The number of days to kill 25% of the fish at 

 the different gas levels is expressed as the lethal 

 exposure — LE 25 , and to kill 50% — LE 50 . 



TEST APPARATUS 



Apparatus shown in Fig. 1 was used to produce 

 water with a definite oxygen and nitrogen content. 

 The tank (1) was divided so that two experiments 

 could be carried on simultaneously with the same 

 equipment. Water was circulated by a centrifugal 

 pump (2) with a valve (3) on the effluent side to 

 cause a controlled back pressure as read on a 

 gauge (4). This created a vacuum on the inflow side 

 (5) so that air could be introduced into the water 

 with either oxygen or nitrogen (6) through a "Y" 

 tube. Circulation of the water caused an increase 

 in temperature which was maintained at approxi- 

 mately 13.6°C by means of a refrigeration system 

 (7) and recorded on a thermograph (8). Water level 

 in the tank was maintained by float valves (9). 

 Each trough was supplied with 1 C per min of water 

 regulated with flowmeters (10). The water used 

 was from the municipal supply, was quite soft, 

 and was passed through activated charcoal to re- 

 move the chlorine. A greater depth of water was 

 needed for absorption of the gases than was af- 

 forded by the tank (1), so two towers (11) were 



added to the system. The spout (12) at the top 

 of the towers was to direct possible overflow water 

 back into the system. 



Inside dimensions of troughs in the fish hold- 

 ing area (13) were 104.5 x 23.5 x 20 cm high. Water 

 depth was maintained at 14 cm. Each trough could 

 be separated into three compartments with screens 

 — "A" was at the inflow end of the trough, "B" 

 middle, and "C" outflow end. In a few cases a com- 

 partment was divided longitudinally so that two 

 groups of fish could be subjected to almost identical 

 conditions. 



Effect of Variation in O2/N2 Ratios 



Times to death (LE 25 and LE 50 ) of juvenile coho 

 salmon at various concentrations of 2 and N2 

 during constant total gas saturation of 119% 

 appear in Table 1 and are shown graphically in 

 Fig. 2. With one exception (192% 2 and 100% N 2 ), 

 all increases in ratio of 2 to N 2 resulted in in- 

 creased tolerance to the total gas saturation. A 

 marked increase in tolerance to total gas pressure 

 occurred between concentrations of 159/109 and 

 173/105% saturation of 2 and N 2 (Fig. 3). 



TABLE 1 Time to Death (Days) of Juvenile Coho Salmon (6 cm) 



Exposed to 119% Total Cas Supersaturation at Different 



Levels of 2 and N, in 13.6 C Water 



FIC. 1 Apparatus for subjecting fish to constant-temperature, 

 flowing water with a definite oxygen and nitrogen content. 



'Not reached, 28% mortality in 39 days. 

 •One replicate reached, 24% mortality in 30 days, 

 the other 25% in 32 days. 

 'Not reached, test concluded at 33 days. 

 'Not reached, 20% mortality in 35 days. 



Effect of Size and Stock 



A number of tests were carried out in the water 

 containing 114% 2 and 121% N 2 to determine effect 

 of size and stock of fish on susceptibility to gas 

 supersaturation (Table 2). Two groups of 3.8 cm 

 coho from the Montlake Laboratory which had just 



86 Rucker 



