tests were designed to investigate this type of 

 intermittent exposure that fish experience as they 

 undergo diel migrations in continuously super- 

 saturated reservoirs. Intermittent exposure due to 

 daily changes in depth should, at least theoreti- 

 cally, reduce the effects of supersaturation. 



PROCEDURES 

 Location of Study 



A major objective of the dissolved gas bio- 

 assays was to test the survival of fish in water con- 

 taining moderate, but relatively constant, levels 

 of supersaturation under natural conditions. The 

 Rock Island Dam forebay on the mid region of the 

 Columbia River was chosen as a site where these 

 conditions could be obtained during the 1974 runoff 

 period. This dam provided a location where the live 

 cages could be placed in water at least 20 ft deep 

 and with a desirable current (0.1 to 0.5 m/sec). 

 The moderate current ensured good circulation of 

 water through the live cages without stressing the 

 test fish. 



Submerged Cages 



The cages used to conduct the supersaturation 

 bioassays were of three basic types, volition, fixed- 

 depth, and intermittent-exposure. All of the cages 

 were constructed with horizontal dimensions of 

 4 ft by 4 ft. The volition cages extended from the 

 surface to depths of 2, 3, and 4 m. The four fixed- 

 depth cages were each 1 m deep and were placed 

 at depths of to 1 m, 1 to 2 m, 2 to 3 m and 3 to 4 m. 

 Following the first test, the 3 to 4 m cage was used 

 for testing the recovery of fish showing signs of 

 gas bubble disease (GBD). The intermittent-expo- 

 sure cages were also 1 m deep, but were constructed 

 so that they could be placed at desired depths 

 between to 1 m and 3 to 4 m. In the first test, the 

 upper depth of the intermittent-exposure cages 

 was 1 to 2 m. The upper depth was raised to to 1 m 

 for Tests II and III. The lower depth was 3 to 4 m 

 for all three tests. A schematic of the various live 

 cages is shown in Fig. 1. 



The cages were constructed with a framework 

 of 1 in. angle aluminum held together by stainless 

 and cadmium plated bolts and nuts. Diagonals of 

 1/2 in. aluminum conduit were used to stabilize 

 the deeper cages. The frames for the volition and 

 fixed-depth cages were constructed with 6-ft cross 

 pieces at the top to suspend the cages within the 

 floats. Suspension of the intermittent-exposure 

 cages was accomplished by placing removable 

 6-ft pieces of aluminum tubing horizontally under 

 the cross pieces which were located at the appro- 

 priate depths. 



Knotless nylon netting of 1/2 in. stretched 

 mesh was used to enclose the cages. Each cage had 



0-2m 0-lm 



WATER 

 LEVEL. 



2m- 



3m- 



16 nr 12 hr 8 hr 



AT3-4m 

 INTERMITTENT EXPOSURE 



FIG. 1 Schematic of fish live cages used in supersaturation 

 bioassays showing the depths occupied by the various cages. 



a zippered opening at the bottom of one side. When 

 the cages were raised each day, these openings 

 permitted removal of dead fish from the bottom of 

 the cage. These openings also permitted observa- 

 tion of the test fish in shallow water when the cages 

 were raised. The specific depth and intermittent- 

 exposure cages were provided with additional zip- 

 pered openings on the top for introducing the fish 

 into the cages. The assembled frames and nets are 

 shown in Fig. 2. 



Two cages were suspended within each float. 

 This provided a minimum distance of 1-1/2 ft 

 between cages in the same float and about 6 ft 

 between cages in adjacent floats. Fig. 3 shows the 

 cage and float assemblies in position adjacent to a 

 barge in the Rock Island forebay. 



Test Periods 



The first test was conducted for a period of 

 10 days while the second and third tests were 

 extended to 20 days each. Because Ebel's (1969 and 

 1970) previous experiments were conducted for 



FIG. 2 Live cages being assembled at the study she; 1 m inter- 

 mittent exposure and fixed depth cages, and 3 m volition cage 

 are shown. 



Live Cage Bioassays at Rock Island 25 



