Table l.-Timetodeathof groups of juvenile coho salmon (about 

 6 cm long) in 13.6°C water with total gas pressure of 119% of 

 saturation and different ratios of 02and N2. 



'Not reached, 28% mortality in 39 days. 



20ne replicate reached 24% mortality in 30 days; the other, 

 25% in 32 days. 

 3Not reached, test concluded at 33 days. 

 "Not reached, 20% mortality in 35 days. 



XO2 %Nj 



101 



20 



40 



Figure 2.— Mortality pattern of 6-cm coho salmon reared at 

 different O2/N2 levels at 13.6°C with a 119% total gas pressure. 



100% N 2), all increases in ratioof02toN2resulted 

 in increased tolerance to the total gas saturation. 

 A marked increase in tolerance to total gas pres- 

 sure occurred between concentrations of 

 159%/ 109% and 173%/ 105% saturation of 02andN2 

 (Figure 3). 



Effect of Size and Stock of Fish on Mortality 



A number of tests were carried out in the water 

 containing 114% Ogand 121% N2 to determine ef- 

 fect of size and stock of fish on susceptibility to gas 

 supersaturation (Table 2). Two groups of 3.8-cm 

 coho from the Northwest Fisheries Center which 

 had just started feeding were initially tested. One 

 group of 96 fish reached LEgsin 22.9 days and LEgg 

 after 30 days. The other group of 50 fish reached 

 LE25 in 10.9 days. No further losses occurred until 

 the 27th day. Loss at 30 days was 34%. Averages of 

 the two groups placed LE25 at 16.9 days. Average 

 loss at 30 days was 32%. 



Two groups of 50, 4.6-cm fish from the Quilcene 

 National Fish Hatchery were also tested. These 

 tests produced LE25of 15.1 and 18.3 days. LE50 was 



40n 



30- 



20- 



10- 



•LE50 

 ALE25 





 %N2-^0 



% 02-^0 



138 

 50 



131 

 75 



121 



114 



1 1 1 



109 105 100 



159 173 192 



Figure 3. -Relationship between Oo/N^levels and time to death 

 of 6-cm coho salmon fingerlings at 13.^C and total gas concen- 

 tration of 119%. 



Table 2.— Time to death of groups of juvenile coho salmon of 

 different body length and stock composition in 13.6° + 0.1°C 

 water with gas concentrations of 114% 02and 121% N2. 



Average body length 



Time to death (days) 



reached in 24.7 and 30 days. Averages of the above 

 placed LE25 at 16.7 and LE50 at 27.4 days. 



Five groups of fish (8, 12, 16, 16, and 16 in 

 number), and approximately 10 cm long, from the 

 Northwest Fisheries Center were then tested. The 

 average for all groups gave an LE25 of 2.1 days and 

 an LEgQof 2.6 days. 



Three groups of 12 fish each approximately 10 

 cm long from the Quilcene National Fish Hatchery 

 were similarly tested. Averages were 2.9 days for 

 LE25and 4.2 days for LEg^. 



These results indicate that the larger 

 fingerlings approximately the same age are 

 definitely more subject to harm from excess air in 

 the water than the smaller fish. These data agree 

 with those of Meekin and Turner (1974) and 

 Dawley etal.' 



'E. Dawley, B. Monk, M. Schiewe, and F. Ossiander. 1974. Sal- 

 monid bioassay of supersaturation of dissolved gas in water. 

 Northwest Fish Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, 

 Wash., unpubl. manuscr. 



917 



