Table 5. — Mortality of juvenile fall chinook salmon of 

 wild (native) origin held in cages at various depths 

 in the forebay of Ice Harbor Dam, 15-22 June 1970.' 



Holding 

 time 



Cage depth (m) and mortality (%) 



Surface 



.75-1.0 m 



1.5-2.0 m 



3.0-4.0 m 

 (control) 



0-4.5 m 



24 hours 



48 hours 



7 days 



58 



82 



100 



74 



100 



'50 



' Water temperature was 13.8° C and dissolved nitrogen 

 concentration was 132% of saturation at start of test; water 

 temperature was 15.8° C and dissolved nitrogen concen- 

 tration was 127% of saturation at end of test. 



' 69% of survivors had symptoms of gas bubble disease. 



° 25% of survivors had symptoms of gas bubble disease. 



* 88% of survivors had symptoms of gas bubble disease. 



Table 6. — Mortality of juvenile fall chinook salmon of 

 wild (native) origin held in cages at various depths 

 in the forebay of Ice Harbor Dam, 24 June-2 July 

 1970.^ 



^ Water temperature was 17.1° C and dissolved nitrogen 

 concentration was 127% of saturation at start of test; water 

 temperature was 19.4° C and dissolved nitrogen concen- 

 tration was 134% of saturation at end of test. 



' Survivors had no symptoms of gas bubble disease. 



= 12% of survivors had symptoms of gas bubble disease. 



disease. Population estimates made by Ray- 

 mond (1971)' of juvenile chinook in the Salm- 

 on River at Whitebird, Idaho, and those arriv- 

 ing at Ice Harbor Dam indicated that about 

 70% of the migrating chinook were lost be- 

 tween these points. A similar estimate for 

 steelhead from Dworshak Hatchery indicated 

 a 15% loss to Ice Harbor Dam and about a 90% 

 loss to McNary Dam. These independent ob- 

 servations of the effect of supersaturation of 

 nitrogen gas on juvenile migrant chinook and 

 steelhead and the estimate of losses recorded 

 by Raymond clearly indicated that substantial 



^ Raymond, H. L. 1971. Survival and timing of the 

 1970 and 1971 migration of juvenile chinook salmon 

 and steelhead trout from the Snake River. Natl. Mar. 

 Fish. Serv., Biol. Lab., Seattle, Wash. (Unpublished 

 manuscript.) 



losses of juvenile steelhead and chinook mi- 

 grants resulted from supersaturation of dis- 

 solved nitrogen in the Snake River. 



Less data are available to determine the 

 effect of dissolved nitrogen on adult fish than 

 are available for juvenile fish. Examination 

 of adult chinook at Rapid River Hatchery 

 showed that after 6 July about 30 ''r of the 

 arriving fish had symptoms of gas bubble di- 

 sease. Searches along the Columbia and Snake 

 Rivers for dead salmon and steelhead during 

 each sampling survey indicated no significant 

 mortality until 7 July. On that date, 27 dead 

 adult chinook were counted between Little 

 Goose and Lower Monumental Dams on the 

 Snake River. Although most of the fish were 

 too decomposed to observe symptoms, symp- 

 toms of gas bubble disease on three of the fish 

 and high nitrogen concentrations in the river 

 led me to believe gas bubble disease was the 

 primary cause of the mortality. Apparently 

 there was considerable delay between the time 

 these fish were subjected to high levels of ni- 

 trogen gas and the time that the mortalities 

 were found. Concentrations of nitrogen gas 

 were high in the Snake River in early April 

 but did not become high in the Columbia until 

 early June. The appearance of mortalities in 

 early July was probably caused by the addition- 

 al exposure of the later migrating fish to high 

 concentrations in both the Snake and Columbia 

 Rivers. These limited observations cannot 

 measure the extent of the losses to adult mi- 

 grants. However, a more comprehensive study 

 (Mallet et al, 1971) indicated that 45% of the 

 adult spring chinook in the Snake River were 

 lost before they spawned and that the loss was 

 caused by the delayed efl['ects from exposure to 

 supersaturation of nitrogen gas. 



LITERATURE CITED 



BEININGEN, K. T., and W. J. EBEL. 



1970. Eff'ect of John Day Dam on dissolved 

 nitrogen concentrations and salmon in 

 the Columbia River, 1968. Trans. Am. 

 Fish. Soc. 99: 664-671. 



1 97 1 . Supersaturation of dissolved nitrogen 

 in the Columbia and lower Snake Rivers, 

 1965-69. U.S. Dep. Commer., Natl. 

 Oceanic Atmos. Admin., Natl. Mar. Fish. 



