 term (2-4 mo) holding in water saturated 100 to 

 127% Ng+Ar is currently underway. Results of 

 these tests may provide further information on 

 effects of gas supersaturation on growth rate. 



791 



