LEGGETT and WHITNEY: WATER TEMPERATURE AND SHAD MIGRATIONS 



Table 1. — Time of median shad passage at Bonneville 

 compared with time when temperature exceeded 15.5 °C 

 and flow diminished to 325,000 cfs and number of days 

 counted from May 31 (shown in parentheses).^ 



Date on which: 



Shad-temperature partial correlation r = 0.58, P<0.01 



Shad-flow partial correlation r = 0.44, P-C0.05 



Temperature-flow partial correlation r = —(3.004 not sig., df = 29 



1 Method is that of Talbot (1953) as are data 1938-1950, data 1951- 

 1969 from Corps of Engineers (1951-1969). 



analysis suffers from the usual defects of data 

 on two or more factors which may trend in the 

 same direction without necessarily being related 

 in a cause-and-effect manner. For example, we 

 do not believe that the significant correlation of 

 flow and the timing of the runs has any particu- 

 lar meaning beyond the fact that the shad run 

 occurs after flows begin to decline. Shad mi- 

 grate into streams of various sizes, and we do 

 not think that they select a particular flow in a 

 particular stream. On the other hand, the fact 

 that they select the same water temperature in 

 different streams and, as will be shown later, that 

 in their oceanic migration they remain in water 

 of the same temperature points to the conclu- 

 sion that the timing of the shad run is related 

 to water temperature. 



To find the particular temperature preferred 

 by shad, we tabulated the temperature when 

 each fish was counted at the Bonneville fish lad- 

 ders. For each year we then calculated an aver- 

 age of these temperatures. This amounts to de- 

 termining the temperature at the peak of the 

 shad run (Table 2). Such a procedure is justi- 

 fied by the large samples and the fact that the 

 frequency distribution is approximately normal. 

 Very little variation in the temperature at the 

 peak of the run occurred from year to year. Most 

 of the time the peak occurred at 18.0°C. In 78% 

 of the years the peak appeared in the range 

 16.5° to 19.0°C. (Table 3) . In the average year, 

 90% of the shad run was counted when the 

 temperatures ranged between 15.5° and 19.5°C. 

 In 26 of the 32 years the temperature varied 

 only 4.0 °C or less during the time when 90% 



Table 2. — Water temperatures (°C) associated with the 

 peak of the shad run and lower and upper temperatures 

 associated with the middle 90% of the shad counted at 

 Bonneville Dam fishways,' Columbia River. 



Year 



Lower 



Peak 



Upper 



Difference 



Mean 



16.0 



18.0 



19.5 



3.5 



1 From Corps of Engineers (1948-1969) and unpublished data for which 

 we are indebted to Ivan Donaldson, Corps of Engineers, and Kingsley G. 

 Weber, National Marine Fisheries Service. 



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