Figure 6. --Dissolved oxygen levels at study area A (11-foot tide 

 level of Harris River). Samples were obtained 7 to 10 incties 

 beneath gravel surface at points shown. 



during the early September spawning period, 

 i.e., average oxygen levels differed spatially 

 between two large spawning areas in late 

 Slimmer. 



3. There was no significant difference 

 in oxygen levels between the two sampling 

 areas in midautumn. 



ROUTINE EVALUATION OF 

 OXYGEN LEVELS 



An important objective of the study of 

 dissolved oxygen content of intragravel water 

 is to determine the importance of oxygen 

 level as a factor associated with natural 

 mortality of salmon embryos. Field observa- 

 tions of dissolved oxygen levels and mortality 

 are not intended to define oxygen levels 

 lethal to embryos. Instead, they are designed 

 to establish general relationships between 

 oxygen level and mortality in natural en- 

 vironments. Determination of rates of oxygen 

 supply necessary to sustain embryos is pri- 

 marily a laboratory problem, and some prog- 

 ress has been reported on the study of the 

 oxygen requirements of embryos (Alderdice, 



Figure 7. --Dissolved oxygen levels at study area B (upstream 

 Harris River). Samples were obtained 7 to 10 inches beneath 

 gravel surface at points shown. 



Wickett, and Brett, 1958; Doudoroff, 1957; 

 Silver, 1960; Shumway, 1960). 



There was evidence that the low dissolved 

 oxygen levels observed in late summer 1957 

 were associated with high mortality; whereas, 

 the high dissolved oxygen levels observed in 

 late summer 1958 were associated with low 

 mortality. The ratio of dead to total pink 

 salmon eggs collected from 18 random points 

 in Indian and Twelvemile Creeks in November 

 1957 was 68.6 percent. In November 1958, 

 the ratio of dead to total pink salmon eggs 

 collected from 20 random points in Indian 

 and Twelvemile Creeks was only 11.4 percent. 

 This evidence suggested that low oxygen ]evels 

 observed in 1957 were indicative of environ- 

 mental conditions detrimental to the survival 

 of salmon embryos. 



It has been shown that spatial and temporal 

 variations in oxygen levels may be of great 

 magnitude. These variations are apparently 

 influenced by complex environmental factors 

 that are not well understood. 



Sampling methods described in this report 

 when used with statistically designed sampling 



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