FIG. 2 Mortality pattern of 6 cm coho salmon reared at different 

 O./N , levels at 13.6 C with a 119% total gas pressure. 



%N 2 

 %0, 



FIG. 3 Relationship between O ,/N , levels and time to death of 

 6 cm coho salmon fingerlings at 13.6 C and total gas concentra- 

 tion of 119%. 



TABLE 2 Resistance of Juvenile Coho Salmon to Gas 



Supersaturation (114% 2 + 121% N .,) in Relation to Size and 



Stock of Fish. Lethal exposure levels (LE.., and LE , ) in days 



to death, water temperature 13.6 C + 0.1 C 



and test duration 30 days 



Days to death by size and stock 



Lethal 3.8 cm 4.6 cm 10 cm 10 cm. 



exposure Montlake Quilcene Montlake Quilcene 



LE, 



LE 5 



16.9 



Not reached 

 in 30 days 



16.7 

 27.4 



2.1 

 2.6 



2.9 



4.2 



started feeding were initially tested. One group of 

 96 fish reached LE 25 in 22.9 days and LE 30 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 place LE 25 at 16.9 days. Average loss at 

 30 days was 32%. 



Two groups of 4.6 cm fish from the Quilcene 

 National Fish Hatchery were also tested. These tests 



produced LE 25 's of 15.1 and 18.3 days. LE 50 was 

 reached in 24.7 and 30 days. Averages of the above 

 place LE 25 at 16.7 and LE l 50 at 27.4 days. 



Five groups of fish, approximately 10 cm long, 

 from the Montlake Laboratory were then tested. 

 The average for ail groups gave an LE 25 of 2.1 days 

 and an LE 50 of 2.6 days. 



Three groups of 10 cm fish from the Quilcene 

 National Fish Hatchery were similarly tested. Aver- 

 ages were 2.9 days for LE 25 and 4.2 days for LE 50 . 



These results indicate that the larger fingerlings 

 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). 



Although the data are limited, there appears 

 to be little difference between susceptibility of the 

 Montlake and Quilcene stocks. 



Effect of Reduced 2 Concentration on 

 Mortality Rate 



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 compart- 

 ment "A" (inflow end). Compartment "B" in the 

 central part of the trough would have O2 levels 

 somewhere between those in "A" and "C". Nitrogen 

 levels in these compartments, however, were the 

 same. To demonstrate the effect of reduced oxygen 

 in relation to gas bubble disease, 48 coho of 8.5 cm 

 fork length were randomly distributed into com- 

 partments "A", "B", and "C". Two additional repli- 

 cates of the "C" compartment tests were run using 

 32 coho (8.5 cm) in each trial. These are listed as C, 

 and C 2 in Table 3. 



TABLE 3 Mortality of 8.5 cm Coho Salmon Subjected to 



a Constant Temperature (13.6 C) and 121% N 2 with 



a Variation in Oxygen Tension 



Inspection of these data indicates that when 

 121% N 2 is maintained, oxygen plays a more sig- 

 nificant role above 110% than below 110%. 



Some of the data obtained when the oxygen- 

 nitrogen ratio tests were done also illustrated the 



Oxygen-Nitrogen Ratio Variation 87 



