Although the data are limited, there appears to 

 be little difference between susceptibility of the 

 Montlake and Quilcene stocks. 



Effect of Reduced O^ Concentration on Mortality 



Fish held in compartments in a trough utilize 

 oxygen so that the water in compartment C (out- 

 flow end) would have less oxygen than in com- 

 partment A (inflow end). Compartment B in the 

 central part of the trough would have 0, levels 

 somewhere between those in A and C. Nitrogen 

 levels in these compartments, however, were the 

 same. To demonstrate the effect of reduced 

 oxj'gen in relation to gas-bubble disease, 48 coho of 

 8.5 cm fork length were randomly distributed into 

 compartments A, B, and C. Two additional 

 replicates of the C compartment tests were run 

 using 32 coho (8.5 cm) in each trial. These are listed 

 as Ci and C2 in Table 3. 



Table 3. -Time to death of groups of juvenile coho salmon (about 

 8.5 cm long) in 13.6°C water with 121% N2 and different concen- 

 trations of 02- 



Inspection of these data indicates that when 

 121% N2 is maintained, oxygen plays a more sig- 

 nificant role above 110% than below 110%. 



Some of the data obtained when the oxygen-ni- 

 trogen ratio tests were done also illustrated the 

 effect of reduced oxygen on the mortality rate. 

 This was apparent in the e.xperiment using 173% 

 Ogand 105% Ng. At 173% 2 there were losses of 26 

 and 30% in 39 days, whereas slightly larger fish at 

 the lower end of the troughs subjected to 169% O2 

 had losses of only 7% in 39 days. 



Pathology 



Generally the fish died suddenly in the higher 

 nitrogen concentrations. Never was tissue damage 

 or any progressive pathology demonstrated. The 

 fish always seemed to die from gas embolism, re- 

 stricting the flow of blood through the gills. When 

 the nitrogen was near normal and the oxygen 



high, the fish were moribund for many days before 

 succumbing. These fish had blebs in the mouth 

 which interfered with feeding and caused 

 emaciation. 



Summary 



Coho salmon fingerlings were subjected to a to- 

 tal gas pressure of 119% at 13.6°C with the O2/N2 

 varying from 50%/138% to 229%/90%. The small 

 fish (3.8 to 6 cm) were the most resistant and the 

 larger fish (8 to 10 cm) the least resistant to gas- 

 bubble disease at the gas concentrations used. A 

 drastic decrease in lethal effect of individual ra- 

 tios of Ogto Naoccurred between 159% 02/109% N2 

 and 173% 02/105% N2at the same total gas pres- 

 sure (119%). 



Acknow^ledgments 



Facilities for this work were furnished by the 

 Western Fish Disease Laboratory, U.S. Bureau of 

 Sport Fisheries and Wildlife, Department of the 

 Interior, Seattle, Wash. The work was started 

 while the author was an employee of this 

 organization. 



The information in the section on pathology was 

 furnished by William T. Yasutake, pathologist. 

 Western Fish Disease Laboratory. 



Literature Cited 



Anonymous. 



In press. Total dissolved gases (supersaturation). In David 

 Gates et al., Water quality criteria, p. 135-139. Natl. 

 Acad.Sci. 

 Meeking, T. K., and B. K. Turner. 



1974. Tolerance of salmonid eggs, juveniles, and squawfish 

 to supersaturated nitrogen. Wash. Dep. Fish., Tech. Rep. 

 12:78-126. 

 RUCKER, R. R. 



1972. Gas-huhhle disease of salmonids: a critical 

 review. U.S. Fish Wildl. Serv., Bur. Sport Fish. Wildl. 

 Tech. Pap. 58, lip. 



Robert R. Rucker 



Northwest Fisheries Center 



National Marine Fisheries Service, NOAA 



2725 Montlake Boulevard East 



Seattle, WA 9^112 



918 



