FISHERY BULLETIN: VOL. 71, NO. 3 



DISSOLVED OXYGEN Img /I) 



</ 



o — o— oo 



/ 



Figure 6. — Surface pH at station 1, March 1961 to March 

 1962. Summer increases reflect blooms of blue-green algae. 



_ 130 1 



O 



u 120 



D 

 U 



E IIOH 

 a 



- too 



$ BO 



70 

 60 

 50 



40 



Figure 7. — Methyl orange alkalinity expressed as ppm 

 CaCOs at station 1, April 1961 to March 1962. 



swarms of adult Diptera hovered above the 

 reservoir. 



EFFECTS OF RESERVOIR 

 CONDITIONS ON SALMON 



Happy Valley Reservoir, a eutrophic im- 

 poundment with warm summer conditions, is 

 markedly different as a rearing site from the 

 streams naturally inhabited by juvenile chinook 

 salmon. Spring chinook salmon normally make 

 the parr-to-smolt transformation during the 

 spring after 1 yr of freshwater residence. Salmon 

 reared 1 yr in the reservoir met severe mid- 

 summer conditions. They were driven upward 

 by low oxygen concentrations and downward 

 by high temperatures, high pH, and possibly by 

 algal toxins released by decomposing blue- 

 green algae. As an example of midsummer 

 conditions, temperature and dissolved oxygen 

 profiles are presented in Figure 8 for 1 August 

 1961. Under these conditions, which likely did 

 not represent the extreme for that day, fish 



TEMPERATURE (°C) 



Figure 8. — Midday conditions of temperature and dis- 

 solved oxygen on 1 August 1961 at station 1. Fish die-off 

 occurred on this date. 



seeking temperatures below 20°C would have 

 had to accept dissolved oxygen concentrations 

 below 5 mg/liter. The preferred temperature is 

 12° to 13°C (Brett, 1952), at which depth oxy- 

 gen levels would not have sustained life. 



Scores of observations of fish "echoes" on 

 the dial of a depth-sounder indicated that the 

 fish sought the least extreme combination of 

 these low oxygen and high temperature stresses 

 and congregated near the 1.7-m (5- to 6-foot) 

 depth. Similarly, gill nets set at various depths 

 in August 1962 captured fish almost exclusively 

 at depths of 1.8 m (6 feet) or less (Table 1). Water 

 at 1.8 m usually held 5 mg/liter or more oxygen 

 and was just above the zone of rapid oxygen 

 decline (Figure 9). These observations are in 

 accord with those by Whitmore, Warren, and 



Table 1. — Frequency of capture by gill net at approxi- 

 mate depth of capture, 8 to 15 August 1962. Surface 0.6 m 

 was not fished. Most sets were made in waters more than 

 4.6 m deep. 



882 



