JUVENILE SALMON 



Examination of juvenile chinook salmon that 

 suffered excessive mortality in holding tanks (used 

 for marking fish) at Priest Rapids Dam in 1966 

 revealed that most fisli had symptoms of gas bub- 

 ble disease. Although the fish otherwise appeared 

 to be sound, they were definitely distressed and, if 

 held, eventually died from gas embolism. Dissolved 

 nitrogen concentrations at Priest Rapids Dam 

 ranged from 114 to 133.9 percent saturation. Tem- 

 I>erature in the gatewells of the dam from which 

 fish were collected ranged from 13.9° C. (57° F.) 

 on July 7 to 17.8° C. (64.0° F.) on August 31 

 wlien operations ceased. The water temperature 

 in the holding tanks usually rose about 1° to 2° C. 

 before marking began. This slight increase was 

 sufficient to liberate enough gas from the blood of 

 the fish to cause embolism and death. The symp- 

 toms of gas bubble disease and distress were 

 eliminated if these fish were held in water slightly 

 cooler than that in the river where they were col- 

 lected. Usually a decrease of 1° or 2° C. was suffi- 

 cient to reduce the mortality. 



Apparently, juvenile chinook salmon, equili- 

 brated to supersaturation of nitrogen in this tem- 

 perature range, have considerably less tolerance 

 to increases in temperature than fish that are 

 equililirated to normal saturation. Brett (1952) in 

 his studies of the tolerance range of juvenile salm- 

 on to temperature changes showed that spring 

 chinook salmon acclimated to 10° C. (50° F.) 

 could tolerate temperatures up to 24.6° C. (76.1° 

 F.) before mortalities began. The fish at Priest 

 Rapids Dam, which were acclimated to higlier 

 temperatures (13.9-17.8° C), could not tolerate 

 increases of 1° to 2° C. 



It was apparent from observations at Priest 

 Rapids Dam that juvenile migrants were in a pre- 

 carious situation in July and August. If they did 

 not remain at sufficient depth to compensate for 

 the supei-saturation of dissolved nitrogen, they 

 died of gas bubble disease. Their tolerance to tem- 

 perature increases was also lowered; conceivably 

 sudden temperature increases of only a few de- 

 grees centigrade would have caused mortality. 



We conducted an experiment in the forebay of 

 Priest Rapids Dam to determine the depth at 

 which juvenile fish must remain to avoid gas bub- 

 ble disease wlien subjected to supersaturation of 

 nitrogen. Juvenile coho (0. Msutch) and chinook 



salmon were held in i^ens submerged at different 

 depths in the forebay and then observed by 

 SCUBA divers at various intervals for symptoms 

 of gas bubble disease. Three tests from 200-280 

 hours' duration were conducted from May 28 to 

 August 14, 1967, a period when saturation of dis- 

 solved nitrogen changed from a high of 143 per- 

 cent in late May and June to 118 percent in Au- 

 gust. Hatchery-reared coho salmon were used in 

 tests 1 and 2 and wild chinook in test 3. One 

 hundred fish were placed in each test cage and in 

 a control cage held in river water of normal sat- 

 uration at the dam. Tlie first observation was made 

 about 12 hours after placing the cages at the 

 selected depths in the forebay. Other observations 

 were made at 24-hour intervals for the first 4 days ; 

 the time between observations was increased 

 thereafter, depending on rate of mortality. Dead 

 fish were brought to the surface by divers and 

 examined for symptoms of gas bubble disease. 

 Neither test nor control fish were fed during the 

 tests. In one test 100 percent of the coho salmon 

 in a surface pen and 70 percent in a pen lowered 

 to a depth of 2 to 3 m. died from gas bubble dis- 

 ease (fig. 5) . Three percent was the maximum mor- 

 tality for control fish held during the test. The 

 experiment showed that at 130 to 140 percent sat- 

 uration fish must remain below 2.5 m. if they are 

 to be free of symptoms of gas bubble disease. From 



TEST I 

 COHO 



TEST 3 

 CHINOOK 



0.5- 2 2 5 

 1,5 3 3 5 



DEPTH OF CAGES 



2.0 2 5 0- 

 3 3 5 GO 



(M.) 



05- 20 25- 0- 

 I 5 3.0 3 5 6.0 



Figure .5. — Percentage mortalit.v caused by gas bubble 

 disease during holding exixriment in forebay of Priest 

 Rapids Dam May 28 to August 14, 1967. Saturation of 

 dissolved nitrogen ranged from 14.3 iiereent in test 1 

 to lis i>ercent in test 3 ; flsli were held about 200 hours 

 in tests 1 and 2,280 hours in test 3. 



SUPERSATURATION OP NITROGEN IN COLUMBIA RIVER 



