sure to the higher negative pressures were 24% 

 at 61 cm and 20% at 71 cm mercury vacuum. 

 The response of coho salmon fingerlings to 

 decompression is shown in Figure 3. Symptoms 

 of partial immobility developed in 2, 8, and 5% 

 of the fish decompressed at 51, 61, and 71 cm 

 of mercury vacuum, respectively. Ten minutes 

 were required for development of the symptoms, 

 compared to 5 min for the chinook salmon. No 

 fish were completely immobilized after exposure 

 to 51 cm of mercury vacuum ; peak percentages 

 were only 2% at 61 cm of mercury vacuum and 

 5% at 71 cm. Only 3% died from exposure to 

 71cm. 



10 



5 



- 



" 10 

 o: 



UJ 

 Q. 



- 5 



o: 10 



PARTIAL IMMOBILITY 



CENTIMETERS OF MERCURY VACUUM 

 51 



AH 



COMPLETE IMMOBILITY 



DEATH 



5 10 20 30 40 50 60 70 80 90 1440 

 TIME AFTER EXPOSURE (MINUTES) 



Figrure 3. — Coho salmon fingerlings partially immo- 

 bilized, completely immobilized, or dead after exposure 

 to various levels of negative pressure, by time after 

 exposure. 



Wild chinook salmon were more susceptible to 

 the effects of decompression than were hatchery 

 coho salmon. The difference between species 

 varied with level of vacuum; the greatest dif- 

 ference occurred within 5 min after exposure 

 to 71 cm of mercury vacuum, when 69% of chi- 

 nook salmon were either partially or completely 

 immobilized as compared with 6% of the coho 

 salmon. At 41 cm of mercury vacuum, no re- 

 sponse to decompression was observed for either 

 species. 



The duration of exposure to negative pressure, 

 or the rate at which fish subjected to reduction 

 from atmospheric pressure to a specific level of 

 negative pressure, probably influenced the ab- 

 solute if not the relative results. Holmes and 

 Rucker (cited by Holmes and Davidson, see foot- 

 note 2) found that chinook salmon fingerlings 

 lived if allowed 1 sec to adjust to decompression 

 from 40 psi to atmospheric pressure. In the 

 present tests, 2.5 sec were required to reach 

 71 cm of mercury vacuum from a.tmospheric 

 pressure and another 0.5 sec to relieve the vac- 

 uum. The test was executed in slightly less time 

 at 51 and 61 cm of mercury vacuum. 



IMMOBILIZED FINGERLINGS BELOW 



ICE HARBOR DAM 



During the turbine studies at Ice Harbor Dam, 

 recovered test fish were closely observed for 

 symptoms of immobilization similar to those 

 manifested by fish decompressed in the labora- 

 tory. The first immobilized fish were seen on 4 

 November during the fourth test (Table 1). On 



' Remained immobilized for several hours. 



