capture in the barge, and of recovery or death 

 were recorded for each fish. All live immobilized 

 fish were transferred at the end of the work day 

 to the holding tanks near the dam and held for 

 at least 3 days to evaluate delayed effects. 



LABORATORY STUDY OF 

 DECOMPRESSION 



Some fish were examined at the highest vac- 

 uum attainable with the system employed (71 cm 

 of mercury vacuum) . Other groups were then 

 tested at successively lower levels to the lowest 

 level of vacuum that produced immobility (61, 

 51, and 41 cm of mercury vacuum) . For valid 

 comparison of the laboratory results with those 

 in the field, it was desirable to simulate field 

 conditions by decompressing the fish as rapidly 

 as when passing a turbine (one-half second or 

 less). In exploratory trials with a small pres- 

 sure chamber (10.2 X 121.9 cm), a high level 

 of negative pressure could be achieved in a frac- 

 tion of a second. The test fish, however — par- 

 ticularly the wild Chinook salmon — did not ap- 

 pear to acclimate to the restricted environment. 

 Subsequently, a larger pressure chamber (30.5 

 X 144.8 cm) was used, although the time re- 

 quired to reach high negative pressures was in- 

 creased slightly. 



The term "precondition" as used here means 

 allowing the fish time to readjust the gas volume 

 of the swim bladder to achieve neutral buoyancy 

 with respect to the water density at atmospheric 

 pressure in the excitement of being caught and 

 transferred to the pressure chamber. Harvey 

 (1963) described this behavior as a fright re- 

 action whereby fish force gas from the swim 

 bladder out through the pneumatic duct, de- 

 creasing buoyancy and facilitating escape by de- 

 scent to greater depths. In the tests this be- 

 havior was more pronounced among wild chinook 

 salmon than among hatchery coho salmon. Im- 

 mediately after being placed in the chamber, 

 the fish descended to the bottom where they re- 

 mained relatively motionless. The hardy indi- 

 viduals generally swam to the surface after about 

 5 min, gulped air, returned to the bottom, and 

 repeated the process until neutral buoyancy was 

 established. About 1 hr was required to re- 

 equilibrate all members of a test group; this 

 amount of time was subsequently adopted in the 

 standard test procedure. 



Some chinook salmon fingerlings developed 

 partial immobility soon after decompression 

 (Fig. 2); peak levels of 17% at 51 cm, 43% at 

 61 cm, and 60% at 71 cm of mercury vacuum 

 were reached about 5 min after exposure. These 

 fish either recovered from partial immobility or 

 developed further complications leading to com- 

 plete immobility. The symptomatic transition 

 proceeded slowly but at a relatively uniform rate 

 of slightly less than 1% per min, although small 

 percentages were partially immobilized after 

 several hours. 



Complete immobility occurred among some 

 chinook salmon fingerlings within a few seconds 

 after exposure to 61 and 71 cm of mercury vac- 

 uum. Others developed the condition after sev- 

 eral minutes of partial immobility (peak per- 

 centage 60 min after exposure to 61 cm and 30 

 min after exposure to 71 cm). None were com- 

 pletely immobilized by 51 cm of mercury vac- 

 uum. Most fish died that had developed complete 

 immobility after several minutes of partial im- 

 mobility. The percentages of deaths from expo- 



PARTIAL IMMOBILITY 



CENTIMETERS OF MERCURY VACUUM 



51 



61 



71 



5 10 20 30 ^O 50 60 70 80 90 1440 

 TIME AFTER EXPOSURE (MINUTES) 



Figure 2. — Chinook salmon fingerlings partially immo- 

 bilized, completely immobilized, or dead after exposure 

 to various levels of negative pressure, by time after 

 exposure. 



