stress from supersaturation on their ability to trans- 

 fer to salt water; a second group was examined for 

 signs of gas bubble disease. Groups of chinook and 

 one group of steelhead exhibiting signs were then 

 placed in equilibrated water (100% T.D.G.) for a 

 2-week recovery period and subsequently re- 

 examined for signs of gas bubble disease. 



Deep water tanks were 2.44 m (8 ft) deep which 

 provided a maximum hydrostatic compensation of 

 0.27 atm or 27% of saturation, and shallow tanks 

 were 0.24 m (10 in.) deep providing only 0.025 atm 

 of pressure compensation or 2.5% of saturation. The 

 shallow water tests on both chinook salmon and 

 steelhead trout consisted of two replicates at 120, 

 115, 110, 105, and 100% (control) total dissolved gas 

 (T.D.G.). Deep water tests with chinook salmon 

 included tests at 127% (1 tank), 124% (1 tank), 120% 

 (2 tanks), 115% (2 tanks), 110% (2 tanks), 105% 

 (1 tank), and 100% (1 tank). Deep water tests with 

 steelhead trout consisted of two replicates at 127%, 

 120%, 115% and at 110% T.D.G. (Previous work 

 indicated that tests at 110% of saturation in deep 

 tanks could serve as a quasi-control). 



Juvenile fall chinook were acquired from the 

 Spring Creek National Fish Hatchery in early 

 February 1973 as buttoned up fry for use in the first 

 experiment, and juvenile steelhead were captured 

 during their seaward migration down the Snake 

 River on April 30, 1974, for use in the second bio- 

 assay conducted in 1974. Steelhead were 1+ yr of 

 age. Chinook and steelhead populations were ac- 

 climated to our laboratory water system at 10°C, 

 for 19 and 6 days, respectively, prior to testing. 

 Before initiation of tests, random samples were 

 taken from each population to obtain average 

 weights, lengths, and condition factors (Table 1). 



The bioassay with chinook began February 20, 

 1973, with the introduction of 220 fish per tank 

 and was terminated on July 8 (127 days of ex- 

 posure to concentrations of dissolved gas plus 

 13 days of subsequent tests). Steelhead tests began 

 May 6, 1974, with the introduction of about 80 fish 

 per tank; these were terminated after 21 days (7 days 

 of exposure to concentrations of dissolved gases 

 plus 14 days of subsequent tests). 



Once testing began, each tank was examined 

 four times daily for the first 4 days followed by 

 three, two, or one times daily throughout the re- 

 mainder of the test period. During each observa- 

 tion mortalities were removed, their length and 

 weight recorded, and signs of gas bubble disease 

 noted. Vertical distribution of the fish in each deep 

 tank was also noted in percentage of total popula- 

 tion at four levels of depth; 0-0.6 m, 0.6-1 m, 1.2- 

 1.8 m, and 1.8-2.5 m. Subsampling of each test 

 group for condition factor and disease signs was 

 done each 28 days at a rate of 10% (but not less 



TABLE 1 Means and Standard Deviations of Weights, 



Lengths, and Condition Factors of Randomly Sampled 



Fall Chinook and Steelhead Taken from Test Populations 



Before Testing 



Fall chinook 



n WL (g) Ln (mm) Condition factor 



60 .43 + .06 40 + 1.3 .67 



84 .42 + .06 40+1.2 .65 



Steelhead 



_n Wt. (g) Ln (mm) Condition factor 



24 56.5 + 12.7 180+14 .922 



26 54.5 + 13.3 180+15 .890 



29 53.7 + 13.7 180+15 .907 



28 54.8 + 14.6 180+16 .917 



than five individuals) of the surviving population. 

 Fish from each subsample were weighed, mea- 

 sured and examined for signs of gas bubble dis- 

 ease (none were returned to the tests). Fish were 

 fed an Oregon Moist Pellet® ration 5 days a week, 

 at a rate of 4% of body weight/day. Rations for 

 each test tank were corrected daily for numbers 

 of surviving fish and corrected each 28 days for 

 weight change (calculated from size of fish sub- 

 sampled every 4 weeks). 



Dechlorinated water from the Seattle city 

 water system which is supplied by the Cedar River 

 was used in these tests. Temperature was main- 

 tained at 10° ± 0.5°C, by mixing hot (27°C) and 

 cold (7°C) water in a reservoir tank. Water for the 

 shallow tank system was supersaturated by inject- 

 ing 0.5C/min air and 0.23 C/min 2 into the suction 

 side of two centrifugal pumps which were plumbed 

 with a recirculation loop to two closed receivers (52 

 gal each). Hydraulic pressure within the receivers 

 was maintained at 2.1 kg/cm 2 (30 psi) where dis- 

 solved gas content was increased to about 122% 

 of saturation T.D.G. (Fig. 1). Water for the deep 

 tank system was recirculated by a pump through 

 an open reservoir tank 9 m deep x 3 m in diameter 

 which was tapped at the bottom for distribution 

 to the test tanks. Air and oxygen were injected into 

 the recirculating pump at about 2.0 C/min and 0.2 

 £/min respectively. This resulted in a stable satura- 

 tion level of 128% T.D.G. Both sources supplied 

 individual test tanks through PVC lines which 

 directed the supersaturated water to a vertical 

 stack of aluminum trays (28 x 41 cm) placed 5 to 

 10 cm above one another. One half of each tray 

 was perforated with 500-3 mm holes and the per- 



2 Dawley, Schiewe, Monk 



