% mortal II y /^ — Coho 



5 10 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25 



APRIL MAY JUNE 



Figure 6. — Nitrogen concentrations and accumulative mortal- 

 ity of fish in shallow test tank (river water, tank 1 m deep) 

 during test. 



Table 5.— Number of dead fish and percentage with symptoms 

 of gas-bubble disease in each holding tank. 



Chinook 



Coho 



Holding 

 tank 



Dead fish 

 Total with symp- 

 dead toms (%) 



Dead fish 

 Total with symp- 



dead toms (%) 



salmon and day 67 for the coho salmon. In this tank, 

 79% of the Chinook salmon and 96% of the coho 

 salmon that died had definite external symptoms of 

 gas-bubble disease (Table 5). 



Weekly checks for gas-bubble disease (Table 4) 

 showed that from 25 April through 2 May, symptoms 

 completely disappeared on the test fish. It was during 

 this period that nitrogen concentrations in the river at 

 Prescott were decreased by the closure of spillway 

 gates at the dams (the spill closure was planned to aid 

 downstream migrating juvenile salmon and trout 

 released from hatcheries). 



Mortality and Symptoms in Relation 

 to Species and Size of Fish 



There were differences in rate of mortality and 

 incidence of disease symptoms between the two 

 species of test fish. The chinook salmon were the first 

 to show external symptoms, although to a lesser 

 degree of severity than the coho salmon. However, the 

 chinook salmon were the first to begin dying — a trend 

 which persisted throughout the test (Figs. 5, 6). For 

 example, in the shallow tank, 50% accumulative mor- 

 tality was recorded for chinook salmon on test day 50, 

 whereas 50% accumulative mortality of coho salmon 

 occurred on test day 67. The coho salmon showed a 

 higher incidence of symptoms of gas-bubble disease 

 but seemed to withstand the effects of the disease 

 longer than did the chinook salmon. 



A slight increase in length of fish was recorded 

 throughout the test period (Fig. 7); the trend is more 



DEEP TANK 



CONTROL TANK 

 150- 



E 

 E 



I 125 



l- 



(9 



Z 



|»j 100 



UJ 



S 75 



<£ 

 Ui 



25 



Coho 



April 



May 



April 



May 



Figure 7. — Fitted regression lines 

 for average body length of samples 

 (20 fish each) of juvenile coho and 

 chinook salmon during test. 



SHALLOW TANK 



150- 



E 

 X 



o 125 



Coho 



OOP 



100- 

 tu 

 o 



Qc 75- 

 > 



" 50- 



25'^ 



April 



May 



