ability of these fish to make this transition. Chi- 

 nook groups of 50 fish from each gas level and 

 from each series (deep and shallow) were trans- 

 ferred on test day 127. A combined total of 98% 

 mortality occurred in 3 days. Only 8 fish survived 

 for a longer time; 1 fish from the 105% shallow tank 

 and 7 fish (14%) from the 110% deep tank; these 

 lasted the entire 13 days. Results of a statistical 

 comparison of fork lengths of the survivors (X = 

 67.3 mm) to those of mortalities (X = 52.5 mm) 

 indicate a definite size correlation with ability to 

 make the transfer (T = 5.73, 46 df, P < 0.001). This 

 suggests that the majority of the experimental stock 

 had not yet reached smolting size and their ability to 

 transfer to salt water was thus severely lessened. 



Steelhead test groups were likewise subsampled 

 and groups of 10 to 20 fish were placed into salt 

 water. Mortality varied from 0-16 with no correla- 

 tion to previous stress experience. However, a size 

 comparison between mortalities and survivors indi- 

 cated that mortalities were the smaller of the popu- 

 lation (T = 1.925, 51 df, P < 0.06) again suggesting that 

 the dead fish may not have been up to smolting size. 



/ 



53mm 

 112% TDG 



NMFS 

 42mm 

 115% TDG , 



40mm •' 



112% TDG • 



: ^ 



/ . / NMFS 



•' y " 42mm 

 _ -&' 110% TDG 



30 40 50 



NUMBER OF DAYS 



60 



70 



DISCUSSION 

 Test Results 



The mortality curves (Fig. 2 and 3) may be 

 affected by synergistic effect of C. psychrophila 

 after day 64; however, incidence rate and types of 

 gas bubble disease signs of dead fish showed no 

 apparent difference for individual tests between 

 the first 60 days and the last 67 days indicating that 

 the effect was not large. We, therefore, assume the 

 mortality curves (adjusted for control mortality) 

 are generally representative of death rates caused 

 by gas bubble disease at the dissolved gas con- 

 centrations indicated. The first 60 days have no 

 qualifications, but the last 67 days may represent a 

 fish stock with less than normal tolerance to excess 

 dissolved gas pressure. 



As shown in Fig. 10, the death rates and curve 

 shapes correlate well with experiments done by 

 Meekin and Turner (1974), in which they exposed 67, 

 53, and 40 mm fall chinook to 122% N 2 + Ar plus 74% 

 O2 (112% T.D.G.). Fish tested by these researchers 

 showed a definite inverse correlation between 

 resistance times in supersaturated conditions and 

 age and growth. Larger fish (53 mm, 67 mm) suc- 

 cumbed much more rapidly than 40 mm fish, 

 tested at the same level of percent saturation (T.D.G.). 

 This same trend was also shown by Shirahata 

 (1966), testing rainbow trout from hatching to fry 

 stage. From this evidence we conclude that the 

 times to death at indicated gas concentrations pre- 

 sented here are typical for these species at this 

 size, and that the increase in mortality rates of fall 



FIG. 10 Mortality versus time curves for bioassay of dissolved 

 gas in shallow tanks (0.25 m or less) at 122% N 2 + Ar and 74% of 

 saturation O2 (resulting in 112% T.D.G.) with fall chinook at 

 various sizes (Meekin and Turner 1974) and curves at 115 and 

 110% T.D.G. with fall chinook at 42 mm (NMFS data). 



chinook groups as the experiment progressed was 

 mainly commensurate with aging and growth. 



In tests done by Dawley and Ebel (1974) the re- 

 sistance times of steelhead in shallow water tanks at 

 115% supersaturation was 400% longer than our 

 tests with steelhead at the same saturation level. 

 Fish tested at that time were hatchery reared and 

 smaller (130 mm compared to 180 mm) which is 

 probably the reason for their greater resistance. 



The order of magnitude of this difference, how- 

 ever, is small compared to the 1- to 2-month dif- 

 ferences in resistance times we observed between 

 fall chinook and steelhead. This difference correlated 

 A/ett with data by Meekin and Turner (1974) which 

 also indicates that fall chinook were more tolerant 

 to exposure to supersaturation than were steelhead 

 of comparable size and age. This same order of 

 ranking was noted by Ebel, Dawley, Monk (1971) 

 and Dawley and Ebel (1974). 



Some prominent signs of gas bubble disease 

 occurred on dead chinook in association with certain 

 stages in physical development or stress experi- 

 ence. Cutaneous blisters in the buccal cavity and on 

 the body surface and hemorrhages in and around 

 the eye required more time to develop than other 



8 Dawley, Schiewe, Monk 



